Estuarine, Coastal and Shelf Science 169 (2016) 207e215 Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Quantifying the dispersal potential of seagrass vegetative fragments: A comparison of multiple subtropical species E.J. Weatherall a, E.L. Jackson a, *, R.A. Hendry a, M.L. Campbell a, b a School of Medical and Applied Sciences, Central Queensland University, CQUniversity Gladstone Marina Campus, Building 604, Bryan Jordan Drive, Gladstone QLD 4680, Australia b Environmental Research Institute, Faculty of Science and Engineering, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand a r t i c l e i n f o a b s t r a c t Article history: Received 12 June 2015 Received in revised form 16 November 2015 Accepted 28 November 2015 Available online 1 December 2015 Seagrass meadows are threatened by anthropogenic and natural disturbances on both a local and global scale. Understanding the potential for seagrasses to disperse, connecting populations separated by unsuitable habitat is important to assess the resilience of regional populations. This study investigated the relative dispersal potential of vegetative fragments of seagrass from five subtropical species (Zostera muelleri, Halodule uninervis, Halophila ovalis, Halophila spinulosa, Halophila decipiens). Five questions were examined: 1) do vegetative fragments of different species settle at different velocities; 2) does a species morphometric variables influence settling velocities; 3) is a species settling velocity related to the species local distribution; 4) does temperature stress affect settling velocity; and 5) what is the composition and potential viability of seagrass fragments floating in the bay. A proportional distribution index for each species was determined using data from a habitat prediction model. It was found that H. spinulosa settled significantly faster than the remaining species and Z. muelleri settled the slowest. Variables influencing settling velocity included rhizome length, weight and surface area. In both Z. muelleri and H. ovalis settling velocities were significantly greater at higher temperatures (although there was no significant difference between approximately 5 and 10  C above ambient temperature). H. uninervis was not significantly influenced by temperature. There was a significant negative correlation between species settling velocities and their distribution. © 2015 Elsevier Ltd. All rights reserved. Keywords: Movement ecology Recruitment Morphometrics Intertidal Dispersal 1. Introduction Understanding the potential for seagrasses to disperse, connecting populations separated by unsuitable habitat, is of prime importance to assessing the resilience of regional populations. The dispersal of a species is only effective (or successful) when a pathway exists between a donor and a settlement site, and where the site of settlement is suitable for growth and reproduction (Reynolds et al., 2013; Campbell, 2002). Successful dispersal and recruitment can create a seagrass metapopulation: spatially separated populations that are linked by the effective dispersal of viable propagules. Connectivity in the metapopulation can be facilitated by ocean currents (e.g., Harwell and Orth, 2002), dislodgement and hitchhiking on species (e.g., Baldwin and Lovvorn, 1994; Dos Santos et al., 2012) and movement via vessels (Ruiz and Ballantine, 2004). Yet, what occurs when a classic metapopulation (sensu Hanski and * Corresponding author. E-mail address: emma.jackson@cqu.edu.au (E.L. Jackson). http://dx.doi.org/10.1016/j.ecss.2015.11.026 0272-7714/© 2015 Elsevier Ltd. All rights reserved. Gilpin, 1997; Hanski, 1998) is interrupted by disturbance? Does local species extinction occur or does the disturbance create opportunities for different recruitment and colonisation possibilities? Within the literature studies have investigated the dispersal potential of seeds (Orth et al., 1994, 2006; Berkovi
c et al., 2014), yet few have examined the dispersal of vegetative fragments and even fewer compared multiple species. Understanding how a species disperses and recruits is particularly important in coastal areas where both natural and anthropogenic stressors have the potential to fragment seagrass meadows at a landscape scale (Short and Wyllie-Echeverria, 1996). Seagrass have four life stages capable of dispersal: pollen, sexual propagules, vegetative fragments, and vegetative growth of individuals (McMahon et al., 2014). Dispersal can occur on the water surface, in the water column, on or in the sediment, via animal vectors and through clones, and the movement path is influenced by external factors (vectors and environment), internal states and navigation capacity which change over space and time (McMahon et al., 2014). With few exceptions seagrass pollen and seeds tend to be negatively buoyant and have restricted dispersal distances, perhaps to 208 E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 only a few meters (Reusch, 2002; Berkovi
