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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L04607, doi:10.1029/2006GL028069, 2007
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Iron links river runoff and shelf width to phytoplankton biomass along
the U.S. West Coast
Zanna Chase,1 Peter G. Strutton,1 and Burke Hales1
Received 4 September 2006; revised 3 January 2007; accepted 9 January 2007; published 27 February 2007.
[1] A poleward increase in phytoplankton biomass along
the West Coast of North America has been attributed to
increasing river runoff towards the north. We combine
streamflow and shelf width data with satellite-derived
estimates of phytoplankton biomass to quantify the
relationship between these variables. We find that a
combination of winter streamflow and shelf width can
account for over 80% of the spatial variance in summer
chlorophyll within 50 km of the coast. At a given
location, interannual variability in streamflow is not
associated with interannual variability in chlorophyll.
We attribute these relationships to the role of rivers as
suppliers of the micronutrient iron, and the role of the
shelf as a ‘capacitor’ for riverine iron, charging during
the high-flow winter season and discharging during the
upwelling season. Data from the Oregon shelf confirm
that, during winter, a significant fraction of riverine iron
escapes the estuary and reaches the coastal ocean.
Citation: Chase, Z., P. G. Strutton, and B. Hales (2007),
Iron links river runoff and shelf width to phytoplankton
biomass along the U.S. West Coast, Geophys. Res. Lett., 34,
L04607, doi:10.1029/2006GL028069.
1. Introduction
[2] The upwelling system of the West Coast of the U.S. is
an important site of air-sea CO2 exchange and deep ocean
carbon (C) sequestration [Hales et al., 2005, 2006]. Productivity along this margin also supports an economically
significant but fragile fishery. Recent work [Ware and
Thompson, 2005] has documented a significant correlation
between fish yields and phytoplankton standing stock
(chlorophyll a).
[3] Despite its importance in terms of C sequestration
and fisheries, it is not known what determines the
primary productivity in the region or its along-shore
and inter-annual variability. Ware and Thompson [2005]
noted a poleward increase in chlorophyll a (chl), which
they speculated to be driven by a poleward increase in
river runoff. However, productivity is dominated by the
summer upwelling season when river discharge is minimal. We argue here that the gradient in chl is driven by a
poleward increase in winter runoff, which supplies the
micronutrient iron (Fe) to the shelf. A concomitant
increase in shelf width retains riverine Fe where phytoplankton can use it.
1
College of Oceanic and Atmospheric Sciences, Oregon State
University, Corvallis, Oregon, USA.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2006GL028069$05.00
2. Methods
2.1. River Flow and Chlorophyll Data
[4] We divided the U.S. West Coast into 15 sub-regions
with latitudinal dimensions of 1° and cross-shelf dimensions
of 50 km (Figure 1). Satellite chl data were obtained as
9 km, level 3 standard mapped images from SeaWiFS.
Mean monthly chl for 1998 –2003 was calculated for each
sub-region, which contained, on average, 95 pixels. Potential problems with SeaWiFS chl retrievals in coastal areas
include mistaking colored dissolved organic matter
(CDOM) and suspended particles for chl. Fluorescence line
height (FLH) from the MODIS sensor is free from these
interferences. For summer 2005, we found SeaWiFS chl and
FLH are strongly correlated throughout the study region (r =
0.76, n = 15, p < 0.002). This indicates that the SeaWiFS chl
algorithm is not mistaking suspended sediments or CDOM
for chl.
[5] We estimated streamflow to the ocean from the
furthest downstream gauging station (FDGS) for every
USGS-gauged river. Errors associated with this approach
include the effect of un-gauged rivers and losses (gains)
through evaporation (precipitation) between the FDGS and
the ocean. In our study region this approach agrees well
with a global 1° 1° discharge model that corrects for these
uncertainties [Dai and Trenberth, 2002]. For all but the
Sacramento and Columbia Rivers, the discharge latitude
was taken as the latitude of the FDGS. The Sacramento’s
FDGS is located in the latitude bin to the north of San
Francisco Bay. Relocating this gauge was necessary to place
the coastal discharge at the correct latitude. The Columbia’s
FDGS is located close to the northern boundary of the
second most northerly bin. Because the Columbia plume is
directed northward over the WA shelf during winter, when
discharge is highest, we moved its discharge latitude into
the most northerly bin.
