Natural Coastal Protection Series ISSN 2050-7941
The response of mangrove
soil surface elevation
to sea level rise
Anna McIvor, Tom Spencer, Iris Möller
and Mark Spalding
Natural Coastal Protection Series: Report 3
Cambridge Coastal Research Unit Working Paper 42
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Authors:
Anna L. McIvor, The Nature Conservancy, Cambridge, UK and Cambridge Coastal Research Unit, Department
of Geography, University of Cambridge, UK. Corresponding author: anna.mcivor@tnc.org
Tom Spencer, Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, UK.
Iris Möller, Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, UK.
Mark Spalding, The Nature Conservancy, Cambridge, UK and Department of Zoology, University of
Cambridge, UK.
Published by The Nature Conservancy and Wetlands International in 2013.
The Nature Conservancy’s Natural Coastal Protection project is a collaborative work to review the
growing body of evidence as to how, and under what conditions, natural ecosystems can and should be worked
into strategies for coastal protection. This work falls within the Coastal Resilience Program, which includes a
broad array of research and action bringing together science and policy to enable the development of resilient
coasts, where nature forms part of the solution.
The Mangrove Capital project aims to bring the values of mangroves to the fore and to provide the
knowledge and tools necessary for the improved management of mangrove forests. The project advances the
improved management and restoration of mangrove forests as an effective strategy for ensuring resilience
against natural hazards and as a basis for economic prosperity in coastal areas. The project is a partnership
between Wetlands International, The Nature Conservancy, Deltares, Wageningen University and several
Indonesian partner organisations.
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Suggested citation for this report
McIvor, A.L., Spencer, T., Möller, I. and Spalding. M. (2013) The response of mangrove soil
surface elevation to sea level rise. Natural Coastal Protection Series: Report 3. Cambridge
Coastal Research Unit Working Paper 42. Published by The Nature Conservancy and
Wetlands International. 59 pages. ISSN 2050-7941. URL:
http://coastalresilience.org/science/mangroves/surface-elevation-and-sea-level-rise
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Executive Summary
Coastal ecosystems such as mangroves can reduce risk to people and infrastructure from
wave damage and flooding. The continued provision of these coastal defence services by
mangroves is dependent on their capacity to adapt to projected rates of sea level rise. This
report explores the capacity of mangrove soil surfaces to increase in elevation in response to
local rises in sea level.
Historical evidence suggests that mangrove surface elevations have kept pace with sea level
rise over thousands of years in some places, such as Twin Cays, Belize. Rates of surface
elevation increase ranged between 1 mm/yr and 10 mm/yr in different locations and settings.
Key controls on this include external sediment inputs and the growth of subsurface roots.
Recent evidence based on measurements using the Surface-Elevation Table – Marker
Horizon methodology (from studies published between 2006 and 2011) suggest that
mangrove surfaces are rising at similar rates to sea level in a number of locations. However,
surface elevation change measurements are available for a relatively small number of sites,
and most records span short time periods. Longer term mangrove surface elevation datasets
are needed from more locations, and these need to be analysed relative to sea level changes
over the same periods of measurement.
Six sets of processes are known to influence surface elevation change in mangroves:
sedimentation/resuspension; accretion/erosion; faunal processes (e.g. burrowing of crabs);
growth/decomposition of roots; shrinkage/swelling of soils in the presence/absence of water;
and compaction/compression/rebound of soils over time and under the weight of soil/water
above. A variety of factors affect the rates of these processes, including the supply of external
sediment, the types of benthic mats that bind surface sediments together, vegetation
characteristics such as tree density and aerial root structure, nutrient availability to subsurface roots, storm impacts, and several hydrological factors such as river levels, rainfall and
groundwater pressure. The sum of these processes results in surface elevation change.
The number and complexity of processes involved in surface elevation change create
significant challenges to the modelling and prediction of future elevation change in the face
of sea level rise. It is likely that negative feedbacks exist between sea level change and
surface elevation change, but evidence for these feedbacks is currently lacking. Such
feedbacks might enable mangrove soil surfaces to maintain their surface elevation with
respect to local sea level over the longer term. Threshold rates of sea level rise are also likely
to exist, beyond which mangrove surfaces are no longer able to keep up. An improved
understanding of the different processes and feedbacks involved in surface elevation change
will increase our ability to predict the response of surface elevation to sea level rise, and to
manage mangrove areas in ways that enhance their ability to keep pace with sea level rise.
Monitoring and management of mangrove areas is recommended to ensure continued
provision of coastal defence services into the future. In particular, sediment inputs need to be
maintained, mangroves should be protected from degradation, and space should be allowed
for mangroves to colonise landward areas. In many areas, short term anthropogenic losses of
mangroves represent a greater threat to the provision of coastal defence services by
mangroves than the longer term effects of sea level rise.
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Contents
1. Introduction ........................................................................................................................ 5
1.1 The tidal environment, mangroves and accommodation space ................................... 6
1.2 Sea level rise ................................................................................................................ 8
1.3 Surface elevation change in mangroves ..................................................................... 10
1.4 How mangrove surface elevation varies with sea level rise ...................................... 11
2. Can mangrove surface elevation keep pace with sea level rise? ..................................... 13
2.1 Historical evidence..................................................................................................... 13
2.2 Recent evidence ......................................................................................................... 15
2.2.1 Measurements made using the SET-MH methodology ...................................... 15
Box 1. The SET-MH methodology.............................................................................. 16
2.2.2 Comparing surface elevation change data with sea level rise data ..................... 19
2.2.3 Conclusion .......................................................................................................... 20
3. Processes .......................................................................................................................... 21
3.1 Surface processes ....................................................................................................... 21
3.1.1 Sedimentation ..................................................................................................... 21
3.1.2 Accretion ............................................................................................................. 25
3.1.3 Erosion ................................................................................................................ 28
3.1.4 Surface faunal processes ..................................................................................... 30
3.2 Subsurface processes ................................................................................................. 30
3.2.1 Root growth and decomposition ......................................................................... 32
3.2.2 Shrink-swell of soils (dilation water storage) ..................................................... 33
3.2.3 Compaction, compression and rebound .............................................................. 35
3.2.4 Subsurface faunal processes ............................................................................... 35
4. Magnitude of surface and sub-surface contributions to surface elevation change .......... 36
4.1 Accretion, shallow subsidence and surface elevation change ................................... 36
4.2 Interactions between surface and subsurface processes............................................. 39
4.3 Factors affecting surface elevation change rates ....................................................... 39
4.3.1 Forest type ........................................................................................................... 40
4.3.2 Tidal range .......................................................................................................... 41
4.3.3 Tree density......................................................................................................... 41
4.3.4 Nutrient availability ............................................................................................ 41
4.3.5 Mean sea level and hydrological factors ............................................................. 42
4.3.6 Storms and hurricanes ......................................................................................... 42
5. The effect of sea level rise rates on elevation change rates ............................................. 42
5.1 Factors affecting surface elevation change in the face of SLR .................................. 43
5.1.1 Sediment inputs................................................................................................... 43
5.1.2 Tidal range .......................................................................................................... 44
5.2 Feedbacks ................................................................................................................... 44
5.3 Thresholds .................................................................................................................. 46
6. Predicting surface elevation change with future sea level rise ........................................ 47
6.1 A mangrove sediment development model for mangroves in Honduras ................... 47
7. Conclusions ...................................................................................................................... 48
8. Acknowledgements .......................................................................................................... 50
9. References ........................................................................................................................ 51
Appendix A: Data used to create figures, with sources of information. .............................. 58
Appendix B. Location of tide gauges, approximate distances between SET-MH
measurement station and tide gauges, tide gauge measurement period and relative sea level
rise measured there. ............................................................................................................. 59
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
1. Introduction
Coastal ecosystems such as mangroves can reduce risk to people and infrastructure from
wave damage and flooding. The continued provision of these coastal defence services by
mangroves is dependent on their capacity to adapt to sea level rise, either through an increase
in soil surface elevation (Figure 1), or by colonising more landward areas. In this report we
review the response of mangrove soil surface elevation to sea level rise. For a discussion of
the factors affecting the landward migration of mangroves, see Woodroffe (1990), Ellison
(1993), Woodroffe (1995), Gilman et al. (2007), Gilman et al. (2008) and Soares (2009).
Lovelock and Ellison (2007) and Ellison (2012) review other potential effects of climate
change on mangroves, which will also affect the long-term provision of coastal defence
services by mangroves.
An understanding of how mangrove surface elevation is likely to respond to changes in sea
level is needed in order to predict whether mangroves will be able to survive in their current
position as sea levels rise, and to manage mangrove ecosystems in ways that increase their
chance of surviving in the face of rising sea levels. In this report we present the current state
of knowledge, starting with basic descriptions of the key concepts, then describing available
data and discussing various factors that may affect surface elevation change, before finishing
with a description of a sediment development model that could be used to predict future
surface elevation change in mangroves. The information and discussion provided here are by
necessity incomplete, as relatively few studies have explored this topic, few data are
available, and many important questions remain unanswered.
In the first section of this report, we briefly explain how sea level is changing, why this varies
locally, what is meant by “surface elevation change” in mangroves, and how mangrove
surface elevation may be able to keep pace with local sea level rise. In Section 2, we examine
historical and recent evidence for mangrove surface elevation keeping pace with sea level
rise. In Section 3, we summarise the processes involved in mangrove surface elevation
change and the factors that affect these processes. In Section 4, we explore the relative
contribution of surface and subsurface processes to elevation change, and look at factors
known to affect surface elevation change rates. Section 5 then considers the factors affecting
the response of mangrove surface elevation to sea level rise, including possible feedbacks and
thresholds. Section 6 briefly considers a sediment development model that aims to predict
surface elevation change in mangroves. Section 7 concludes by considering what more we
need to know in order to better predict when and where mangroves may be able to maintain
their surface elevation in the face of sea level rise.
Figure 1. Schematic diagram showing how, when mangrove soil surface elevation can keep pace with
sea level rise, mangroves will be able to continue to protect people and infrastructure from waves.
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
1.1 The tidal environment, mangroves and accommodation space
Mangrove forests include a variety of species of trees and shrubs that are able to live in
tidally flooded areas. Mangrove forests occur in intertidal areas, at heights between mean sea
level (MSL) and high tide (mean high water; exact tidal levels vary with species and location;
Ellison, 2009). Therefore they occupy the upper part of the tidal frame, where the ‘tidal
frame’ refers to the area that is flooded by the tides (i.e. it does not include areas that are
always under water or which are only flooded during storms).
Due to the shifting and dynamic nature of the tidal environment, intertidal mudflats both form
and are washed away over relatively short periods of time (a single storm can radically alter a
muddy coastline). When the height of a mudflat reaches a height above mean sea level
suitable for mangroves, and providing mangrove propagules (i.e. seeds) are available, then
mangroves are expected to colonise such an area (Figure 2a). Once mangroves have
established, they may change the environment: by slowing water flows and reducing wave
energy, they may allow further deposition of sediments, and through the growth of subsurface
roots, they may increase the soil volume. Both processes can further increase the height of the
soil surface. If a time comes when soil inputs and losses approximately balance such that the
soil surface height (i.e. the surface elevation) remains relatively stable (e.g. Figure 2b), then
mangroves may remain as the climax vegetation for many years (sometimes thousands of
years e.g. in Twin Cays, Belize). If the height of the soil surface continues to increase due to
soil inputs exceeding soil losses, then the soil surface height may continue to rise until it
reaches the upper limit for mangroves to survive; ultimately, terrestrial vegetation may
outcompete mangroves.
The difference in height between the current soil surface height within a mangrove forest and
the maximum soil surface height that can be achieved with mangroves present (limited either
by the balance of soil inputs and losses, or by mangrove vegetation being outcompeted by
terrestrial vegetation) is referred to as the mangrove accommodation space (Figure 2a).
More generally, the term ‘accommodation space’ describes the available space for soil
expansion or growth, both vertically and laterally, given the current position of the soil
surface, the tidal frame, and erosive forces1. Over a particular stretch of coast, an
accommodation volume may also be defined as the volume of space above the substrate that
could be filled with sediment and allow mangroves to grow there; this allows for a ‘lateral
accommodation space’, meaning seaward areas where mangroves could live if sediment filled
the space (limited also by bathymetry and wave conditions eroding sediment; these factors
limit the seaward edge of the accommodation space shown in Figure 2). The accommodation
concept is widely used in geology (e.g. Schlager, 1993; Miall, 1996); in relation to coastal
ecosystems, it has been applied more frequently to coral reef systems (e.g. Pomar, 2001;
Kennedy and Woodroffe, 2002; Montaggioni, 2005), but only occasionally in relation to
saltmarshes (e.g. French, 2006) and mangroves (Spencer and Möller, 2013).
When sea level rises or land subsides, the volume of accommodation space increases (Figure
2c), as the difference in height between the height of the substrate and mean sea level has
increased. This volume can now be filled with soil if soil inputs are high enough, allowing the
1
The concept of accommodation space is fundamental in the study of sequence stratigraphy in geology, and
Miall (1996, p. 456) offers the following definition from Jervey (1988): “the space made available for potential
sediment accumulation [where] in order for sediments to be preserved, there must be space available below base
level (the level above which erosion will occur)”. In other words, accommodation space refers to the space
between the level of the substrate and the highest level that sediment could remain without being eroded away.
6
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 2. Schematic diagram illustrating the concept of accommodation space (see text for further
description).
soil surface to rise until the newly created accommodation space has been filled. Soil inputs
include organic or inorganic sediments and subsurface roots. The increase in height of the
mangrove soil surface can result in mangroves remaining in their preferred part of the tidal
frame, i.e. between mean sea level and high tide. Without such an increase in soil surface
height, the mangrove surface could end up below mean sea level, creating stress on mangrove
trees, and probably resulting in their death. If the change in soil surface height exactly
matches the change in sea level, this results in the relative height of the mangrove surface
remaining constant within the tidal range (Figure 2b and c).
In Sections 2 to 7, we explore whether mangrove soil surfaces tend to rise in response to rises
in sea level, the mechanisms underlying this, and the factors affecting it.
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
1.2 Sea level rise
Globally, mean sea levels are rising as a result of both the thermal expansion of sea water, as
temperatures rise with climate change, and the melting of the polar ice caps and other land
ice, which add additional water to the sea (Cazenave et al., 2008). Both thermal expansion
and melting of ice increase the volume of water in the oceans, and the resulting rise in sea
level is called eustatic sea level rise. Recent estimates of mean global sea level rise are 3.4 ±
0.4 mm/year over the 14 year period from 1993 to 2007, based on satellite measurements of
sea surface level (Beckley et al., 2007). Taking a longer term perspective, sea levels have
been relatively stable over the last 7,000 years (global mean sea level rose by 3 to 5 m over
this period, i.e. rise rates of 0.4 to 0.7 mm/yr; Fleming et al., 1998). Over the last 20,000
years, sea levels have risen by more than 100 m, and sea levels have fluctuated widely over
the last 250,000 years (Curray, 1965; Chappell and Shackleton, 1986). These fluctuations are
largely related to periods of glaciation, when more water is locked up as ice on land, resulting
in a fall in global mean sea level.