c et al., 2014). Yet, this can be overcome if sexual propagules attached to seagrass fragments are dispersed. For example, in Zostera species, seeds can be transported by positively buoyant flowering branches (rhipidia) hundreds of kilometers for weeks to months (McMahon et al., 2014). Seagrasses invest energy in sexual reproduction as it enhances genetic diversity and hence long-term resilience (Short, 1987). Vegetative reproduction is also employed by seagrasses through rhizome elongation, and the breaking off and recolonisation by viable fragments (Rasheed, 2004; DI Carlo et al., 2005, Short, 1987). Dislodged fragments can occur naturally by storm events and foraging herbivores, and by vessel anchoring and water craft activity (Coyer et al., 2008). Recruitment and establishment of dislodged vegetative fragments has been detected over both short (10e100 s of metres; Campbell, 2003; Touchette and Burkholder, 2000; Hall et al., 2006) and long-distances (100e1000 s of kilo~ iz-Salazar et al., metres; Olsen et al., 2004; DI Carlo et al., 2005, Mun 2006; Berkovi
c et al., 2014). Seeds and reproductive structures can raft on vegetative fragments, transporting genes amongst spatially €llstro €m et al., separated populations (Patterson et al., 2001; Ka 2008). Understanding how recruitment and recolonization occurs in disturbed industrial locations is even less understood. Hence, this study occurs in Port Curtis Bay, which is located on the central Queensland coast in a subtropical climate (Fig. 1). This site is within the Great Barrier Reef World Heritage Area and includes the industrial Port of Gladstone. The Port of Gladstone has recently undergone rapid industrial expansion with three new liquefied natural gas plants on Curtis Island as well as an additional coal export terminal. Associated with these projects are dredging operations, increased shipping movements, land reclamation and both land and marine based construction activity. These disturbances continue to threaten the health of seagrasses, with seagrass monitoring already reporting 50%e75% losses of local seagrass over the last five years (Bryant et al., 2014; Coles et al., 2015). The results of this study provide valuable information on the dispersal potential of five tropical seagrass species that use fragmentation as a method of reproduction, which can be utilised in seagrass dispersal modelling to identify the sources and sinks of detached material and demonstrate population connectivity. The endpoint of this information is to aid prediction potential for restoration (facilitated and un-facilitated) and concentrate conservation and restoration efforts to critical seagrass beds. Thus, this study aims to investigate the settling velocities of vegetative fragments from five subtropical seagrass species growing in Port Curtis, to gain an indication of their relative dispersal potential, and assess how it may vary with morphometric variables and temperature. Specifically, four null hypotheses were tested. Firstly, that there was no significant difference in settling velocities between species of seagrass; that there was no relationship between morphometric variables and settling velocity; that there was no significant difference in settling velocity between fragments in ambient water temperatures compared to plus 5  C and plus 10  C; and that there is no relationship between settling velocity and the distribution of each species. In addition, to provide some qualitative context, surveys were carried out to examine the composition and potential viability of floating seagrass fragments within the bay. 2. Materials and methods 2.1. Study area A lot of effort has been expended mapping and monitoring the seagrass patches in Port Curtis Bay, Queensland (23 550 3800 S, 151 250 2400 E) (Thomas et al., 2010, Davies et al., 2013). There are over 100 distinct seagrass dominated patches (Fig. 1), which occur across a range of environmental gradients and vary in terms of configuration (patch size, species composition), environment (depth, wave and current exposure, turbidity) and human pressures (in particular increased turbidity and sedimentation from dredging activities, land reclamation, nutrient enrichment and boat propeller damage). Patches of seagrass (Fig. 1) include multi-specific and monospecific stands of seagrass