[6] The average shelf width (distance to the 200 m
isobath) was calculated for each latitude bin using the
NOAA/NOS medium resolution coastline data and
ETOPO2 bathymetry data [Smith and Sandwell, 1997].
The adjusted coefficient of determination, denoted R2a
[Zar, 1999], was used to compare multiple linear regression
(MLR) models.
2.2. Field Sampling
[7] In January and February of 2003 samples for Fe
analysis were collected off the OR coast and analyzed as
described by Chase et al. [2005a]. In January 2005 samples
were collected at the mouth of Alsea Bay, OR using a clean
pumping system. Iron was determined as per Chase et al.
[2005a], salinity was measured with a salinometer, total
suspended solids (TSS; > 0.45 mm) were determined gravi-
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CHASE ET AL.: STREAMFLOW, SHELF WIDTH, AND PRODUCTIVITY
Figure 1. Study area. Average chl from SeaWiFS, 1998 –
2003, is indicated in color (log scale). Circles are
proportional to the average annual streamflow at the most
downstream USGS gauging station. Also indicated is the
200m bathymetric contour and the 1° latitudinal bins used
in the analysis. Puget Sound is not included in the
northernmost bin.
metrically and nutrients were determined by continuous
flow analysis using standard colorimetric methods.
3. Results and Discussion
[8] Our analysis covers the years 1998 (the first complete
calendar year of SeaWiFS data) to 2003 (the last complete
calendar year of river flow data) and looks at winter (DecJan-Feb-Mar) river flow and the following late summer’s
(Jul-Aug-Sep-Oct) average chl. This approach minimizes
the potential for river-supplied CDOM and suspended
particles to confound SeaWiFS chl. Note, however, that
Siegel et al. [2005] determined that chl errors due to CDOM
did not appear to be dominated by riverine inputs. Our
analysis (Figure 2a) confirms and quantifies the latitudinal
gradient in chl along the U.S. West Coast [Ware and
Thompson, 2005], with latitude explaining over 70% of the
variance in summer chl averaged across all years (Table 1).
[9] Some potential explanations for the correlation between chl and latitude can be rejected immediately. Summer
insolation shows no positive trend with latitude over this
time period (Figures S1 and S2 of the auxiliary materials).1
Atmospheric pressure-based, in situ and remote-sensing
(QuikSCAT) measurements show upwelling-favorable
winds are greater to the south [Ware and Thompson, 2005;
Risien and Chelton, 2006]. All of these wind products have
shortcomings. However, QuickSCAT winds, geostrophic
winds and model winds are well correlated along the U.S.
West Coast [Pickett and Schwing, 2006]. QuikSCAT-derived
wind stress curl, which also induces upwelling of nutrientrich waters, has a maximum around Cape Mendocino in
summer [Risien and Chelton, 2006]. At least at the level of
Levitus climatology, nitrate concentrations on upwelled horizons appear invariant with latitude. A poleward decrease in
1
Auxiliary materials are available in the HTML. doi:10.1029/
2006GL028069.
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mixed layer depths remains a possibility, but data do not exist
at sufficient resolution to address this mechanism.
[10] We propose a mechanism based on Fe availability
that depends on delivery of riverine Fe during highdischarge winter conditions, followed by retention of reactive Fe in surface sediments on wide shelves until summer.
To examine this, we consider each independent variable
separately. Winter river discharge alone explains 50% of the
variance in summer chl, while shelf width alone explains
76% (Table 1). A multiple linear regression including both
shelf width and river flow further improves the relationship,
explaining 82% of the spatial variance in chl. This is a
significant improvement over the correlation with latitude
alone, and adding latitude as a third independent variable
does not increase the explanatory power significantly
(Table 1). This suggests that most of the latitudinal dependence of chl is a result of the co-varying river discharge and
shelf width. These patterns hold when we consider chl within
100 km from shore, when we consider the median or
geometric mean in space and/or time of chl and/or river flow,
and when we consider annual chl and annual river flow.
[11] In some Fe-limited regions, satellite-derived estimates of biomass from chl overestimate phytoplankton
carbon [Behrenfeld et al., 2006]. This means the true
latitudinal gradient in biomass may be greater than estimated here, as we postulate that Fe limitation (and therefore any
possible overestimate of chl) increases equatorward.
[12] The case for Fe-limitation in U.S. West Coast waters
is complicated. Upwelling-source waters do not contain
sufficient Fe to support full consumption of the ample
available nitrate [Bruland et al., 2001]. Atmospheric inputs
of Fe have been assumed to be minor since offshore wind
events are rare. Direct riverine inputs have also been
considered minor, because although river water typically
Figure 2. Scatter plots of chl and environmental predictor
variables. (a) Summer chl as a function of latitude.