There is significant spatial and temporal variation in eustatic sea level (Cazenave et al.,
2008). Spatial variation in recent sea level trends is shown in Figure 3. Some areas have
experienced much higher rates of sea level rise (e.g. parts of the Philippines), while others
have experienced falls in sea level (e.g. parts of the west coast of North America). The main
cause of regional variation in sea level change is the regional variation in thermal expansion
(Cazenave et al., 2008). Temporal variation in sea levels also occurs, caused by temporary
reorganisation of ocean currents and associated oscillations in regional ocean temperatures
which affect thermal expansion, such as those seen with the El Niño Southern Oscillation
(ENSO), which affects large areas of the Pacific Ocean (Lombard et al., 2005).
Mean sea level rise as measured by tide gauges along the coast also varies because of vertical
land movements, such as glacial isostatic adjustments and lithospheric flexural subsidence
(Pugh, 2004; Yu et al., 2012). These changes in land level result from a wide range of factors,
such as earthquakes and tectonic movements, consolidation of coastal sediments (e.g. in
deltas), the extraction of oil or water, and a change in loading (i.e. weight) on the land surface
or sea floor (e.g. from the melting of glaciers and ice caps or the deposition of sediments
around large deltas) (Pugh, 2004; Mitchum et al., 2010). Rates of uplift/subsidence vary
geographically: for example, uplift rates of up to 20 to 30 mm/yr have been observed in
northeast Canada, while subsidence rates of up to 6 to 7 mm/yr have been observed between
Greenland and northeast Canda (Pugh, 2004).
The combination of eustatic and isostatic changes in sea level results in sea level rise rates
which vary significantly along coasts and over time. The net effect of eustatic and isostatic
sea level changes in a particular location is referred to as Relative Sea Level Rise (RSLR)
(Figure 4, top). It is this local change in sea level that affects coastal ecosystems such as
mangroves and the people who live along these coastlines. Therefore, for the purpose of
understanding the relationship between sea level change and mangrove surface elevation
change, local measurements of sea level are needed.
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 3. Global map of eustatic sea level trends between 1992 and 2012. Map and altimetry data are
provided by the NOAA Laboratory for Satellite Altimetry
(http://ibis.grdl.noaa.gov/SAT/SeaLevelRise/LSA_SLR_maps.php).
Figure 4. Regional and local processes affecting the elevation of the mangrove surface relative to
local mean sea level.
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
1.3 Surface elevation change in mangroves
The elevation of a point on the Earth’s surface is the height of that point measured with
respect to a reference point or datum. The elevation of the soil surface within a mangrove
area is referred to as the surface elevation within the mangrove, and is the height of the
mangrove substrate, usually measured with respect to a local datum such as mean sea level.
Surface elevation change refers to a change in height of the soil surface over a defined
period of time (Figure 5); such changes in surface elevation are usually not referenced to a
local datum, because of the practical difficulties of doing so.
A number of processes may result in changes in the mangrove surface elevation, and these
are illustrated in Figure 4 (lower part). These processes may be divided into surface processes
and sub-surface processes. For the purposes of this report, the soil surface refers to the
interface between the soil and the air (or water, when the tide covers the soil) (Figure 5).
Surface processes refer to those processes which occur at or above the mangrove soil
surface, including sedimentation (the deposition of material on to the surface of the soil),
accretion (the binding of this material in place), and erosion (the loss of surface material).
Subsurface processes refer to processes that occur below the soil surface but above the
basement or consolidated layer (Figure 5); these include growth and decomposition of roots,
swelling and shrinkage of soils related to water content, and compaction, compression and
rebound of soils due to changes in the weight of material above.
Figure 5. Schematic diagram of a mangrove tree and the soil beneath it, showing where accretion,
shallow subsurface change and deep subsidence/uplift occur in the profile, and illustrating how
surface elevation change may occur over time.
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
When sub-surface processes result in a change in volume of the soil, this is called subsurface expansion or shallow subsidence. Shallow subsidence refers to the loss of elevation
caused by these sub-surface processes, which act above the bedrock or consolidated layer; it
is called “shallow” to distinguish it from “deep” subsidence, caused by longer-term
geological processes (Cahoon et al., 1995a), which are accounted for in relative sea level rise
rates (described above in Section 1.2).
These surface and subsurface processes are described in more detail in Section 3; the
combined effect of these processes results in surface elevation change, as described in
Section 4.
1.4 How mangrove surface elevation varies with sea level rise
The following scenarios describe how mangrove surface elevation may change as mangroves
are exposed to sea level rise (some of these are shown schematically in Figure 6):
1. In areas with very high rates of sedimentation, mangrove soil surfaces may rise at a
rate which exceeds the local rate of sea level rise, such that terrestrial species invade
landward areas, and progradation occurs (i.e. new land is formed seaward of the
current mangrove area, which mangroves then colonise); this is likely to occur around
the deltas of large rivers that bring high volumes of sediment to the coast.
2. Sea level rise rates may be matched by a rise in mangrove soil surface elevation,
allowing mangroves to remain in the same location, possibly also colonising more
landward areas if such areas have suitable substrate and topography (this scenario is
illustrated in Figure 6d). An example of where this has occurred is Twin Cays in
Belize, as discussed in Section 2.1.
3. Mangroves soils may be unable to rise as fast as the local rate of sea level rise,
resulting in death of trees in the lower areas and at the seaward edge of the mangrove
area (Figure 6b). Mangroves are likely to invade landward areas which now fall
within the tidal frame, providing suitable substrate and topography are present there.
The deeper water in mangrove areas may also allow waves to penetrate further into
the mangrove area, resulting in erosion particularly at the seaward edge.
Which of these scenarios is observed in any particular location is likely to depend on rates of
sedimentation and sub-surface soil inputs in combination with rates of sea level rise. A
variety of positive and negative feedbacks between changes in sea level and the rates of
surface and subsurface processes that influence soil volume may also be at play (these are
discussed in Section 5).
A useful measure of how mangrove surface elevation is changing relative to local sea level
change is the elevation surplus/deficit (Cahoon et al., 1995a), which is calculated as:
elevation surplus/deficit = surface elevation change – relative sea level rise rate.
(mm/yr)
(mm/yr)
(mm/yr)
An elevation surplus occurs if surface elevation rises more quickly than sea level, while an
elevation deficit reflects that sea level is rising at a faster rate than the mangrove surface.
If both the mangrove surface elevation and the sea level are changing, another useful measure
is the rate of sea level rise relative to the mangrove surface, which we term the “mangrovesurface-relative sea level rise” (MSR-SLR). While this is calculated in the same way as the
elevation surplus/deficit outlined above, it is useful to be able to describe changes in water
level with respect to the mangrove surface, particularly when considering feedbacks between
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 6. Schematic diagram of mangroves to demonstrate tidal range, tidal frame, accommodation
space, and possible scenarios following sea level rise with or without surface elevation change.
12
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
water levels and the rates of surface and sub-surface processes (Section 5). A positive value
means that local sea levels are rising more quickly than the mangrove surface resulting in
deeper water over the mangrove substrate and more frequent inundation; a negative value
would indicate that mangrove surfaces are more than keeping pace with sea level rise and
being inundated less often (scenario 1 above).
The following section explores historical and recent evidence for mangrove surfaces keeping
pace with sea level in different locations.
2. Can mangrove surface elevation keep pace with sea level rise?
There are two sources of evidence for whether mangrove surface elevation can keep pace
with sea level rise: historical evidence of mangrove persistence in the face of sea level rise
over thousands of years, and recent measurements of surface elevation change that can be
compared with known rates of sea level rise over similar periods and in nearby locations. We
will consider these two sources of evidence in turn.
2.1 Historical evidence
In some areas, mangrove surface elevation has kept pace with sea level rise over thousands of
years. The most compelling evidence that mangrove surface elevation is able to keep pace
with sea level rise comes from areas with deep mangrove peats under existing mangroves,
such as in Twin Cays and the Tobacco Range Islands, Belize. The peat layer can be several
metres thick, formed from dead mangrove material that has accumulated over many years.
The age of the peat layers can be estimated using radiocarbon dating techniques (described in
Toscano and Macintyre, 2003). Dating of the deepest layers of peat show that some peat
layers are more than 7000 years old; for example, mangrove peat found at a depth of 8.7 m in
Twin Cays was estimated to be between 7,430 and 7,580 years old (McKee et al., 2007;
Figure 7).
Depth (metres below Mean Sea Level)
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
0
2000
4000
6000
8000
10000
12000
Calendar age (years before 1950)
Figure 7. Mangrove peat depth-age data (x) from Twin Cays, Belize (McKee et al., 2007) plotted on
top of a sea-level history curve (line) derived from separate studies of the age of mangrove peat and
coral material at different depths from the Caribbean region (Toscana and Macintyre, 2003).
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McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
The age of the mangrove peat at different depths gives an indication of the sea level at the
time that the peat was formed, provided that there has not been significant compaction of peat
layers. Mangroves can only live in the intertidal zone, so the age of mangrove peats at
different depths has been used to construct sea level rise curves (Scholl, 1964; Woodroffe,
1990; Toscano and Macintyre, 2003). Dating of mangrove peat formed by the red mangrove
Rhizophora mangle, in combination with coral material formed by the reef crest coral
Acropora palmata, was used to reconstruct a sea level rise curve for the Western Atlantic
region (including the Caribbean) (Toscano and Macintyre, 2003); the curve is shown in
Figure 7. More recent dating of mangrove peats from Twin Cays by McKee et al. (2007) is in
close agreement with the original data used to construct the curve by Toscano and Macintyre
(2003); the data from McKee et al. (2007) are also shown in Figure 7.
The Twin Cays data show that mangroves were not present before 7,600 years BP2, when sea
level rise rates were greater than 3.5 mm/yr (McKee et al., 2007). After this time, the
mangroves accumulated peat at rates of 3 mm/yr between 7,600 and 7,200 years BP, 1.3
mm/yr between 7,200 and 5,500 years BP, and 1.0 mm/yr between 5500 and 500 years BP,
matching sea level rise rates in the region (McKee et al., 2007). This demonstrates that
mangroves in this area have been capable of increasing in surface elevation at a rate of at
least 3 mm/yr. If the mangrove surface elevation had not kept pace with sea level rise, the
substrate would now be several metres below sea level and mangroves would no longer be
present. The absence of mangrove peat more than 7,600 years old could be related to a
number of factors, including an absence of suitable substrate, unsuitable climatic conditions,
a lack of mangrove seeds arriving in the area, or an inability of mangrove surface elevation to
keep pace with the higher rates of sea level rise before this date.
Similar studies exist in other areas (Table 1). These studies show that mangroves in different
areas have been able to keep pace with sea level rise for long periods; some of them were
then drowned as sea level rise rates increased beyond a critical threshold for that site, and
others were replaced by terrestrial vegetation following high rates of sedimentation. These
studies are reviewed in Ellison (2008 & 2009).
Mangrove peat found in cores taken from the sea bed (e.g. Parkinson, 1989; Ellison, 1993;
Ellison, 2008) provides evidence that mangroves may be submerged by rising sea levels. For
example, Parkinson (1989) took sediment cores from a number of locations within Ten
Thousand Islands in Florida, and found a layer of mangrove peat buried beneath other
sediments in areas of open water up to 6 km from the coast and 5 m below mean sea level.
Radiocarbon dating indicated that this peat layer was more than 3,500 years old.
Therefore historical records show that in some locations, mangrove surface elevations have
kept pace with rising sea levels over thousands of years until the present day. In other
locations, surface elevations kept pace with sea level rise for a period of time, but mangroves
were eventually drowned when the rate of sea level rise exceeded some threshold that
mangrove surface elevations could not keep pace with. These thresholds vary with location
and are likely to depend on local conditions (thresholds are discussed further in Section 5.3).
2
BP stands for “Before Present”, where the year 1950 A.D. is taken as the reference point for “Present”.
14
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Table 1. Locations and periods where mangroves kept pace with sea level rise.
Location
Period during which
mangroves persisted
Relative sea
level rise rate
that mangroves
kept pace with
6 mm/yr (12 m
rise in relative
sea level during
this period)
Additional information
Source
Mangrove swamp replaced by
terrestrial vegetation after 5,500 BP
as a result of sedimentary landfill
Woodroffe, 1990;
Ellison, 2009
South
Alligator
River,
Australia
Between 8,000 and
6,000 years BP
Mary River,
Australia
Between 6,500 and
4,000 years BP
Up to 10 mm/yr
Sedimentation caused mangrove
forest to be replaced by freshwater
wetlands
Woodroffe and
Mulrennan, 1993;
Ellison, 2009
Twin Cays,
Belize
Since 7,600 years BP
Up to 3 mm/yr
Described in text above
McKee et al., 2007
Hungry Bay,
Bermuda
Since 2,000 years BP
0.85 to 1.1
mm/yr
Mangrove lost 26% of its area over
previous century due to retreat of
seaward edge
Ellison,1993 &
2009
Fanga'uta
Lagoon,
Tonga
Between 7,000 and
5,500 years BP
1.2mm/yr
Became submerged after 5,500 years
BP with more rapid sea level rise, but
re-established in new locations when
rates slowed
Ellison, 2009
Kosrae,
Federated
States of
Micronesia
Since 2000 years BP
1 to 2 mm/yr
During rapid sea-level rise (10
mm/yr) between 4,100 and 3,700
years BP, mangrove forests retreated
landwards
Fujimoto, 1997, in
Ellison, 2008
2.2 Recent evidence
Recent evidence relating to whether mangrove surface elevation can keep pace with sea level
rise comes from direct measurements of changes in surface elevation using the Surface
Elevation Table – Marker Horizon (SET-MH) methodology, which is described in more
detail in Box 1. This method can measure surface elevation change over periods of months to
years, and measurements made using this methodology are used throughout the rest of this
report. Other methods have been used to measure surface accretion3 in mangroves, including
the use of marker horizons alone and the aging of sediment layers using radionuclides;
however these methods do not account for sub-surface changes in soil volume e.g. due to
compaction, which also affect the level of the soil surface, and therefore they cannot be used
to compare surface elevation change rates with sea level rise rates.
2.2.1 Measurements made using the SET-MH methodology
Cahoon et al. (2006) brought together available mangrove surface elevation change data that
had been measured using the SET-MH methodology for at least a year (Cahoon and Hensel,
2006, also refer to these data). These data were measured in 19 geographical locations in
seven countries (United Sates, Mexico, Belize, Honduras, Costa Rica, the Federated States of
Micronesia and Australia). In each location, a number of different SET-MH stations were set
up to explore elevation change in different forest types (e.g. fringe, basin, riverine or
overwash forests) or in different energy settings (i.e. exposed or protected forests), and
altogether 60 settings were included in the analysis.
3
Accretion refers to the addition of material to the soil surface, and is described in more detail in Section 3.
15
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Box 1. The SET-MH methodology
Surface elevation change is now standardly measured using the Surface Elevation Table –
Marker Horizon (SET-MH) method (also called the Sedimentation-Erosion Table –
Marker Horizon method). The Surface Elevation Table – Marker Horizon methodology
combines a marker horizon (used to measure accretion) with a measurement of the height
of the soil surface above a base layer underground, usually a layer of consolidated
material that a rod or pipe is driven into to the point of refusal (Figure 1.1). The method
thus allows the measurement of surface elevation change relative to the bedrock or
consolidated layer, which becomes the underground benchmark (Cahoon and Lynch,
1997). The combination of surface elevation change and accretion measurements allows
the magnitude of sub-surface change to be calculated (described below).