of varying densities and configurations. Five species of seagrass currently grow in Port Curtis; Halophila spinulosa (R.Brown) Ascherson, Halophila ovalis (R. Brown) J.D.Hooker, 1858, Halophila decipiens Ostenfeld, 1902, Halodule uninervis (Forsskål) Ascherson, 1882 and Zostera muelleri subsp. capricornii Irmisch ex Ascherson, 1867. The species have various global distributions. H. ovalis is the most widespread, found in the Indo-Pacific and has recently been discovered in the Atlantic Ocean on the Island of Antigua (Short et al. 2010). Z. muelleri has a broad but disjunctive distribution around southern and eastern Australia, and is also found in New Zealand and Papua New Guinea (Short et al. 2010). H. decipiens is circum-global and widespread in the tropics, whereas H. spinulosa occurs in the Indo-Pacific, the Philippines, Malaysia, Indonesia, Singapore and northern Australia (Short et al. 2010). Finally, H. uninervis is widespread in the IndoPacific (Short et al. 2010). Locally, all the species are found primarily on the intertidal, with the exception of H. decipiens which occurs primarily in the subtidal. In the tropical Indo-West Pacific H. spinulosa is reported as a sub-tidal species (Waycott et al., 2004), but in Port Curtis, at the time of the study, it was found only on the extreme lower shore. 2.2. Fragment settling velocities The settling velocities of vegetative fragments of Z. muelleri, H. ovalis, H. spinulosa, H. decipiens and H. uninervis were measured. Due to the lack of knowledge on the influence of age of floating fragments, fresh fragments of seagrasses were collected directly from intertidal meadows at Pelican Banks, Port Curtis Bay (23⁰460 S, 151⁰170 E) during October 2013. Fragments were collected by disturbing the seagrass patch and collecting detached fragment. Upon collection vegetative fragments were placed in shaded, seawater filled containers, and returned to the laboratory where they were immediately placed in indoor, seawater mesocosms. The mesocosms were filled with artificial seawater (Aquasonic Ocean Nature) and artificially lit on a 14:10 h light:dark cycle using twin T5 Razor lights with alternate Actinic Blue and Coral Plus light tubes. Average daily PAR for the tanks was 8.16 (±1.28 S.D.) mol m 2 d 1. A maximum of 30 fragments were allocated to each individual mesocosm. The following seagrass fragment morphometric variables were measured: meristem presence, number of shoots, rhizome length, weight and surface area. No action was taken to achieve consistency in the morphometric variables of fragments, with the exception of the presence of a meristem. Surface area was determined by analysing scanned images of the fragments using image analysis software (Adobe Lightroom, version 5). The settling velocity of the fragments was determined by releasing the fragments just under the water-surface, in a horizontal position, in a 400L (1200  600  600 mm) tank of seawater (maintained at 25  C and a salinity of 35). Five-centimetre intervals were marked on the side of the tank, beginning 10 cm below the water surface. The time for the fragment to sink over each interval was measured with a stopwatch and averaged to determine its mean sinking velocity. This ensured that an accurate average speed could be calculated to account for any acceleration as the fragment settled. Fragments that remained floating (that is, no downwards movement) after a period E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 209 Fig. 1. Location of Port Curtis Bay illustrating the November 2009 distribution of seagrass (Thomas et al., 2010). of 4 weeks (28 days) were assigned a settling velocity of zero. The time spent positively (defined as those floating in the upper 0e10 cm of the water column) and neutrally buoyant (those suspended greater than 10 cm from the surface). Forty replicates from each species were chosen at random to achieve a mixture of morphometric variables. 2.3. Influence of temperature stress To assess the influence of temperature stress, six replicates of fragments collected directly from the meadow (as described above) of the species H. uninervis, Z. muelleri and H. ovalis were kept for 28 days at ambient temperature (median 24.1  C), approximately 5  C above ambient temperatures (median 28.9  C) and approximately 10  C above ambient temperatures (mean 33.7  C). Aquarium heaters were used to increase temperatures to above ambient. After 28 days acclimation settling velocities were calculated following the method described above. 