(b) Summer chl as a function of winter river flow, with
each point corresponding to a single year and color-coded
by latitude. (c) As in Figure 2b, except values in each
latitude band have been averaged across all years.
(d) Summer chl and shelf width.
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Table 1. Multiple Linear Regression Analysis for Variables That
May Predict the Distribution of Summer Chl Within 50 km From
Shore Along the West Coast of the United States, 1998 – 2003a
Predictor Variables for Summer Chlorophyll
Ra2
Latitude
Winter River Flow (log)
Shelf Width
Shelf Width and Winter River Flow (log)
Width and Latitude and Winter River Flow (log)
0.71
0.50
0.76
0.82
0.83
a
All Ra2 values are significant at the p = 0.05 level.
contains orders of magnitude higher dissolved Fe concentrations than does ocean water, classical studies have shown
that nearly all riverine dissolved Fe is lost upon mixing in
estuaries [Boyle et al., 1977]. Furthermore, surface dissolvable Fe concentrations off Monterey Bay were roughly sixfold lower in a high river-flow, El Niño year, than in a
normal low-flow year [Johnson et al., 1999].
[13] Our analysis supports the idea that rivers do not
directly supply dissolved Fe to surface waters during the
summer. If this were the case, one would expect to find a
correlation between years of strong streamflow and years of
greater chl. This is not found; for a given latitude bin, interannual variability in chl is not correlated with variability in
streamflow (Figure 2b). This result also holds if we consider
summer instead of winter streamflow and if we look at chl
in the bin to the south of the river flow, to account for mean
equatorward flow during summer.
[14] Data from the OR coast suggest that rivers are the
ultimate source of Fe to the shelf and to upwelled water.
Specifically, we propose that West Coast rivers deliver
significant amounts of Fe to the shelf during high-flow
winter conditions, this Fe accumulates on the shelf, and is
remobilized for phytoplankton use during summer. In order
for this to be the case, riverine Fe must first escape the
estuaries and reach the coastal ocean, in contrast to the
conclusions of earlier studies on east coast estuaries [e.g.,
Boyle et al., 1977]. Several lines of evidence support this.
We observed high concentrations of total dissolvable Fe
(unfiltered samples acidified to pH < 2 for months) on the
OR shelf during winter (Figure 3). These concentrations are
about an order of magnitude higher than concentrations
observed during the summer [Chase et al., 2005a]. Furthermore, the highest Fe concentrations are associated with the
lowest salinity waters. Significant inputs of dissolvable Fe
(<10 mm, pH 3 [Chase et al., 2005a]) are also associated
with low salinity waters affected by winter coastal river
runoff on the OR coast [Wetz et al., 2006]. These observations are consistent with the near-conservative behavior of
total Fe observed in some east coast estuaries [Mayer, 1982].
[15] These high concentrations of river-derived Fe in
coastal waters suggest that during the winter, when river
flow is high and shelf and estuarine productivity is low, the
small estuaries along the OR coast are exporting a significant fraction of the incoming total riverine Fe. Observations at the restricted mouth of the Alsea Estuary (44.5°N)
in January 2005 confirm this (Figure 4). Concentrations of
dissolved Fe (dFe; <0.45 mm), macronutrients and TSS
were all vertically homogeneous and elevated on the ebb
tide relative to the flood tide and high-tide slack. Iron shows
an enrichment at ebb tide of 40% over flood tide.
[16] High-tide samples all appear typical of non-plume
wintertime coastal waters with respect to salinity, nutrients
Figure 3. Relationship between total dissolvable Fe
concentration and light transmission for samples collected
in winter 2003 on the OR shelf. Samples are color-coded by
salinity.
and Fe [Wetz et al., 2006]. The salinity profiles suggest that
the ebb-tide water consists of about 5% fresh river water,
while the dissolved Fe enrichment (about 15 nM) over
ocean conditions amounts to about 2.5% (given USGS
measurements) to 15% (given our measurements; not
shown) of the primary river dissolved Fe concentration.
Thus ebb tide water discharging from the estuary mouth to
the coastal ocean carries at least 50%, and as much as
100%, of the dissolved Fe that would be predicted based
simply on conservative mixing.