The apparatus consists of a long pipe driven into the sediment to the point of refusal,
which is left permanently within the sediment, and the measuring apparatus is attached to
the top of the pipe when it is time to take a reading (Figure 1.1). The pipe thus acts as a
reference point, which is expected to remain stable over time (it will only be affected by
geological uplift or subsidence of the underlying bedrock or consolidated layer).
(continued on next page)
Figure 1.1. The Surface Elevation Table - Marker Horizon apparatus, shown schematically (left)
and in use in a marsh (top right, showing Iris Möller measuring marsh surface elevation, and fresh
kaolinite layers to be used as marker horizons, at Cartmel Sands, Morecombe Bay) and mangrove
(bottom right, showing USGS hydrologic technician Karen Balentine measuring surface elevation
in a mangrove forest near Lostmans River, Everglades National Park). Photos by Ben Evans
(marsh) and USGS (mangrove; used with permission from Thomas J Smith).
16
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Box 1. The SET-MH methodology (continued)
The measuring apparatus consists of an arm attached to the reference pipe (Figure 1.1).
The arm holds up a small table through which nine plastic rods can be lowered gently onto
the substrate surface; the distance from the surface to the table is then measured for each
of the rods, in each of four directions from the pipe, at time intervals ranging from months
to years. These measurements are used to calculate the rate of change of the surface
elevation with reference to the benchmark.
Nearby, markers are placed on top of the sediment in patches (often 50 by 50 cm). The
markers consist of lighter-coloured material such as feldspar or kaolin. After a period of
time, a core is taken through the patch in order to measure the depth of sediment that has
accreted above the patch. This gives a rate of accretion. By subtracting the rate of
accretion from the rate of surface elevation change, it is possible to calculate the rate of
sub-surface change, based on the following equation:
surface elevation change (mm/yr) = accretion (mm/yr) + sub-surface change (mm/yr).
Section 4 describes the range of measurements recorded at several different mangrove
sites.
Full details of this method can be found in the original paper by Boumans and Day (1993)
and on the USGS Surface Elevation Table web-site (Cahoon and Lynch, 2003). The
methodology has been developed more recently to allow measurements of expansion in
different sub-surface layers (Whelan et al., 2005; Cahoon et al., 2011), and different
versions of the SET-MH apparatus now exist such as the rod SET (Cahoon et al., 2002).
Cahoon et al. (2006) compared the change in surface elevation with long term rates of
relative sea level rise measured as close as possible to the SET-MH sites, and found that in
most sites, surface elevation change lagged behind relative sea level rise, resulting in an
elevation deficit (i.e. surface elevation fell with respect to local sea level; see definitions in
Section 1.4). They did not find a relationship between elevation change rates and relative sea
level rise rates, except in embayments (one of five geomorphic classes that the sites were
divided into), where elevation change increased with relative sea level rise (however, the
significance level was low at p = 0.07, n = 8).
We repeated their analysis with more recent data from 15 geographical locations (including
31 settings), using data from 5 studies published between 2006 and 2011 (Table 2, raw data
given in Appendix A). Five sites showed an elevation surplus, while 10 sites showed an
elevation deficit with respect to relative sea level rise for the area (Figure 8 shows the
frequency distribution of elevation surplus/deficit). The mean elevation surplus/deficit was 1.26 mm/yr (mean of 15 values), and this was not significantly different from zero (t = -1.59,
d.f. = 14, p-value = 0.13) (surface elevation change rates varied between -2.6 and 5.64
mm/yr, with a mean value of 0.69 mm/yr; relative sea level rise rates varied between -0.47
and 4.1 mm/yr, with a mean value of 1.95 mm/yr). These more recent data suggest that
mangrove surface elevations are keeping up with relative sea level rise rates in some
locations.
17
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Table 2. Mangrove locations where surface elevation change has been measured and where rates of
relative sea level rise are available.
Surface
elevation
change
(mm/yr)
Record
length
(years)
Relative sea
level rise
rate
(mm/yr)
+0.61 to +3.85
3
2.1
McKee, 2011
Twin Cays, Belize
-3.7 to +4.1
3.5
2.0
McKee et al., 2007;
McKee, 2011
Various sites on Kosrae and
Pohnpei, Micronesia
-5.8 to +6.3
1.4 or
3*
1.8
Krauss et al., 2010
Moreton Bay, Australia
+1.4 to +5.9
3
2.4
Lovelock et al.,
2011a
Several sites in Australia
-2.6 to +5.64
3
-0.5 to +4.1
Location
Rookery Bay and Naples
Bay, Florida, US
Source
Rogers et al., 2006
* Krauss et al. (2010) measure surface elevation change over 1.4 or 3 years, and 5 or 6.6 years. Here
we use the shorter period of measurement because accretion and sub-surface change measurements
were measured concurrently (described in more detail in Sections 3 and 4).
Figure 8. Histogram showing the distribution of elevation surplus/deficit values at 15 locations
described in Table 2 and Appendix A (mean values have been taken for each location, with the
exception of Kosrae and Pohnpei in Micronesia, which are treated as two separate locations).
18
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 9 plots these surface elevation change measurements against relative sea level rise
rates as measured in nearby tide gauges (distance to tide gauges given in Appendix B). Figure
9 shows that there was a high level of variation in surface elevation change measurements in
most sites (raw data given in Appendix A). There was no significant relationship between
surface elevation change and relative sea level rise (linear regression: F(1,13) = 2.81, p = 0.12).
8
several locations, Australia
(Rogers et al., 2006)
Surface elevation change +/- S.E. (mm/yr)
6
Moreton Bay, Australia
(Lovelock et al., 2011)
4
Kosrae and Pohnpei,
Micronesia (Krauss et al.,
2010)
2
0
-1
0
1
2
3
4
5
Twin Cays, Belize (McKee,
2011)
-2
Rookery Bay and Naples
Bay, Florida (McKee, 2011)
-4
Surface elevation change =
Relative Sea Level Rise
-6
Relative Sea Level Rise (mm/yr)
Figure 9. Surface elevation change plotted against relative sea level rise at different locations. The
dashed line shows the case where the rate of surface elevation change equals the rate of sea level rise.
Points above this line represent sites where surface elevation change is more than keeping pace with
sea level rise, while below the line, sites are not keeping pace. Where several points have the same
Relative Sea Level Rise, the points have been slightly staggered to make the error bars visible.
Standard errors are not shown for data points from Moreton Bay, Kosrae, Pohnpei, Twin Cays or
Rookery Bay and Naples Bay, Florida, as the raw data from which to calculate the standard error of
these mean elevation change measurements were not provided in the respective source papers.
2.2.2 Comparing surface elevation change data with sea level rise data
When comparing surface elevation change data with sea level rise data, several potential
issues need to be taken into account, including:
high temporal variability in both surface elevation change and sea level change
measurements, combined with different measurement periods. Temporal variation in
sea level change can be large. e.g. Church et al. (2006) estimate that sea level varied
by more than 300 mm over a 2 year period on the island of Pohnpei in Micronesia
(coinciding with the beginning of Krauss et al.’s (2010) study of surface
elevation.change there). Similarly, surface elevation can both rise and fall over
relatively short periods: e.g. Gilman et al. (2007) measured surface elevation changes
of 50 to almost 200 mm over less than 6 months using stakes in American Samoa.
In the studies in Table 2, surface elevation change was measured over periods of 3.5
years or less, while sea level rise was measured over periods of 10 years or more (data
in Appendix A and B). Even if surface elevation at a particular location closely
tracked sea level rise, any relationship might well be obscured by the different
19
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
measurement periods combined with the temporal variability. Ideally both would be
measured over the same period and this period would be several decades, to average
out inter-annual and inter-decadal variation, due to natural oscillations such as the El
Niño Southern Oscillation (ENSO) and the 18 year tidal cycle. If surface elevation
responds to sea level rise following a time lag, this will further complicate the
interpretation of such data; longer term datasets with regular measurements are
needed to explore whether time lags exist in surface elevation responses to sea level
rise.
Another issue relates to the acceleration of sea level rise rates over the past 140 years
(Church and White, 2006). This could make long term sea level rise measurements
less suitable for comparing with recent short term rates of surface elevation change.
high spatial variability in surface elevation change measurements. The small-scale
variability in surface elevation change measurements is shown by the large error bars
in Figure 9; standard errors of surface elevation means ranged between 0.44 and 2.23
mm/yr (Rogers et al., 2006; Appendix A). Calculating a mean surface elevation
change from these measurements may not provide an accurate spatial average of
elevation change across the site: French and Spencer (1993) demonstrated that spatial
averaging of accretion data across a marsh site provided a poor estimate of total
accretion because accretion varied with height of the substrate and distance from
channel margins; a numerical integration taking these factors into account provided a
better accretion estimate across the marsh habitat.
spatial variability in relative sea level change, combined with variable distances
between SET-MH stations and tide gauges: small-scale variation in relative sea level
rise rates can be caused by local geomorphology and bathymetry (e.g. larger rises in
sea level may be observed in an estuary relative to neighbouring open coast). Largerscale variation is caused by regional variation in rates of thermal expansion of sea
water and isostatic adjustments (as discussed in Section 1.2). This spatial variability in
relative sea level rise means that SET-MH stations need to be placed as close as
possible to the tide gauges measuring relative sea level rise. Most SET-MH stations
used in the studies in Table 2 were less than 25 km from tide gauges (Appendix B).
However the nearest tide gauges to the SET-MH stations on Twin Cays, Belize and
Kosrae, Micronesia were 1075 and 550 km away respectively, and relative sea level
rise rates in these SET-MH locations may differ significantly from the nearest tide
gauge in Key West, Florida and on Pohnpei, Micronesia.
Confounding factors
It is also important to note that other controls on surface elevation change may or may not be
linked to sea level rise, such as changes in sediment supply, and altered wave action or tidal
currents which affect sediment routing and deposition. Where other controls are dominant,
there may not be any correlation between sea level rise and surface elevation change, and
even where the two are correlated, they may not necessarily point towards a direct causal
link.
2.2.3 Conclusion
In conclusion, recent studies suggest that surface elevation change rates are not significantly
different from sea level rise rates, indicating that mangrove surfaces are rising at similar rates
to relative sea level rise in their respective locations. There is high variability in surface
20
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
elevation change rates even within sites, indicating that some areas within each site may be
keeping pace with local sea level rise, while other areas may be lagging behind. More surface
elevation data measured over longer time periods are needed to better understand whether
surface elevation change rates are correlated with local sea level rise rates.
3. Processes
In order to understand when and where mangrove surface elevation is likely to be able to
keep pace with sea level rise in the future, we need to understand the processes involved in
surface elevation change. These processes can be divided into surface processes
(sedimentation, accretion and erosion) and sub-surface processes (growth/decomposition of
roots, shrink/swell of soils, and compaction/compression/rebound of soils) (Section 1.3).
These processes are described in turn below, first giving a brief description of the process,
followed by factors that are likely to affect it. The following sections give an overview of
current knowledge, but do not attempt to provide an exhaustive review or bibliography of
relevant publications because of the large number of processes involved.
It is important to note that surface processes interact with subsurface processes, and both sets
of processes may be influenced by local sea level fluctuations (amongst many other factors,
as described below). We consider some of the likely interactions between surface processes
and subsurface processes in Section 4.2, and interactions with sea level rise are considered in
Section 5.
3.1 Surface processes
Surface processes include all processes which affect the material arriving at the sediment
surface and the fate of this material. Here we divide these processes into sedimentation,
accretion, erosion and faunal processes (i.e. processes mediated by animals that live within
mangrove areas).
3.1.1 Sedimentation
Sedimentation refers to the deposition of inorganic sediments and organic matter onto the soil
surface. The deposited material can be allochthonous (i.e. derived from outside the mangrove
area) or autochthonous (i.e. created within the mangrove area).
Allochthonous material can be:
terrigenous material from the land brought down by rivers; for example, the Sundarbans
receive billions of tonnes of sediment per year from the Ganges-Brahmaputra-Meghna
system (Woodroffe and Davies, 2009); small rivers can also deliver significant
quantities of sediment;
brought in through the creeks during high tides and then deposited when the creeks
overspill onto the surrounding area; such sediment may have been carried along the
coast (long-shore transport), as seen along the coast of French Guiana, north of the
Amazon delta (Allison and Lee, 2004), or advected from offshore by wave and tidal
processes, particularly in macrotidal systems (i.e. systems with a large tidal range),
such as those along the coast of northern and north-western Australia (Woodroffe and
Davies, 2009); large quantities of off-shore material may also be brought in during
high-magnitude storm or tsunami events (Ellison, 2009);
biologically produced, for example coral sand generated in nearby coral reef
ecosystems; or
21
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
precipitated, for example, solid calcium carbonate can be precipitated from dissolved
carbonate in the water, and the calcareous muds of the Great Bahama Bank are
produced in this way (Woodroffe and Davies, 2009).
When mangrove sediments are made up of predominantly coral sands or precipitated
carbonate, the mangroves are classed as being in a carbonate setting (Woodroffe and Davies,
2009); examples include mangroves in Florida, Caribbean islands and many other low-lying
islands. Alternatively mineral sediment inputs may dominate, and most often this is made up
of terrigenous material brought down by rivers; such settings are classed as minerogenic, to
distinguish them from carbonate settings. Examples include many mangrove areas in
Australia and south-east Asia.
Autochthonous material includes leaf litter, dead twigs, branches and roots from the
mangrove vegetation, as well as the benthic mats that grow on the sediment surface (Cahoon
et al. 2006; McKee, 2011). These materials may become incorporated into the soil by
bioturbation e.g. by crabs, or be buried under deposited sediments. The build-up of this
material is influenced by detritivores such as crabs, amphipods and gastropod molluscs,
which consume leaf litter (Middleton and McKee, 2001; Nagelkerken et al., 2008) (see
Section 3.1.4 on faunal processes).
Excess sedimentation, for example during storms or caused by construction projects, may
result in reduced vigour of mangrove trees or even death, depending on the amount and type
of sedimentation (Ellison, 2009). This topic is reviewed in Ellison (2009).
Factors affecting sedimentation
The factors likely to affect sedimentation rates in mangroves are shown in Figure 10. The
most important influences on sedimentation rates are likely to be the amount of incoming
sediment and locally generated material, the period of inundation when external material can
settle out, and factors affecting whether particles are able to settle out or are quickly
resuspended, including flow rates and flocculation of particles.
Factors affecting the amount of incoming material
The most important factor affecting the amount of incoming allochthonous material is likely
to be proximity to a source of material, e.g. a river mouth. The delivery of this sediment into
mangrove areas will depend on water currents and flow pathways, and may vary seasonally
or during storms. For example, Saad et al. (1999) found that seasonal variation affected the
rate of sedimentation and accretion rates in Kememan, Terengganu, Malaysia: accretion rates
were 2.6 mm/month (equivalent to 31 mm/year) during the monsoon period between
November and January, compared to 1.2 mm/month outside the monsoon period (equivalent
to 14 mm/year). This may be explained by the higher river discharge and river sediment load
during the monsoon season, with suspended sediment concentrations in the river between 50
to 100 ppm (parts per million) at this time, compared to 8 to 20 ppm outside the monsoon
season.
Storms and hurricanes (and particularly the storm surges associated with them) can bring in
large pulses of sediment: for example, after Hurricane Wilma in 2005, a mangrove area on
Shark River, Florida, increased in elevation by 48 mm (Smith, unpublished data, in Cahoon,
2006), due to an influx of sediment (accretion was 77 mm, accompanied by 29 mm of
shallow subsidence). Cahoon (2006) notes that the degree of sediment mobilization is usually
related to the intensity of the storm, the size of the storm surge and the local geomorphic
22
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 10. Sources of sediment and processes affecting sedimentation. Disturbances such as storms
and waves can either increase sedimentation and carry away material, resulting in unpredictable
effects of storms on sedimentation/erosion.
setting combined with the storm track. For example, while the storm surge from Hurricane
Wilma deposited 77 mm of sediment in mangroves along Shark River, mangroves at Big
Sable Creek, located to the south of Shark River, on the lee side of Cape Sable and therefore
more protected from the surge, received only 1 mm of sediment (Cahoon, 2006). In marsh
settings, single storms can deposit more sediment than would otherwise be deposited
annually, and such low frequency pulses of sediment may be critical for maintaining surface
elevations in areas with low sediment inputs and high rates of subsidence (Cahoon et al.,
1995b). The relative importance of such sediment pulses in mangroves is not known, but is
likely to be similar.
Factors influencing the amount of incoming autochthonous material include forest
characteristics and the local climate: Saenger and Snedaker (1993) found that litterfall was
related to both height of vegetation and latitude. Storms can also result in large quantities of
autochthonous material being dislodged and arriving on the substrate, e.g. if leaves are blown
off trees or epiphytic algae are washed off tree roots. However some of this organic detrital
material may be carried out to sea by the ebb tidal currents: Wolanski et al. (1980) note that
the outgoing tide at Coral Creek, Queensland, Australia, was strong enough to carry all leaves
into tidal creeks and hence out to sea. The amount of litterfall that accumulates also depends
on how much is consumed by detritivores such as crabs and amphipods, and on rates of
microbial decomposition (Middleton and McKee, 2001).
23
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Processes involved in particle settling
Flocculation
Suspended particulate matter entering mangrove forests includes particles of various sizes,
from clay particles (particle diameter less than 3.9 μm) to aggregated flocs (aggregations of
smaller particles) that can be very large (sometimes more than 100 μm in diameter). Particle
size is important because it influences the rate at which particles settle in water; very small
particles settle very slowly, and may not have time to settle during tidal inundations. Large
particles settle rapidly, even in slowly flowing water. In fast flowing turbulent water, large
flocs usually break up as the forces holding them together are relatively weak. The size and
nature of flocs varies in different mangrove settings: in calcareous settings, flocs may be
much larger than in clay-dominated settings, where floc density is higher and flocs are
stronger (i.e. they do not break up as easily) (Wolanski, 1995).
Flocculation rates (i.e. the rate at which small particles stick together to form larger particles)
are dependent on the concentration of suspended particulate matter: Verney et al. (2009)
observed maximum floc sizes above concentrations of 0.1 g/l, and no flocculation was
observed below concentrations of 0.004 g/l (this was measured in an estuarine environment in
France). They noted that diatom blooms speeded up rates of flocculation. Flocculation rates
depended on the types of particles present and on the content and concentration of organic
matter. Turbulence limited the maximum floc size. Salinity was found to have less effect on
flocculation than suspended particulate matter concentration.
Settlement of flocs
Furukawa et al. (1997) measured particle sizes over three spring tides in mangroves at
Middle Creek, Cairns, Australia. They found that the median particle size of flocs entering
mangroves on the flood tide was 20 μm, with individual flocs often exceeding 100 μm in
diameter. At ebb tide, no large flocs were seen; median floc size was 2 μm with the largest
flocs still less than 20 μm. This suggests that the large flocs had settled during the high tide
period. Furukawa et al. (1997) measured the exact timing of settlement using an upwardlooking nephelometer (this measures the thickness of a sedimented layer by measuring the
reduction in light level; the nephelometer wiped the sediment off the sensor every 5 minutes,
enabling continuous measurements of sedimentation). Sedimentation peaked sharply
approximately 30 minutes before high tide (slack water), and the bulk of sedimentation
occurred over a 20 minute period.
Furukawa et al. (1997) also explored the currents within the mangrove area at Middle Creek.
Using observations of fine-scale flow patterns around Rhizophora prop roots and a numerical
model (VORTEX) to simulate flows around mangrove trunks and roots, they estimated flow
rates among the roots. The field observations showed that the roots generated eddies, jets and
stagnation zones. Using the model, they found that at a flow rate of 0.2 m/s, particles
remained within the mangrove area for longer than when the flow rate was lower at 0.05 m/s,
due to particles being trapped in stagnation zones behind roots; these stagnation zones
resulted from a reduction in laminar flow at the higher flow rates. This implies that the faster
flow rates resulted in higher rates of sedimentation as particles become trapped in these
stagnation zones.
Distance from coast or creek
In tidal areas of southeast Queensland, Australia, sedimentation was highest in the seaward
fringe mangroves (Adame et al.; 2010); however in riverine settings, Adame et al. (2010)
observed a more homogeneous pattern of sedimentation across the intertidal zone. This is
24
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
likely to be related to a longer hydroperiod (i.e. time under water), during which time
sediment can settle out of the water column. Saad et al. (1999) also noted that the mean
particle size of sedimented material was highest in the fringe zone (because the largest
heaviest particles settle out first), and decreased further into the mangrove area in Kemaman,
Malaysia.
Furukawa et al. (1997) found that net sedimentation rates during spring tides in mangroves
surrounding Middle Creek, Cairns, Australia decreased exponentially with distance from the
tidal creek. Sedimentation rates were measured with sediment traps, and were as high as 300
g/m2/spring tide next to the creek, decreasing to almost zero 200 m from the creek. They
estimated that 10.4 kilograms of sediment per metre length of tidal creek per spring tide were
retained in the mangroves, out of a total incoming sediment load of 12.5 kg/m length
creek/spring tide.
Sediment trapping by roots
Mangrove aerial root architecture may influence sediment trapping in mangroves:
Kathireesan (2003) found differences in sediment trapping efficiency in the Vellar estuary,
India, between mixed stands of Avicennia and Rhizophora and areas with just one species or
the other, and he suggests this relates to the different aerial root structures (Avicennia spp.
have pneumatophores, pencil like projections sticking out of the substrate, while Rhizophora
spp. have prop roots). In areas with both Avicennia and Rhizophora, 30% of the total
suspended sediment received at high tide was trapped at low tide, while in areas with only
one species, only 20 to 25% of the suspended sediment was trapped. The trapping effect
probably relates to flow modifications around aerial roots; if stagnant areas form (as
suggested by Furukawa et al. (1997) above), then particles can settle out and are likely to be
retained in the mangroves.
Furukawa and Wolanski (1996) modelled the influence of tree species on sedimentation rate,
and predicted that most sedimentation should occur around trees that form a complex matrix
of roots, such as Rhizophora spp., and least sedimentation around isolated trees such as
Ceriops spp. that lack extensive aerial roots.
3.1.2 Accretion
Sedimentation contributes to surface accretion, which occurs when the deposited material
becomes fixed in place (i.e. it can no longer be washed away by the tides or waves). It is
usually measured relative to a marker horizon (Box 1).
Processes which contribute to accretion include:
the growth of surface mangrove roots into the newly deposited layer, binding
sediments in place (Cahoon and Lynch, 1997), and preventing them from being
washed away by waves and tidal flows;
the formation of benthic mats, made up of single-celled organisms (diatoms and
bacteria), filamentous algae and cyanobacteria, mineral sediment, leaf litter and other
organic matter (McKee, 2011), which cover and incorporate sediments, holding them
in place;
dewatering and consolidation of fluid muds, increasing soil shear strength and ability
to resist resuspension/erosion by waves (Wells and Roberts, 1980).
The distinction between sedimentation and accretion is often unclear, and the terms are
sometimes used interchangeably in the literature. The difference lies in their temporal scale:
25
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
sedimentation can be measured over a period of hours or days (and may be followed by
resuspension of deposited but unbound material), while accretion can only be measured over
months or years, when the deposited material is more firmly bound in place.
Factors affecting accretion
The rate of accretion depends on the balance of the rates of sedimentation and resuspension,
and on processes which bind deposited material so that more force is required to resuspend it.
The factors affecting sedimentation have been discussed in the previous section; resuspension
rates are likely to be affected by waves and currents, and are discussed further in the
following section on erosion. The binding of deposited material will depend on the growth of
near-surface roots into the newly deposited material, and the formation of benthic mats and
mucilaginous layers, both of which can ‘fix’ the material in place.
Benthic mats
Benthic mats can form on the soil surface of wetlands, and they may consist of filamentous
algae, plant roots, microbial communities or any combination of these (Cahoon et al., 2006).
McKee (2011) recognized three types of benthic mat in Caribbean mangrove systems: turf
algal mats, consisting of filamentous algae; leaf litter mats, containing a higher proportion of
mangrove leaf litter alongside filamentous algae; and microbial mats, containing mixtures of
cyanobacteria, diatoms, other microalgae and other amorphous organic matter. Turf algal
mats accreted faster than leaf litter mats, and at similar rates to microbial mats, but there was
high variability across sites (McKee 2011). McKee (2011) found different types of benthic
mat in different forest types in Caribbean mangrove systems; for example, microbial mats
were common in dwarf mangrove forests and in shallow protected ponds where the tree
canopy was open or absent and the soil surface remained flooded. Turf algal mats are often
seen in Rhizophora mangle forests throughout the Caribbean (Cahoon et al., 2006).
Rates of vertical mat growth can vary from 1 mm/yr (e.g. turf algal mats along the shoreline)
to 6 mm/yr (microbial mats in interior dwarf mangrove stands) (Cahoon et al., 2006).
Variation in accretion rates of different types of benthic mat may contribute to different rates
of accretion and elevation change.
McKee (2011) also noted that benthic mats in Belize contained up to 30% (by volume) live
mangrove roots.
Factors affecting sedimentation and/or accretion
Several studies report factors that have been shown to affect accretion rates, but it is likely
that these factors affect accretion primarily through their influence on sedimentation rates.
Aerial root type and density
Krauss et al. (2003) investigated the influence of root type on vertical accretion in three river
basins in Micronesia. They looked at three different functional root types: prop roots in
Rhizophora spp., knee roots in Bruguiera gymnorrhiza, and pneumatophores in Sonneratia
alba. In the Enipoas River basin, Pohnpei, accretion rates were higher among prop roots
(11.0 mm/yr) than among pneumatophores (7.2 mm/yr), knee roots (9.3 mm/yr) and bare soil
controls (9.4 mm/yr).
Young and Harvey (1996) placed arrays of artificial pneumatophores within mangroves in the
Hauraki Plains, New Zealand, to investigate how accretion rates are affected by the density of
26
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
pneumatophores. Accretion rates were 4 mm/yr with 100 pneumatophores/m2, and 25 mm/yr
with 350 pneumatophores/m2. They also measured accretion along 2 transects within the
mangroves: 102 accretion measurements were taken along the 500 m length transects over a 5
month period, with maximum accretion rates of 14 mm. They found a significant positive
correlation between the density of Avicennia marina var. australasica pneumatophores and
accretion rates, but the correlation between mangrove stem density and basal area with
accretion was not significant.
It is likely that the prop roots of Krauss et al.’s (2003) study, and the higher densities of
pneumatophores in Young and Harvey’s (1996) study, are more effective at promoting
sedimentation and reducing resuspension through slowing water flows from waves or
currents.
Tree density
Mangrove seedling density can influence accretion rates: in an experiment where Rhizophora
mucronata seedlings were planted at different densities in Palakuda, Sri Lanka, accretion
rates were highest among the highest densities of mangrove seedlings over a period of 3 years
(Table 3; Huxham et al., 2010; Kumara et al., 2010). However such high densities are only
possible with seedlings; older trees could not exist at such densities.
Table 3. Accretion rates and surface elevation change rates measured over 3 years at different
seedling densities of Rhizophora mucronata in Palakuda, Sri Lanka (from Kumara et al., 2010). Older
plants could not survive at the higher densities used here.
Seedling density (no.
of seedlings/m2)
0
0.95
1.93
3.26
6.96
Accretion rate
(mm/yr)
5.7
6.9
8.4
10.5
13
Standard
error
0.3
0.5
0.3
0.9
1.3
Surface elevation
change (mm/yr)
-0.3
0.6
1.1
1.6
2.8
Standard
error
0.1
0.2
0.2
0.1
0.2
Amount of mangrove leaf litter present
Cahoon et al. (2006) found that the standing stock of litter present on the mangrove surface in
a forest in southwest Florida affected vertical accretion in basin forests, with a significant
positive correlation between litter biomass (g/m2) and vertical accretion (mm/yr). However
no relationship was seen in fringing mangroves, where tidal action may wash leaves away
and the drier conditions may allow leaves to decompose more quickly.
Frequency and period of inundation (hydroperiod)
The period of time that mangroves are flooded (hydroperiod) and the frequency of flooding
affects sedimentation and accretion rates because allochthonous sediment arrives suspended
in the water column. Rogers et al. (2005) found that sediment accretion rates were directly
related to inundation frequencies in Homebush Bay, Australia: in areas inundated by 5% of
tides per year, sediment accretion rates varied between 1 and 2.6 mm/yr, while in areas
inundated by 13% of tides per year, accretion rates varied between 4.6 and 8.6 mm/yr.
27
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Forest type
Cahoon and Lynch (1997) measured elevation change and accretion in fringe, basin and
island mangroves of Rookery Bay, Florida. They identified three distinct accretionary
environments based on hydroperiod and soil properties: fringe forests with regularly-flooded
mineral soils, basin forests with irregularly-flooded organic soils, and overwash island forests
that were flooded regularly and had mixed mineral-organic soils. Accretion rates were
highest in the fringe forests (7.2 and 7.8 mm/yr) and lowest in the overwash forest on the
sheltered island (4.4 mm/yr).
Tidal range
Rogers et al. (2006) found a significant positive relationship between mangrove surface
accretion and tidal range using data from 10 wetland sites in south-eastern Australia; they
attribute this to stronger tidal currents in areas with larger tidal ranges, which can re-suspend
sediments and facilitate deposition within wetlands. Cahoon et al. (2006) also found a
significant positive relationship between accretion and tidal range in a global dataset
including 38 mangrove locations.
3.1.3 Erosion
Erosion refers to the loss of surface material caused by the top layer of the sediment surface
being sheared off by water flows, leading to a loss in elevation. Surface erosion can occur
when waves or currents scour the sediment surface, for example during intense storms. Small
waves (even capillary waves) can also result in erosion of the very fine muds found in some
mangroves areas (Winterwerp et al., 2005).
Surface erosion is sometimes called “sheet erosion” to distinguish it from lateral or bank
erosion, which occurs at the edge of the mangrove where it borders the sea or a tidal channel.
Factors affecting surface erosion
Surface erosion is difficult to measure as surface markers are lost (Cahoon and Lynch, 1997)
and any lowering of surface elevation as measured using the SET-MH methodology (Box 1)
could also have been caused by shallow subsidence resulting from sub-surface processes
(Section 3.2). Therefore, in order to understand the factors affecting surface erosion, we are
obliged to study the factors affecting the erodibility of the mangrove surface and the bottom
shear stresses (i.e. the hydrodynamic forces) acting on the surface. These are reviewed in Le
Hir et al. (2007) in relation to a wide range of coastal systems.