[17] River flow during this time period was about 60 m3 s 1,
less than two-thirds of the average January discharge for the
prior decade. The high dissolved Fe efflux from this
estuary—several-fold greater than predicted by Boyle et
al. [1977]—does not appear to require extreme flood events
as studied by Wetz et al. [2006]. Our measurements from the
OR coast therefore indicate that during winter a large
fraction of riverine dissolved Fe reaches the coastal ocean.
[18] In order for winter riverine Fe to support high
productivity during summer upwelling conditions, it must
be retained on the shelf in intervening months. Wintertime
Figure 4. Chemical properties at the mouth of Alsea Bay,
OR, 12 January 2005. Samples were collected at ebb tide
(right-facing triangles), flood tide (left-facing triangles), and
high tide (squares).
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circulation patterns facilitate this, with a strong downwelling front isolating shelf waters from exchange with the
open ocean [Wetz et al., 2006]. Winter total dissolvable Fe
concentrations are positively correlated with particle concentration, suggesting that this Fe is primarily associated
with particles (Figure 3). If this reactive Fe phase is stored
in seafloor sediments where it is remobilized by suboxic
diagenetic processes [Berelson et al., 2003], it can be
released to the water upwelled through the bottom boundary
layer during summer [Perlin et al., 2005]. The shelves thus
act as a capacitor for Fe, charging with river-source Fe in
winter and discharging it during upwelling conditions.
[19] Several studies have suggested a link between a
narrow continental shelf and susceptibility to Fe limitation
[Johnson et al., 1999; Bruland et al., 2001; Chase et al.,
2005b]. The wider the shelf, the greater the likelihood of
retention of riverine Fe, and the greater the opportunity for
upwelled waters to interact with shelf sediments and become enriched in Fe. The good correlation we find between
shelf width and chl is consistent with this interpretation.
[20] In summary, the data presented here show that the
poleward increase in chl along the U.S. West Coast is
correlated with a poleward increase in shelf width and
streamflow, both of which contribute to greater Fe availability to the north. Winter river runoff supplies Fe to the
shelf, in both dissolved and particulate form. Downwelling
conditions during winter isolate the coastal ocean from the
open ocean and help retain this Fe on the shelf. A large
fraction of the Fe exported from rivers is thus ultimately
delivered to the sediment as flocculants or via scavenging
rather than lost to the open ocean. This winter-derived Fe is
then remobilized during the summer upwelling season,
where it fuels phytoplankton productivity. The shelf acts
as a large capacitor, charging in winter with riverine Fe and
dampening year-to-year fluctuations in riverine Fe flow. In
regions of the coast with a relatively broad shelf and large
riverine inputs (OR and WA coasts), phytoplankton productivity is not limited by Fe [Chase et al., 2005a; Lohan and
Bruland, 2006] and nearly all available NO3 and considerable amounts of CO2 are consumed within about 20 km of
the coast [Hales et al., 2005, 2006]. Regions with narrower
shelves and reduced river discharge can experience incomplete macronutrient uptake and Fe-limited productivity
[Hutchins and Bruland, 1998; Bruland et al., 2001].
[21] Several approaches can be taken to test our hypothesis. First, the availability of high-resolution, in situ observations of chl, productivity, wind stress and curl, mixed
layer depth and nutrient concentrations would remove any
doubts about the validity of remotely-sensed measurements.
Second, verification through in situ observations of a
poleward increase in Fe stress during summer would
confirm the proposed mechanistic link between shelf width,
river flow and phytoplankton biomass. Finally, additional
insight could be gained by examining large-scale relationships between shelf width, chl and river runoff in other
eastern boundary current regions such as South America
and Africa.
[22] Acknowledgments. Roseanne Schwartz and Linda Baker were
critical for success at sea, and Roseanne Schwartz, Dale Hubbard and Joe
Jennings assisted with analytical work. We thank Rob Wheatcroft, Jim
McManus, Dudley Chelton and Craig Risien for helpful discussions.
Comments from two anonymous reviewers significantly improved the
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manuscript. This work was supported by the National Science Foundation
(grants OCE9907953 to A. van Geen and OCE9907854 to Burke Hales).
SeaWiFS chlorophyll data were provided by the SeaWiFS Project, NASA/
Goddard Space Flight Center and ORBIMAGE: ftp://oceans.gsfc.nasa.gov/.
Stream flow data were obtained from the USGS (http://nwis.waterdata.usgs.
gov/usa/nwis/monthly).
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