The rate of erosion ɛr (m/s) can be described by the following equation (Hanson and Cook,
2004):
(Equation 1)
where kd is the erodibility or detachment coefficient (m3/N-s), τe is the effective hydraulic
stress (Pa) and τc is the critical stress (Pa). This equation shows that the rate of erosion
depends on a coefficient of erodibility, and on the difference between the hydraulic stress τe
(i.e. the shear force acting on the substrate as the water flows over it) and a critical or
threshold stress level (τc) above which the substrate gives way and starts to be eroded.
In mangroves, several studies have measured the shear strength of soils using a device which
measures the torque required to shear or deform the soil (McKee and McGinnis, 2002;
McKee and Vervaeke, 2009; Cahoon et al., 2003a&b; McKee, 2011; these studies are
28
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
described below). The shear strength of a soil is thus related to the critical stress (τc) in
Equation 1. Soils with higher shear strengths have higher erosion thresholds above which
erosion will occur.
Waves and currents generate hydraulic stress (τe) at the sediment surface (also referred to as
bed shear stress). Factors which result in reduced currents or waves, and thus reduced
hydraulic stress, are also expected to reduce the rate of erosion at the mangrove surface.
Factors that either increase the shear strength of mangrove soils or decrease the flow rates
within mangroves will reduce erosion rates. The factors affecting the shear strength of
mangrove soils and the water flows within mangroves are discussed below.
Factors affecting the erodibility of mangrove soils
McKee and Vervaeke (2009) measured the shear strength of mangrove soils on mangrove
islands in the Pelican Cays and Twin Cays Ranges, Belize; the main mangrove species was
Rhizophora mangle. They found that the shear strength of undisturbed mangrove soils was
higher than that of degraded soils (mangroves were classed as degraded in areas where clearcutting of mangroves had taken place, followed by filling with sediment dredged up from
nearby coastal areas). The substrate in the undisturbed mangrove forests consisted of a strong
matrix of living and dead fibrous roots, with mats of filamentous algae on the soil surface.
This material was extremely resistant to shearing and retained its integrity even when agitated
in water (McKee and Vervaeke, 2009). These results imply that the network of living and
dead roots of healthy mangroves increase the shear strength of the mangrove soil surface,
presumably by binding the soil together (Scoffin, 1970; Spenceley, 1977; Cahoon and Lynch,
1997).
McKee and McGinnis (2002) studied the shear strength of mangrove soils 14 months after
the passage of Hurricane Mitch in 1998; they found that impacted sites had lower shear
strength than unimpacted sites, and this was associated with reduced sub-surface root
densities or death of the root system. The effect of hurricane impacts on soil shear strength
varied with the level of impact, the depth of the soil, and whether the soil was in a fringe or
basin forest. Soil shear strength was highest in fringe forests, in deeper soil layers (5 cm and
below) and in less impacted forests (Cahoon et al., 2003b). Reduced shear strength of soils is
expected to make the soils more vulnerable to erosion.
Benthic mats that form on mangroves soils also affect the shear strength of the surface
(Cahoon et al., 2006; McKee, 2011). Based on a study in Belize, Cahoon et al. (2006) found
that the soil shear strength of algal mats made up of filamentous algae and roots was
generally higher than for microbial mats (soil shear strength of filamentous algae and roots
varied between 0.25 and 0.45 kg/cm2, while for microbial mats it was less than 0.05 kg/cm2).
McKee (2011) compared the shear strength of different types of benthic mat found on the
mangrove soil surface in sites in Belize and Florida; she also found that benthic mats made of
“turf algae” (primarily filamentous algae) had higher shear strength than benthic mats
containing more leaf litter or microbial matter. Turf algal mats were found in fringe, scrub
and restored forests.
Factors affecting the hydraulic stress on mangrove soils
The hydraulic stress (or bed shear stress) on mangroves soils is caused by the water flows
within surface wind and swell waves, and flows caused by water currents related to tides or
storm surges. These vary according to local or distant weather systems. Generally, mangroves
29
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
only receive small wind and swell waves, as they live in sheltered areas. However, during
storms, both waves and currents may generate stronger water flows. The presence of
mangrove vegetation such as aerial roots and trunks can reduce wave energy and height
(Massel et al., 1999; Mazda et al., 1997 & 2006; Quartel et al., 2007; reviewed in McIvor et
al., 2012a). Likewise, the mangrove vegetation slows water flows during storm surges,
resulting in peak water level reductions (Krauss et al., 2009; Zhang et al., 2012; reviewed in
McIvor et al., 2012b). Therefore the mangrove vegetation reduces the hydraulic stress on the
sediment surface, reducing the frequency of occasions when the shear stress exceeds the
critical threshold (τc) for erosion to occur (as described in Equation 1 above).
However, mangrove roots and trunks may also create eddies and jets in flowing water
(Furukawa et al. 1997), resulting in very localised areas that may experience higher shear
stress, possibly scouring out sediment and increasing erosion rates in these areas.
3.1.4 Surface faunal processes
The soil surface of mangrove forests hosts a wide variety of animal species, amongst which
crabs and molluscs are very common. These organisms affect surface processes in a variety
of ways:
sesarmid (Grapsidae) and fiddler (Ocypodidae) crabs consume mangrove leaf litter,
reducing export of such leaf litter by outgoing tides, and retaining nutrients contained
in the leaf litter within the mangrove ecosystem (Kristensen, 2008; Nagelkerken et al.,
2008; Alongi, 2009). Molluscs, particularly snails such as the mud whelk Terebralia,
and other organisms such as copepods and nematodes, play a similar role, recycling
nutrients within the system (reviewed in Spalding et al., 2010). These nutrients can
then be taken up by mangrove trees, enhancing growth, including the growth of above
and below ground roots.
many crabs live in burrows in the mangrove substrate, into which they drag the leaves
(reviewed in Spalding et al., 2010). These burrows alter the surface topography,
potentially altering the shear strength of soils, water flows over the surface and
sedimentation rates.
Crabs may also affect benthic mat formation and persistence: Kristensen and Alongi
(2006) found that the fiddler crab, Uca vocans, depressed the abundance and
productivity of microalgal mats in an Avicennia forest in experimental mesocosms in
Queensland, Australia.
The importance of faunal processes may vary in different mangrove regions; for example,
crabs avoid eating mangrove leaves and seeds in temperate Australian mangrove-salt marsh
ecosystems and in some Caribbean mangroves (Alongi, 2009).
3.2 Subsurface processes
Various subsurface processes have been discussed in the mangrove surface elevation change
literature; the names given to these processes vary to some extent between authors. In this
report, we consider the following three groups of subsurface processes:
the growth and decomposition of mangrove roots and other organic matter;
the swelling and shrinkage of soils and the live mangrove roots within them in the
presence or absence of water or changes in groundwater pressure (also referred to as
dilation water storage);
the compaction or compression of soils, due to the sorting of particles or the weight of
material above them (sediment, organic matter, or water e.g. a storm surge), followed
30
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
in some cases by the rebound of soils when this weight is removed (e.g. after a storm
surge).
While changes in sub-surface thickness (depth below marker horizon to benchmark; Box 1)
can be measured using the SET-MH methodology, it is not possible to measure the
contributions of different sub-surface processes using this methodology. Therefore our
understanding of the different subsurface processes has to be derived from other measures,
such as the mass or volume of live and dead root matter in soil samples, or it has to be based
on correlations, such as changes in subsurface volume in different soil layers that are
correlated with rainfall or groundwater levels. Our understanding of these processes is
growing, but there remains much to learn about how these processes work and their
contribution to surface elevation change in mangroves.
Of these processes, the shrink-swell response and the compression-rebound responses of
mangrove soils to local weather-related and tidal events (rainfall, tidal flows, droughts,
floods, storm surges) are expected to act over short time-scales of hours to months, while root
growth, organic matter decomposition and soil compaction are expected to have long-term
consequences for surface elevation over many years. Our interest here is primarily in the
longer-term processes affecting mangrove surface elevation. However we also consider the
short-term processes because of their potential effects on other longer-term processes; for
example, a drought may reduce surface elevation through the shrinkage of soils, which will
result in an increased hydroperiod when soils are flooded by tides, possibly causing increased
sedimentation and accretion.
The processes described below and the factors affecting their contribution to surface
elevation change are summarized in Table 4.
Table 4. The factors affecting sub-surface processes within mangroves, and their effects on surface
elevation change. These processes and factors are described in more detail in the text.
31
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
3.2.1 Root growth and decomposition
The growth of mangrove roots results in an increase in soil volume and sub-surface
expansion (Cahoon et al. 2006; McKee, 2011). Conversely, when roots decompose, they take
up less space, causing a reduction in soil volume and resulting in shallow subsidence
(Cahoon et al., 2003b). In a study in Twin Cays, Belize, McKee et al. (2007) found that root
inputs explained more than 50% of the variation in surface elevation change (42% from fine
roots and 10% from coarse roots), with subsidence (compaction) and accretion explaining
36% and 2% respectively. In a separate study, McKee (2011) found that both fine and coarse
root mass accumulation was positively correlated with elevation change in mangrove sites in
Belize and Florida. Cahoon et al. (2006) also found a positive correlation between subsurface
root production and elevation change, using data from 18 mangrove forests in different
geographic locations and three soil types (mineral, organic and peat).
Factors affecting sub-surface root growth and decomposition
The growth and decomposition of mangrove vegetation, including sub-surface roots, are
influenced by tree health, salinity (mangroves usually grow faster in lower salinities),
temperature (mangrove species are generally intolerant of cold temperatures), nutrient
availability (related to riverine inputs and regional geological influences), tree species, and
soil aeration, amongst many other factors. Some of these factors are explored in more detail
below.
Evidence for the importance of tree health comes from areas where trees have died following
lightning strikes or the passage of hurricanes. After Hurricane Mitch hit Honduras in 1998,
mangrove areas where trees had been destroyed by the high winds experienced peat collapse;
Cahoon et al. (2003a) measured a fall in elevation of 11 mm between 18 months and 33
months after the hurricane. They attributed this to the death and decay of sub-surface
mangrove roots leading to shallow subsidence.
After Hurricane Andrew in 1992, some mangroves in southwest Florida lost 20 mm elevation
because of peat decomposition (Wanless, unpublished data in Cahoon, 2006). The likelihood
of peat collapse may be related to the organic content of soils; those with higher organic
content may be more likely to suffer collapse following tree death (Cahoon et al., 2003a).
Gaps in the mangrove canopy caused by lightning strikes to trees can also lead to localised
elevation losses of up to 60 mm in Everglades National Park, Florida (Whelan, 2005).
Availability of nutrients may also affect root growth and decomposition. In Twin Cays,
Belize, McKee et al. (2007) found that application of fertilizers altered both the direction and
rate of change of surface elevation through the effects on root growth. Addition of
phosphorus as superphosphate increased rates of root accumulation in interior mangrove
zones: fine roots contributed substantially to soil volume and explained a significant amount
of the variation in elevation change. Conversely, when a nitrogen fertilizer (in the form of
urea) was applied in the same zone, there was a significant increase in root mortality, and
these plots had higher rates of shallow subsidence. The effect of nutrients on subsurface
change and elevation change varied with mangrove zone, and addition of nutrients to
transition and fringing zones did not produce the same effect as in interior zones (e.g. in the
fringing zone, addition of both nitrate and phosphate resulted in shallow subsidence, while
the control zone still showed sub-surface expansion; fertilized plots showed a smaller
increase in surface elevation than control plots in this zone). Elevation change was
significantly correlated with sub-surface change in these sites (r = 0.94; p < 0.0001),
showing that sub-surface processes were the primary controls on surface elevation change
32
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
(McKee et al., 2007). It is unknown if natural variation in nutrient inputs has similar effects
on root growth and decomposition.
The importance of root growth contributions is not limited to organic or peat soils, but is also
important in mineral soils (i.e. soils with a greater percentage of non-organic material):
Cahoon et al. (2006) found that in 5 out of 7 mineral settings, root growth contributed to soil
expansion, compared with 15 out of 17 peat settings.
Decomposition rates are also affected by the degree of aeration of the soil: most mangrove
soils are anaerobic, reducing rates of decomposition. However air may reach further into
mangrove soils via tree roots, or if the soils dry out, or through the action of burrowing
invertebrates, such as crabs.
3.2.2 Shrink-swell of soils (dilation water storage)
Dilation water storage refers to the expansion or contraction of soils when the soil water
content increases or decreases respectively (Cahoon et al., 2011). Dilation water storage
results in the shrink-swell response of wetland soils to flooding and drying. Mechanisms for
this are likely to include changes in the osmotic pressure within mangrove roots in the
shallow soil layers, and increases in groundwater pressure in the deeper soil layers that result
in an increase in volume in these layers. The effect may also be more pronounced in soils
with a higher organic content.
Cahoon and Hensel (2006) suggest that the effects of water availability on surface elevation
are usually transitory and may not affect longer-term trends in surface elevation. However,
this may not be the case where water flows have been permanently altered through drainage,
diversion, the building of dams upstream and abstraction, or where precipitation patterns are
changing as part of on-going climatic changes.
Factors affecting soil swelling and shrinkage
Soils swell and shrink in response to the presence of water and groundwater pressure over a
variety of timescales, from very rapid changes related to tidal levels, to changes over months
to years in response to longer-term climate variations, such as those associated with the El
Nino Southern Oscillation (ENSO).
Tidal levels
Rogers and Saintilan (2008) measured surface elevation repeatedly over a four hour period on
23 January 2004 between high tide and low tide in a mangrove at Homebush Bay, Australia.
During the first 210 minutes (over which time measurements were taken approximately every
hour), surface elevation decreased by 3.7 mm; the reduction was almost linear, and this
equates to a fall in surface elevation of 1.1 mm/hr. Over the last 15 minutes of observation,
surface elevation started to rise again. This demonstrates that tidal levels can cause short term
changes in surface elevation of relatively large magnitude compared to annual surface
elevation changes, which are often of a similar order of magnitude (between 2000 and 2003,
the surface elevation change rate in Homebush Bay was measured as 5.6 mm/yr by Rogers et
al., 2006). Rogers and Saintilan (2008) note that similar short term changes in surface
elevation have been measured in tidal marshes by Paquette et al. (2004), where they are
attributed to changes in soil water content (and hence soil volume) influenced by tidal
inundation and evapotranspiration.
33
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Rainfall and groundwater level
Smith and Cahoon (2003, in Whelan et al., 2005) measured surface elevation in a mangrove
forest along Shark River in the Everglades National Park, Florida, over a 3 year period. They
found that mangrove surface elevation increased linearly with increasing water levels in the
Shark River as measured 15 to 30 days previously, indicating that water levels can strongly
influence mangrove surface elevation.
Rogers et al. (2005) monitored surface elevation in a mangrove area in Homebush Bay at
approximately 6 month intervals over a 43 month period between 2000 and 2003 (the
following discussion refers only to the control area of their experimental design). They found
that surface elevation varied significantly over the measurement period: after an initial
increase which then plateaued (a 12 mm increase over 3 years), surface elevation then rose
again sharply (by 10 mm over 6 months) at the end of the measurement period. The change in
surface elevation correlated with rainfall, where rainfall was adjusted to include a 3-month
time lag (this time lag was introduced to allow for a delay between rain falling and it
influencing groundwater levels). Rogers et al. (2005) concluded that the reduced rainfall
associated with an El Niño event which occurred in the middle of their measurement period
had a measurable effect on the rate of surface elevation change seen within the mangrove.
(See below for further discussion of the effects of the El Niño Southern Oscillation on
mangrove surface elevations.)
In a separate study at the same location, Rogers and Saintilan (2008) measured surface
elevation every 2 weeks over a 4 month period in 2004, and found a strong correlation
between groundwater depth and surface elevation (where surface elevation was averaged
from one SET measurement in the saltmarsh zone, three SETs in the mangrove zone, and one
in a mixed zone, all measured at low tide to control for tidal variations in surface elevation).
They note that groundwater level reflected monthly rainfall. Mean surface elevation increased
by 2.5 mm over the first month (groundwater depth rose by 200 mm during this period and
rainfall exceeded 100 mm); surface elevation then fell by 3.5 mm over the next 2 months
(groundwater depth fell by approximately 60 mm over this time), and finally increased again
by 2 mm over the last month (groundwater depth increased by 100 mm).
Whelan et al. (2005) studied the effect of groundwater pressure and river level on mangrove
surface elevation along Shark River in the Everglades National Park, Florida. They explored
the response of mangrove soils at different depths by measuring surface elevation change in a
way that allowed them to separate out soil volume changes in deep, middle and shallow soil
layers (using 3 SETs whose benchmarks were buried to depths of 6 m, 4 m and 0.35 m). The
SETs were monitored on a monthly basis over 1 year (March 2002 to March 2003), with all
measurements taken at low tide; hourly measures of ground water level and river levels were
also recorded. Over this time the highest mean elevation above initial surface elevation (15.1
mm) was seen at the end of the wet season (November 2002) and the lowest mean elevation
was seen during the dry season (-0.1 mm in January 2003). Changes in groundwater pressure
were strongly correlated with changes in soil elevation; the change in thickness of the bottom
soil zone (4 m to 6 m) accounted for most of the change in surface elevation. Whelan et al.
(2005) concluded that hydrology and groundwater pressure have a large influence on
mangrove surface elevation, and that it is important to consider the differential effects on
different soil zones.
34
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
The El Niño Southern Oscillation (ENSO)
Over a three year study period, Rogers and Saintilan (2008) found that surface elevation fell
at several sites in southeastern Australia, despite sustained vertical accretion over the same
time period. Their study period coincided with the onset of an El Niño drought in 2001-2002.
To explore the relationship between the El Niño event and mangrove surface elevation, they
plotted surface elevation against the Southern Oscillation Index (SOI; a measure of the
changes associated with the El Niño Southern Oscillation). They found that the SOI
accounted for 70 to 85% of the variability in surface elevation. However, the two deltaic sites
did not fit into this pattern, which they attribute to lower groundwater inputs at these sites.
3.2.3 Compaction, compression and rebound
Compaction of soils usually refers to the consolidation of soils over time, as soil particles are
packed closer together and moisture is forced out of the soil. The term “autocompaction” was
used by Kaye and Barghoorn (1964) in relation to wetland peats to refer to “the compression
of peat beneath its own weight”. Peat is very compressible because of its high porosity and
weak skeletal framework of vegetable fibre (Kaye and Barghoorn, 1964). Mangrove peat
includes both organic and inorganic material (Cahoon et al., 1995a); as the weight of the
sediment above increases due to accretion and growth of mangroves, autocompaction is
expected to increase in a similar way to that seen in tidal marshes.
The weight of sea water can also compress peat soils, and this is particularly noticeable after
large storm surges where the soil has been under several metres of water (Cahoon, 2006).
This compression is assumed to occur through the squeezing out of air from the shallow
aerated layer of soil just below the soil surface (Cahoon, 2006). Surface elevation may be
able to rebound following this type of short-lived compressive load (Cahoon, 2006), and this
may contribute to sub-surface expansion after large storm surges. This rebound may be
caused by the regasification of the shallow aerated layer by microbial activity (Cahoon,
2006).
Little is known about the factors affecting the compaction and compression of mangrove
soils. The most important factors are likely to be the weight of material or water pressing
down on the soil, the relative volumes of particles and pores, the soil composition (and
particularly the organic content), and the depth of different soil layers.
3.2.4 Subsurface faunal processes
Many animals live in the mangrove substrate in burrows, such as crabs and bivalves. Their
burrowing activity can increase oxygenation of the soil, mix the surface layers (bioturbation),
and allow flows of dissolved nutrients into and within the soil (Hogarth, 2007). Mangrove
substrates may be underlain by the tunnels these animals create. Many crabs bury organic
material, helping incorporate it into the soil. A crab exclusion experiment in Australia found
changes in soil chemistry and reduced growth of trees in their absence (Hogarth, 2007). The
tunnelling activity and the effects on tree growth could potentially affect soil volume and
hence surface elevation change.
35
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
4. Magnitude of surface and sub-surface contributions to surface
elevation change
Surface elevation change is the result of surface and subsurface processes (Table 5), and also
the interactions between them. In this section we will first focus on the contributions from
surface and subsurface processes to elevation change, as measured using the SET-MH
methodology described in Box 1 in the studies listed in Table 2. We then consider possible
interactions between the surface and subsurface processes, and the evidence for the existence
of interactions. Finally we briefly explore factors that have been shown to influence surface
elevation change rates.
Table 5. The contribution of surface and subsurface processes to surface elevation change, showing
the direction of change.
Contribution to surface elevation change:
Surface elevation rises
Surface elevation falls
Surface processes
(above ground)
Sub-surface processes
(below ground)
Sedimentation and accretion
Erosion
Sub-surface expansion:
swelling, root growth,
rebound
Shallow subsidence: root
decomposition, shrinkage,
compaction, compression
4.1 Accretion, shallow subsidence and surface elevation change
The magnitudes of surface and subsurface contributions to elevation change varied
considerably in the different locations covered by the studies listed in Table 2, as shown in
Figure 10 (data disaggregated within locations where such data are given in the source
references; data shown in Appendix A). The magnitude of surface processes was measured as
accretion, and rates of accretion in these studies varied from 0.7 to 20.8 mm/year (rates of
erosion cannot be distinguished from rates of shallow subsidence using the SET-MH
methodology). The term “shallow subsidence” was generally used to describe all sub-surface
processes (it is generally not possible to distinguish between the sub-surface processes
involved), and shallow subsidence measurements ranged from -19.9 to +2.4 mm/year
(positive measurements indicate that sub-surface expansion has occurred).
Accretion rates and surface elevation change rates were not correlated (Figure 11). However
there was a strong correlation between accretion rates and shallow subsidence rates (Figure
12), and this is discussed further in the following section.
These data show that sub-surface processes are as important as surface processes in
determining the overall rate of surface elevation change in mangroves. Notably, accretion
rates are rarely similar to surface elevation change rates (few points fall near the line of
equality in Figure 11), and as such, accretion rates are not a good predictor of surface
elevation change rates in most locations. In general, accretion rates are higher than surface
elevation change rates, the balance being made up by shallow subsidence. In a small number
of cases, accretion rates are lower than surface elevation change rates (points above the
dashed line in Figure 11), implying that subsurface expansion has taken place.
The relationships between surface elevation change, accretion and shallow subsidence have
also been explored at individual locations. Krauss et al. (2010) found a significant
relationship between surface elevation change and surface accretion in riverine mangrove
36
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
zones in Micronesia, where the highest rates of surface accretion were consistently recorded
(however, these sites still experienced high rates of shallow subsidence). This suggests that in
minerogenic settings with high sediment inputs, there may be a stronger correlation between
accretion and elevation change rates. In some saltmarshes with high mineral sediment inputs,
only low rates of shallow subsidence have been seen, and accretion rates may match
elevation change rates more closely (Cahoon et al., 2000a).
In Twin Cays mangroves, McKee et al. (2007) found that elevation change was not correlated
with surface accretion over a 3 year period; accretion rates varied between 0.71 and
3.5 mm/yr while elevation change rates varied between -3.7 and 4.8 mm/yr, depending on the
mangrove zone and on various experimental treatments. It is likely that sub-surface change
makes a larger contribution to surface elevation change in carbonate settings with low
sediment input.
Figure 10. Rates of surface elevation change, accretion and shallow subsidence from the studies
included in Table 2, with frequency histograms above the bars. Raw data and sources given in
Appendix A; data now disaggregated within locations where these data are given in the source papers
(e.g. surface elevation change was measured in 7 sites on Kosrae, Micronesia (Krauss et al., 2010);
the mean of these 7 measurements was used in Section 2.2 to allow comparisons with other locations
where data had already been averaged in the source papers).
37
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
8
Surface elevation change (mm/yr)
6
several locations, Australia
(Rogers et al., 2006)
4
2
Moreton Bay, Australia (Lovelock
et al., 2011)
0
Kosrae and Pohnpei, Micronesia
(Krauss et al., 2010)
0
5
10
15
20
25
-2
Twin Cays, Belize (McKee, 2011)
-4
Rookery Bay and Naples Bay,
Florida (McKee, 2011)
-6
-8
Accretion (mm/yr)
Figure 11. A plot of surface elevation change rates against accretion rates (data in Table 2 and
Appendix A). The dashed line indicates the expected relationship if elevation change rates and
accretion rates were equal.
24
22
several locations, Australia
(Rogers et al., 2006)
Accretion (mm/yr)
20
18
16
Moreton Bay, Australia (Lovelock
et al., 2011)
14
12
Kosrae and Pohnpei, Micronesia
(Krauss et al., 2010)
10
8
Twin Cays, Belize (McKee, 2011)
6
4
Rookery Bay and Naples Bay,
Florida (McKee, 2011)
2
0
-2
-22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2
0
2
4
Sub-surface change (mm/yr)
Figure 12. The annual rate of shallow subsidence plotted against annual accretion rate (raw data
given in Appendix A). The line shows a linear regression through the data (see text).
38
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
4.2 Interactions between surface and subsurface processes
In over half of these measurements (17 out of 31 measurements, i.e. 54%), accretion and
shallow subsidence were of approximately equal magnitude, resulting in an overall elevation
change between -2 and 2 mm/year (Figure 10). The mean rate of surface elevation change
across all these sites was 0.38 mm/yr, and the difference between accretion rate and shallow
subsidence rate was not significantly different from zero (paired t-test, t = 0.7282, d.f. = 30,
p-value = 0.4721, where shallow subsidence was multiplied by -1 so that absolute rates of
surface and sub-surface change could be compared).
Figure 12 shows that accretion and shallow subsidence are highly correlated, and a regression
analysis of accretion on sub-surface change gave F(1,29) = 79.4, p < 0.0001. Such a strong
relationship between surface and subsurface rates of change implies that the two sets of
processes are not independent of each other, but are influencing each other in some way.
Little is known about interactions between surface and subsurface processes in mangroves:
no studies have been found that investigated how these processes affect each other in
mangroves. Some possible ways in which surface and subsurface processes may interact
include:
i. the weight of matter accreted at the surface presses down on subsurface material,
resulting in the compression of this material and causing shallow subsidence (i.e.
autocompaction within the upper layers);
ii. areas with higher subsurface expansion have a higher surface elevation, and therefore
a shorter hydroperiod, so that accretion rates are reduced (and vice versa);
iii. newly sedimented material may bring nutrients allowing subsurface roots to grow
more vigorously, enhancing sub-surface expansion (Lovelock et al., 2011a); however,
this would result in a positive relationship between accretion and subsurface
expansion, which is not seen here.
Numbers i and ii are expected to result in a negative correlation between accretion and
shallow subsidence, as seen in Figure 12. Interactions between processes are discussed
further in Section 5, in relation to positive and negative feedbacks between sea level change
and elevation change.
4.3 Factors affecting surface elevation change rates
In Section 3 we explored the factors affecting the different processes contributing to surface
elevation change in mangroves. However, as our understanding of many of these processes
and factors affecting them is incomplete, we are not yet able to bring together information on
factors and processes to predict elevation change (see also Section 6 for models that attempt
to do this). Therefore, it is also useful to explore the relationship between environmental
factors and surface elevation change itself, as surface elevation change represents the
synthesis of the effect of environmental factors on the many processes involved, and also any
interactions between these processes (this is shown schematically in Figure 13). A small
number of studies have investigated the relationship between various factors and elevation
change. In this section we briefly describe these studies, and we speculate about which of the
processes explored in Section 3 may be involved in bringing about the observed changes in
elevation.
39
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 13. The different levels (blue arrows) at which we can explore the effects of environmental
and biological factors (red box) on the response of surface elevation in mangroves to sea level rise.
4.3.1 Forest type
Mangrove forests are highly variable in their physical structure, ranging from tall, closedcanopy forests to open areas with sparsely distributed dwarf trees and shrubs. These different
formations are usually related to frequency of tidal flooding and river flooding (Figure 14).
The different types of mangrove forest have been associated with different rates of surface
elevation change.
Krauss et al. (2010) found significant differences in elevation change rates in fringe, riverine
and interior mangrove forests in the Pacific High Islands of Micronesia, with the largest
increases in elevation being seen in the interior mangroves in three out of four sites (data in
Appendix 1).
In a different study of mangroves in carbonate settings in Belize and Florida, McKee et al.
(2011) observed a lowering of surface elevation in dwarf and scrub forest types, while
surface elevation increased in fringe and basin mangroves (data in Appendix 1). They
40
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
attributed this to high root contributions to sub-surface change and/or rapid growth of living
benthic mats which contributed to surface accretion.
No clear pattern has yet emerged linking mangrove forest type to elevation change rates. It is
likely that the type of forest combines with other factors (e.g. geomorphic setting or
carbonate/minerogenic setting) to influence the rate of surface elevation change. More
research is needed to understand how these factors interact.
Figure 14. Mangrove settings, showing the types of forest most commonly referred to in studies on
elevation change (i.e. fringe, basin, scrub and riverine forests). Adapted from Woodroffe (2002).
4.3.2 Tidal range
Cahoon et al. (2006) in their review of SET-MH mangrove datasets found that elevation
change in embayments increased with increasing tidal range; however no relationship was
found between elevation change and tidal range in other geomorphic settings.
4.3.3 Tree density
The density of mangrove seedlings was found to influence surface elevation change rates and
accretion rates in an experiment in Palakuda, Sri Lanka (Table 3 in Section 3.1.2; Huxham et
al., 2010; Kumara et al., 2010). Rhizophora mucronata seedlings were planted at different
densities, and elevation change rates were highest among the highest densities of mangrove
seedlings over a period of 3 years. The increase in surface elevation could be related either to
an increase in sedimentation and accretion caused by slower flows through the higher density
of vegetation, or by increased sub-surface root growth in the higher density areas.
4.3.4 Nutrient availability
McKee et al. (2007) showed that addition of nutrients influenced surface elevation change
through its effect on sub-surface root growth and decomposition in Twin Cays, Belize
(Section 3.2.1). The effect depended on both the type of nutrient (nitrate or phosphate) and
the mangrove zone (interior, transition, fringing mangrove), and it was not possible to
generalise the effect of nutrient addition on surface elevation change.
41
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
4.3.5 Mean sea level and hydrological factors
Lovelock et al. (2011a) found a significant relationship between mangrove surface elevation
change and mean sea level measured over the same period in the western part of Moreton
Bay (but not in the eastern part of the bay). This was based on surface elevation
measurements at approximately 6 monthly intervals compared to mean sea level over the
same intervals. This implies that mangrove surface elevations may respond rapidly to
changes in sea level in some areas.
The effects of various hydrological factors on surface elevation change, acting via subsurface
processes, have already been described in Section 3.2 and were summarized in Table 4.
These studies showed that tidal levels (over periods of hours), river levels (with a 15 to 30
day time lag), rainfall (with a 3 month time lag), groundwater depth, groundwater pressure
and the El Niño Southern Oscillation (measured using the Southern Oscillation Index)
strongly influence surface elevation change through their effects on sub-surface processes,
primarily the swelling and shrinkage of soils. These factors are expected to change surface
elevation over the short term (hours to years).
4.3.6 Storms and hurricanes
The effects of storms and hurricanes on processes contributing to mangrove surface elevation
change have been discussed in the sections on sediment deposition (Section 3.1.1), erosion
(3.1.3), root decomposition resulting from tree death (Section 3.2.1), and compaction and
compression of mangroves soils (Section 3.2.3). Cahoon (2006) reviewed the effects of
storms on wetland surface elevations. While large pulses of sediment can result in raised
surface elevations (e.g. a 48 mm increase in mangrove surface elevation along Shark River
following Hurricane Wilma in 2005), tree death and subsequent decomposition of subsurface
roots can result in lowering of surface elevations (e.g. mangroves in southwest Florida and
Guanaja, Honduras, experienced a reduction in surface elevation of -20 mm/yr and -9 mm/yr
respectively, following Hurricane Andrew (1992) and Hurricane Mitch (1998) (Wanless,
unpublished data, in Cahoon, 2006; Cahoon et al., 2003b)).
5. The effect of sea level rise rates on elevation change rates
Section 3 explored the various processes that govern elevation change in mangroves, and the
factors which are known to affect those processes. Section 4 then examined the contribution
of surface and subsurface processes to surface elevation change, and summarised factors
likely to influence elevation change rates. One important factor that was not explored in
either Sections 3 or 4 was the rate of sea level rise; we have chosen to explore this factor
separately here, because of its central place in understanding mangrove responses to sea level
rise.
Sea level rise is expected to affect several of the processes contributing to mangrove surface
elevation change:
a rise in sea level will result in an increased hydroperiod, during which time
allochthonous sedimentation can occur, possibly resulting in increased accretion;
accreted sediments may bring in nutrients which may affect mangrove sub-surface
root growth and decomposition (McKee et al., 2007; Lovelock et al., 2011b)
a rise in sea level will increase water depth, allowing waves to penetrate further into
mangrove areas, and possibly resulting in increased resuspension and erosion of both
autochthonous and allochthonous sediments, or alternatively in increased delivery of
allochthonous sediment into mangroves;
42
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
a rise in sea level will increase water logging in some mangrove areas, resulting in
increased anoxia and possibly affecting root growth of some mangrove species and
autochthonous sediment inputs (McKee, 1996); and
a rise in sea level is expected to result in a rise in groundwater levels, and possibly
saline intrusion, affecting plant growth, including sub-surface root growth.
While few studies have investigated these hypothetical interactions between local sea level
rise and surface elevation change processes, it is clear that sea level rise could influence
surface elevation change rates in multiple ways.
In this section we first explore some of the factors that are likely to influence the response of
mangrove surface elevation to sea level rise. We then consider some potential feedbacks
between sea level and mangrove surface elevation, which might result in mangrove surface
elevation tracking changes in sea level under some circumstances. Finally, we consider
possible thresholds affecting these feedbacks, above or below which mangrove surface
elevation might no longer be able to track sea level. Due to the paucity of data, much of this
section is speculative; where data are available to support possible mechanisms, these are
highlighted.
5.1 Factors affecting surface elevation change in the face of SLR
Several factors may affect whether mangrove surface elevation keeps pace with sea level rise,
the most important of which are likely to be sediment inputs, tidal range and
geomorphological setting (note that these factors are not independent of one another).
5.1.1 Sediment inputs
The delivery of allochthonous sediment is often cited as one of the most important factors
contributing to the ability of mangroves to maintain their extent, location and zonal
organization during sea level rise (Ellison and Stoddart 1991; Woodroffe 1995; Soares,
2009). Mangroves in large river deltas and other areas with high sediment inputs are expected
to increase in elevation in pace with sea level rise, as sufficient sediment is available to fill
the increase in accommodation space created by sea level rise.
In support of an increase in sedimentation with sea level rise, Cahoon et al. (2006) found that
accretion rates increased with sea level rise rates, based on a linear regression using data from
41 sites (p < 0.0001). When they separated out the data by geomorphological setting, they
found that estuarine settings showed the strongest linear relationship between accretion and
relative sea level rise (20 sites, p < 0.001); there was a weaker relationship between accretion
and relative sea level rise in embayments (8 sites, only significant at p = 0.08).
Lovelock et al. (2011a) also found a correlation between accretion rates and sea level at two
sites in Moreton Bay, Australia, where accretion was measured approximately every 6
months over a 3 year period; accretion rates were then correlated with mean sea level over the
same time interval (sea level fluctuated over a 200 mm range during this 3 year period).
However, both Cahoon et al. (2006) and Lovelock et al. (2011a) found that elevation change
rates did not show the same relationship as accretion with sea level rise in some areas.
Lovelock et al. (2011a) found that elevation change increased with mean sea level on the
sandy western side of Morton Bay, but not on the muddy eastern side. Cahoon et al. (2006)
found some evidence for increasing elevation change rates with rates of sea level rise in
embayments but not in other geomorphological settings (data described in Section 2.2). These
43
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
results suggest that in most sites, shallow subsidence played an important role in determining
elevation change, overwhelming the effects of sea level rise on sedimentation and accretion.
Therefore, while it is anticipated that areas with high sediment inputs should be better able to
keep pace with sea level rise, high sediment inputs are not sufficient to ensure this, because
other processes, such as shallow subsidence, also influence mangrove elevation change. The
strength of the relationships between accretion, elevation change and relative sea level rise
rate are likely to depend on geomorphological setting (as found by Cahoon and Hensel, 2006)
and also on sediment type.
5.1.2 Tidal range
Wetlands along macro-tidal coastlines (with tidal ranges greater than 4 m) have been
considered less vulnerable to the impact of sea level rise (Alongi, 2008; Day et al., 2008),
following early observations that accretion deficits (defined as sea level rise minus accretion
rate) decreased with increasing tidal range (Harrison and Bloom, 1977). However, data are
lacking to confirm this relationship, and recent studies have not found any relationship
between tidal range and elevation change in mangroves (Rogers et al., 2006), except in
embayments (Cahoon et al., 2006).
5.2 Feedbacks
Feller et al. (2010) proposes that the persistence of mangroves on Belizean islands (McKee et
al., 2007; Toscano and Macintyre, 2003), and the close similarity between current surface
elevation change rates and sea level rise rates in some mangrove areas, suggests the existence
of a feedback mechanism that allows mangrove surface elevation to adjust to changing sea
levels. Gilman et al. (2008) also propose the existence of feedback mechanisms “where
processes that control the mangrove sediment elevation interact with changes in sea-level”
(Gilman et al., 2008, p. 240). The feedback mechanisms put forward by Gilman et al. (2008)
and Feller et al. (2010) are shown in Figure 15; they are based on the processes and factors
already described in Sections 3 and 4 of this report.
Gilman et al. (2008) focus on feedbacks between accretion, surface elevation and tidal
inundation, which they propose may operate as follows: an increased hydroperiod (i.e.
increased duration, frequency and depth of inundation, as would occur with a rise in sea
level), may result in increased sedimentation and accretion; as sedimentation can stimulate
plant growth through increased nutrient inputs (Lovelock et al., 2011b), this could possibly
result in more organic debris such as leaves contributing to a further increase in
sedimentation (i.e. a positive feedback, where sedimentation may set into a motion a chain of
processes that result in more sedimentation). Additionally, Gilman et al. (2008) suggest that
increased plant growth might result in the growth of more aerial roots, which would further
slow the flow of water through the mangroves, and thus further increase sedimentation (a
second positive feedback). However, as sedimentation increases (and provided that it results
in an increase in surface elevation, i.e. that subsurface processes do not result in an equal or
greater loss in elevation), then the hydroperiod will be reduced, resulting in reduced
sedimentation, i.e. a negative feedback loop, which could maintain the mangrove surface
elevation within a particular part of the tidal range. Feller et al. (2010) also note the likely
feedback between sedimentation, hydroperiod and position of the mangrove surface relative
to sea level.
44
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 15 Feedback mechanisms proposed by Gilman et al. (2008) and Feller et al. (2010) that may
govern the response of mangrove surface elevation to changes in sea level.
This feedback mechanism is likely to be most important in areas with high external sediment
inputs. In salt marshes, numerical models suggest this feedback operates in areas that are
dominated by external inputs of sediments (Allen, 1990; French, 1993).
A further potential negative feedback suggested by Gilman et al. (2008) is that the increased
hydroperiod could increase the soil pore water storage, potentially resulting in an increase in
surface elevation, which would then result in a decreased hydroperiod.
Feller et al. (2010) propose another negative feedback mechanism related to subsurface root
growth and peat formation, which may be dependent on flooding conditions (i.e.
hydroperiod): with moderate flooding, sub-surface root production is high, and root
decomposition is slow, resulting in peat formation. This then results in a rise in surface
elevation, resulting in reduced flooding and reduced peat formation.
These feedback mechanisms remain putative. They are likely to operate only under some
circumstances, such as within certain geomorphological settings (Gilman et al., 2008).
Despite uncertainty over when and where they may operate, they are included here because
45
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
of their potential importance in governing the behaviour of mangrove surfaces in response to
sea level rise.
Certainly they do not always operate, as evidenced by mangrove areas that have either been
drowned by sea level rise (e.g. fringing mangroves in Bermuda; Ellison, 1993), or that have
gained elevation faster than sea level rise and have become new terrestrial environments (e.g.
some mangrove areas in the Sundarbans increased in elevation until they were rarely flooded
by the tides, and were invaded by non-mangrove tree species; Saenger and Siddiqi, 1993).
To better understand the role played by such feedbacks, observations are needed that span
much longer time scales (decades or longer) and that come from numerous sites from a range
of settings that experience variations in sea level (rises, falls and periods of stability) (Gilman
et al., 2008).
5.3 Thresholds
These negative feedbacks may allow mangrove surface elevations to track changes in sea
level under certain conditions. These conditions are likely to be bounded, i.e. to have
thresholds, beyond which the feedbacks no longer function. One threshold which has been
discussed extensively in the literature is the critical rate of sea level rise that mangrove
surfaces can keep up with. While mangrove surfaces may be able to keep up with low rates of
sea level rise, beyond a certain critical threshold they may no longer able to keep pace
(Ellison and Stoddart, 1991; Ellison 1993; McKee et al., 2007).
Some attempts have been made to estimate critical rates of sea level rise and to understand
what factors affect these critical thresholds. Ellison and Stoddart (1991) suggested that rates
of sea level rise above 12 cm/100 years (1.2 mm/year) would cause the collapse of mangrove
ecosystems in locations that did not receive significant allochthonous sediment input.
However the mangroves at Key West in Florida have experienced relative sea level rise rates
of 19 cm/100 years (based on data showing a mean sea level rise rate of 1.94 mm/yr between
1846 and 1992; Maul and Martin, 1993; some mangrove areas may have been lost, but
mangroves are still present here). Rise rates of 26cm/100 years have also been calculated for
this area in the early Holocene (Parkinson, 1989). McKee et al. (2011) have also recorded
surface elevation rise rates of greater than 3 mm/year in carbonate settings in Florida and
Belize, suggesting that the critical sea level rise threshold is likely to be greater than this rate.
Where there is a large supply of externally derived sediment, mangroves may be able to keep
pace with higher rates of sea level rise: mangroves in the South Alligator tidal river in
Australia have kept pace with sea level rise rates between 0.2 and 6 mm/year (Woodroffe
1990) and mangroves in northern Australian estuaries tolerated rise rates of 8-10 mm/year in
the early Holocene (Woodroffe 1995).
It is likely that the combination of sediment input and sea level rise rates, as well as locationspecific above and below-ground productivity and the frequency of events (e.g. storms) that
remove or resuspend deposited materials, ultimately determine the ability of mangroves to
keep pace with sea level rise in different locations.
46
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
6. Predicting surface elevation change with future sea level rise
As our understanding of the different processes and feedbacks increases, so the ability to
predict how mangroves respond will improve; this may also increase our ability to manage
mangrove areas in ways that will enhance their ability to keep pace with sea level rise.
Numerical models can be used to help us understand how the processes work together to
bring about elevation change, and to predict likely changes in the future with different rates
of sea level rise. A number of such models have been developed for coastal wetlands
(reviewed in Rybzyk and Callaway, 2009). However the majority of these models deal with
salt marshes (e.g. Allen, 1990; French, 1993, 2006; Temmerman et al., 2004; Kirwan and
Temmerman, 2009). Only one model has been found that attempts to model surface elevation
change in mangroves. This model is described below.
6.1 A mangrove sediment development model for mangroves in Honduras
Cahoon et al. (2003a&b) developed a sediment development model to predict surface
elevation change in mangroves in Honduras following the passage of Hurricane Mitch. Their
model was based on a similar model developed by Rybczyk et al. (1998) to predict surface
elevation change in a subsiding coastal forested wetland in Louisiana, USA, and used a
similar basis to other sediment development models used in marshes (Morris and Bowden,
1986; Callaway et al., 1996; Day et al., 1999; Rybczyk and Cahoon, 2002).
Cahoon et al. (2003a&b) used the wetland sediment development model to predict the longterm effects of tree death on surface elevation in a basin mangrove forest on Guanaja, Bay
Islands, Honduras, following the passage of Hurricane Mitch. The model used a cohort
approach, tracking discrete packages of sediment through depth and time to simulate organic
and mineral matter accretion, decomposition, compaction and below-ground productivity
(Cahoon et al., 2003a). It used the following initialization parameters: sea-level rise, deep
subsidence rate, initial wetland elevation, mineral input, root standing crop, above ground
standing crop, sediment bulk density at surface, per cent organic matter at surface, labile
fraction of above-ground biomass and the decomposition rates of: deep refractory organic
matter, labile organic matter, surface labile organic matter and refractory organic matter.
Cahoon et al. (2003a) found that the simulated sediment columns from the model were in
general agreement with observed soil characteristics; Figure 16 shows simulated and
observed soil organic matter with depth. In this case, the model predicts the decrease in per
cent organic matter with depth, but does not predict the sudden drop in organic matter content
at 40cm depth seen in soil samples.
The model predicted a rapid sediment collapse of 37 mm/yr in the first 2 years after the
hurricane, followed by a decrease of 7.4 mm/yr over the next 8 years. Cahoon et al. (2003a)
recorded a loss in elevation of 11 mm/yr between 18 months and 33 months after the storm.
This is not directly comparable to the outputs from the model because of the different time
frames, but it is similar in value to the 7.4 mm/yr loss in elevation predicted by the model
between 2 and 10 years after the passage of the hurricane.
Therefore Cahoon et al. (2003a) conclude that the model is able to make general predictions
about the evolution of soil characteristics and surface elevation in mangroves soils after the
passage of the hurricane. However detailed local measurements are required to calibrate the
model, and the model needs to be tested at other sites. The model was not used to predict the
response of mangrove surface elevation to the effects of sea level rise.
47
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Figure 16. The results of the model used by Cahoon et al. (2003a) compared with observed changes
in soil organic matter with soil depth (from Cahoon et al., 2003a).
7. Conclusions
Historical evidence based on the dating of mangrove peats demonstrates that mangrove
surface elevations have kept pace with sea level rise over thousands of years in some
locations. Rates of rise that mangroves kept pace with ranged between 1 mm/yr and 10
mm/yr in different locations and settings (Table 1). While the reasons for this variation are
poorly understood, it is likely that rates of delivery of allochthonous sediments and rates of
sub-surface root growth are the major controlling factors in minerogenic and carbonate
settings respectively.
Recent evidence using the Surface Elevation Table – Marker Horizon methodology (from
studies published between 2006 and 2011) suggests that mangrove surfaces are rising at
similar rates to local sea level rise in a number of locations; however this is in contrast to the
conclusions of Cahoon et al. (2006) based on studies before 2006, where surface elevations
were lagging behind sea level rise rates. However, surface elevation change measurements
are available for a relatively small number of sites, and most records span short time periods
(Table 2) relative to the longer time scales over which sea level rise has generally been
measured. Therefore, currently available surface elevation data are insufficient to draw
conclusions about the long term capacity of mangrove soil surfaces to keep pace with sea
level rise. Longer term mangrove surface elevation datasets are needed, and these need to be
analysed relative to sea level changes over the same periods of measurement. Additionally
these need to cover a greater variety of locations, including different geomorphological
settings.
Understanding the processes that govern mangrove surface elevation change can help us
understand how mangrove soils respond to sea level rise. A multitude of processes contribute
48
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
to surface elevation change in mangroves. While understanding of these processes and the
factors affecting them is increasing, many of these processes are currently poorly understood.
Whereas surface processes have been studied for many decades, the importance of subsurface
processes has only been recognized in the last two decades, after the Surface Elevation Table
– Marker Horizon made it possible to measure the relative contributions of accretion and subsurface change to surface elevation change. Consequently, relatively few studies have
explored the role of subsurface processes in mangrove surface elevation change.
Nevertheless, it is now clear that at least six sets of processes influence surface elevation
change in mangroves: sedimentation/resuspension; accretion/erosion; faunal processes;
growth/decomposition of roots; shrinkage/swelling of soils in the presence/absence of water;
and compaction/compression/rebound of soils over time and under the weight of soil/water
above. These processes act at the local scale, and some of them vary even over a few
centimetres of substrate; however, they are driven by larger scale processes such as wave
climate and sediment supply, which vary along much longer stretches of coast (100s of
metres to 100s of kilometres).
Recent measurements using the SET-MH methodology have demonstrated that sub-surface
change is often similar but opposite in magnitude to surface change: measurements of surface
change and sub-surface change show a strong negative correlation. Clearly sub-surface
processes are as important as surface processes in determining elevation change in
mangroves. In this respect, mangroves may differ from salt marshes, where accretion rates
and elevation change rates are often more similar, particularly in minerogenic settings. The
close correlation between accretion and sub-surface change suggests that surface and subsurface processes interact with each other, although the nature of these interactions is
currently unknown.
The deep layers of peat lying beneath some mangroves in the Caribbean suggest that under
some circumstances, mangrove soil surfaces are able to track sea level rise over extended
periods and variable rates of sea level rise. This points to the possible existence of negative
feedbacks between sea level change and surface elevation change. The most likely feedback
relates to the change in hydroperiod and sedimentation that is expected to occur as local sea
level rises: rising sea levels result in mangrove surfaces being under water for longer,
allowing increased allochthonous sedimentation, which may then result in an increase in
surface elevation (depending on subsurface change). It is also possible that there may be time
lags in the operation of these feedbacks; however these are likely to be relatively short (days
or months) based on Lovelock et al.’s (2011a) observation that surface elevation change
measured over 3 to 6 month periods was correlated with the mean sea level over the same
period.
Such feedbacks are routinely included in numerical models of wetland soil development.
While several such models have been developed for salt marshes, only one model has been
found that aims to predict surface elevation change in mangroves, and this model was used to
understand surface elevation change after a hurricane, rather than as a result of sea level rise.
There remains a major gap in our ability to predict surface elevation change rates in
mangroves in response to sea level rise. However, the lack of understanding of several of the
processes affecting surface elevation in mangroves, and how these processes are affected by
local environmental factors, remains an obstacle to developing such models. Such models
also need validating with long term datasets, and few such datasets are available for
mangrove surface elevation.
49
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Monitoring of mangrove surface elevations and relative sea level change near mangrove areas
needs to be extended into areas where such data are currently lacking and into the future, so
that, in time, much needed longer term records become available, allowing a clearer
understanding of the processes influencing mangrove surface elevation change, and the
responses of mangrove surface elevation to local sea level rise.
Despite evidence suggesting that some sites are experiencing an elevation deficit with respect
to sea level rise, there are few published records of mangroves being drowned by sea level
rise (e.g. Ellison, 1993). This may be because there will be a time lag between the recent
increases in sea level rise rates and their effects on mangroves, or because such observations
have not been made or published, or because the discrepancy in the time scales of
measurement of surface elevation change and sea level rise do not allow for accurate
calculation of elevation deficits. While some loss of mangrove areas may be expected as sea
levels rise, current rates of loss due to anthropogenic habitat conversion and other threats are
very high, and these losses probably represent a greater threat to mangroves and to the
continued provision of coastal defence services than sea level rise.
To support mangrove soils in maintaining their surface elevation in the face of sea level rise,
sediment inputs need to be maintained, e.g. by ensuring that local rivers continue to bring
down sediments or that long-shore transport of sediments remains possible. River flows and
groundwater levels also need to be protected, as reduced flows and lowering of groundwater
levels has been shown to result in lowered mangrove surface elevations; if these combine
with rises in sea level, loss of mangroves in lower areas becomes more likely. Related to this,
mangroves in areas that are predicted to suffer droughts under climate change scenarios are
likely to be at greater risk from sea level rise because of the effect of reduced rainfall and
groundwater levels on surface elevations. Efforts should also be made to ensure that
mangrove trees remain healthy by protecting freshwater inputs and reducing eutrophication
or other forms of pollution; healthy trees are expected to be better able to generate subsurface roots that can also contribute to increasing surface elevation.
Where possible, space should be allowed behind mangroves for their landward migration in
the face of sea level rise. This will hopefully ensure that mangroves can continue to exist
along a stretch of coast, even if they are not able to remain in their current location. For as
long as some mangrove areas remain intact, they can be expected to continue to provide
coastal defence services, such as wave reduction, and other ecosystem services, such as
supporting fisheries.
8. Acknowledgements
We gratefully acknowledge Femke Tonneijck, Pieter van Eijk, Han Winterwerp, Jan van
Dalfsen, Eric Wolanski, Catherine Lovelock, Karen McKee, Colin Woodroffe, Joanna
Ellison, Norio Tanaka, Mai Sỹ Tuấn, Bregje van Wesenbeeck, Denise Reed, Evamaria Koch,
Mike Beck, Filippo Ferrario, Trevor Tolhurst, Pam Rubinoff, Ty Wamsley, David McKinnie,
Dolf de Groot, I. Nyoman Suryadiputra and Jo Wilson for their comments on this research
and report, which they gave either through correspondence or at the Mangroves as Coastal
Protection workshop held in Bogor, Indonesia, 19-22 January 2012 and the Natural Coastal
Protection workshop help in Cambridge, UK, 27-29 March 2012.
50
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
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Appendix A: Data used to create figures of surface elevation change throughout this report, with sources of information.
. Location
Rookery Bay and Naples Bay, Florida, US
Rookery Bay and Naples Bay, Florida, US
Rookery Bay and Naples Bay, Florida, US
Twin Cays, Belize
Twin Cays, Belize
Twin Cays, Belize
Yela River, Kosrae, Micronesia
Yela River, Kosrae, Micronesia
Yela River, Kosrae, Micronesia
10 Utwe River, Kosrae, Micronesia
11 Utwe River, Kosrae, Micronesia
12 Utwe River, Kosrae, Micronesia
13 Enipoas River, Pohnpei, Micronesia
14 Enipoas River, Pohnpei, Micronesia
15 Enipoas River, Pohnpei, Micronesia
16 Sapwalap River, Pohnpei, Micronesia
17 Sapwalap River, Pohnpei, Micronesia
18 Sapwalap River, Pohnpei, Micronesia
19 Pukusruk, Kosrae, Micronesia
20 Moreton Bay, Australia
21 Moreton Bay, Australia
22 Ukerebagh Island, Australia
23 Kooragang Island, Australia
24 Homebush Bay, Australia
25 Minnamurra River, Australia
26 Cararma Inlet, Australia
27 Currambene Creek, Australia
28 French Island, Australia
29 Kooweerup, Australia
30 Quail Island, Australia
31 Rhyll, Australia
58
Site/area
Basin 1
Basin 3
Fringe 3
Fringe
Transition
Interior
Fringe
Riverine
Interior
Fringe
Riverine
Interior
Fringe
Riverine
Interior
Fringe
Riverine
Interior
Backswamp
West, muddy
East, sandy
Rate of change (mm/yr)
Measurement
Surface Standard
Standard
SubStandard Relative Elevation
length elevation error of
error of surface error of sea level surplus /
(years)
change
SEC
SSC
rise
deficit
Accretion Accretion change
>3
3.85
0.9
2
0.35
1.85
0.58
2.08
1.77
>3
1.06
0.88
7.55
0.94
-6.48
-6.48
2.08
-1.02
>3
0.61
1.84
5.74
0.78
-5.13
-5.13
2.08
-1.47
3.5
4.1
2.2
1.6
0.7
2.4
2.9
2
2.1
3.5
-1.1
1.5
2
1.3
-3.1
2.6
2
-3.1
3.5
-3.7
1
0.7
0.3
-4.4
1.1
2
-5.7
3
-3
0.8
11.6
1.3
-14.6
1.5
1.8
-4.8
3
-2.7
0.6
12.9
2.1
-15.6
1.1
1.8
-4.5
3
1.3
0.7
12
1.2
-10.7
1
1.8
-0.5
3
1.2
0.3
11.9
1.7
-10.7
0.7
1.8
-0.6
3
6.3
0.5
18.7
2.2
-12.4
0.7
1.8
4.5
3
1.3
0.2
12.9
4.3
-11.6
1.9
1.8
-0.5
1.4
-5.8
0.9
6.6
3.1
-12.4
2.3
1.8
-7.6
1.4
-1.4
2.2
6.3
0.9
-7.7
1.2
1.8
-3.2
1.4
-2.8
0.4
2.9
1.4
-5.7
0.1
1.8
-4.6
1.4
-2.3
0.6
4.1
1.5
-6.4
1.3
1.8
-4.1
1.4
-0.6
0.8
14.1
1.7
-14.7
0.9
1.8
-2.4
1.4
0.9
0.5
8.2
1.2
-7.3
0.7
1.8
-0.9
1.4
0.9
0.4
20.8
2.4
-19.9
1.8
1.8
-0.9
3
1.4
9.6
-8.2
2.358
-0.958
3
5.9
8.5
-2.6
2.358
3.542
-0.4
2.8
3
2.4
1.39
2.21
0.3
0.19
0.33
1.65
3
1.98
0.54
4.72
0.05
-2.74
0.91
3
5.64
2.15
4.58
0.28
1.06
4.73
-0.47
3
0.61
0.44
6.64
0.52
-6.03
1.08
4.12
3
-0.81
1
3.03
0.41
-3.84
-4.93
4.12
3
0.29
2.02
0.65
0.34
-0.36
-3.83
2.66
3
-2.13
1.66
9.49
2.69
-11.62
-4.79
2.66
-2.69
3
-0.03
2.23
7.2
0.85
-7.23
2.66
3
-2.6
2.07
6.77
0.79
-9.37
-5.26
2.66
3
0.92
1.87
5.1
0.72
-4.18
-1.74
Source
McKee, 2011
McKee, 2011
McKee, 2011
McKee et al ., 2007; McKee, 2011
McKee et al ., 2007; McKee, 2011
McKee et al ., 2007; McKee, 2011
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Krauss et al., 2010
Lovelock et al ., 2011
Lovelock et al ., 2011
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
Rogers et al ., 2006
McIvor et al., 2013. The response of mangrove soil surface elevation to sea level rise.
Appendix B. Location of tide gauges, approximate distances between SET-MH measurement station and
tide gauges, tide gauge measurement period and relative sea level rise measured there.
Surface elevation
measurement location
Rookery Bay, Florida
Naples Bay, Florida
Twin Cays, Belize
Homebush Bay, Australia
Ukerebagh Island, Australia
Kooragang Island, Australia
Minnamurra River, Australia
Cararma Inlet, Australia
Currambene Creek, Australia
French Island, Australia
Kooweerup, Australia
Quail Island, Australia
Rhyll, Australia
Moreton Bay, Australia
Kosrae, Micronesia
Pohnpei, Micronesia
59
Tide gauge location
Naples, Florida (NOAA, guage #8725110)
Naples, Florida (NOAA, guage #8725110)
Key West, Florida (NOAA, gauge #8724580)
Fort Denison tide gauge, Sidney (60370, GLOSS
no. 57)
Tweed River at Letitia Spit (ARWC 201429)
Hunter River at Hexham Bridge (ARWC 210448)
Macquarie Rivulet at Princes Highway (ARWC
214402)
Jervis Bay at HMAS Cresswell (ARWC 216470)
Jervis Bay at HMAS Cresswell (ARWC 216470)
Stony Point (Westernport) (BoM 586268)
Stony Point (Westernport) (BoM 586268)
Stony Point (Westernport) (BoM 586268)
Stony Point (Westernport) (BoM 586268)
Bishop Island (PSM 21764)
a combination of satellite altimetry and tide
gauge measurements
a combination of satellite altimetry and tide
gauge measurements
Linear
distance to
tide gauge
(km)
13
2
1075
14
Measurement
period length
RSLR
(years)
(mm/yr) Source
35
2.08
McKee, 2011
35
2.08
McKee, 2011
87
2
McKee et al., 2007; McKee,
2011 (RSLR from 2011)
90
0.91
Rogers et al., 2006
1
16
14
16
19
19
-0.4
0.33
-0.47
Rogers et al., 2006
Rogers et al., 2006
Rogers et al., 2006
15
8
16
22
17
12
5 to 30
(6 sites)
549 to 556
(3 sites)
18 to 20
(2 sites)
10
10
10
10
10
10
10
4.12
4.12
2.66
2.66
2.66
2.66
2.36
Rogers et al., 2006
Rogers et al., 2006
Rogers et al., 2006
Rogers et al., 2006
Rogers et al., 2006
Rogers et al., 2006
Lovelock et al., 2011a
26
1.8
26
1.8
Krauss et al., 2010; Church
et al., 2006
Krauss et al., 2010; Church
et al., 2006