2.4. In-situ fragment seagrass material composition Quantitative and qualitative sampling to examine the composition and potential viability of fragments found suspended in the water column was conducted on 17th and 18th October 2013 and 16th and 17th January 2014 at Pelican Banks (at the start and end of the austral growing season), in Port Curtis Bay. Collection occurred on the ebb tide in shallow channels and at meadow edges, where dislodgement of fragments had been commonly observed. Quantitative methods included towing plankton nets (30 cm diameter at 5 knots over ground for 5 min), dip netting (38 cm  38 cm aperture held static for 10 min, with boat speed at 5 knots over ground) and drag netting (60 cm  150 cm aperture held static for 10 min). The flow rate of the tidal current was determined by recording the time for a floating object to travel a set distance. The material 210 E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 collected through these quantitative methods was used to calculate the density of fragment material in a set volume of seawater. Floating material was also collected qualitatively (opportunistically) using a dip net to retrieve potentially viable fragments seen floating near the boat for the duration of the field work at each site. Qualitative drag netting was also performed while walking around in areas of shallow water. Qualitative material was combined with the quantitative fragments to assess the following variables: rhizome length, number of shoots and meristem presence. 2.5. Statistical analysis 2.5.1. Differences in settling velocities between species The fragment settling velocities of each species were compared using a one-way ANOVA (n ¼ 40, Power > 0.6). The settling velocity data could not be transformed to satisfy Cochran's test for homogeneity of variance; as a result, the p value was reduced to 0.01 to reduce the risk of making a Type I error. The SNK post hoc test was then used for pairwise comparisons. To account for the fact that fragment size varied within and between species a second one-way ANOVA was performed using the Reynold's numbers of the fragments. The Reynold's number is a dimensionless value that represents the ratio between inertial force and the viscosity of the medium acting upon an object and acts as a standardisation for size (Purcell, 1977). The density and viscosity of the tank water at the measured temperatures and salinities was determined using preexisting tables (Ramsing and Gundersen, 2013). The Reynold's numbers of the fragments were transformed using the arcsine transformation to satisfy Cochran's test for homogeneity of variance. Again, SNK post hoc pairwise comparisons of the means were performed. 2.5.2. Influence of morphometric variables on settling velocities It was recognised that there were not only differences between species, but also within species due to variations in morphometric variables. The contribution of morphometric factors as predictors of settling velocity were evaluated using a Multiple Linear Regression model (Statistica version 12), for each species. Shoot number, rhizome length, surface area and wet weight were standardised and entered into the model as continuous predictors. The ridge regression method was chosen (Lambda 0.1) due to the high colinearity of morphometric variables. No model was developed for Z. muelleri due to the large proportion of floating fragments. A t-test was used to examine if there were significant differences in settlement velocity between fragments with an apical meristem versus those without, for each species. 2.5.3. Influence of temperature stress on settling velocity To determine whether temperature stress affects the settling velocity of seagrass fragments, a one-way ANOVA was performed. The ANOVA compared the mean settling velocities of the fragments that had been stored under ambient temperature conditions (the control) and those that had been stored in temperatures 5  C and 10  C above ambient temperature, with salinity maintained at 35. The settling velocity data met the assumption of homogeneity of variance. The StudenteNewmaneKeuls (SNK) post hoc test was then used for pairwise comparisons. All ANOVA were carried out using Windows GMAV. 2.5.4. Composition of in-situ floating material The floating material collected quantitatively was used to calculate the density of potentially viable seagrass material in the water column. This was calculated by using the current velocity measurements to determine the flow rate of water passing through each type of net and dividing it by the amount of fragments, from each species, collected to give a number of each species per m3 of seawater that passed through each type of net. The material collected quantitatively was measured in terms of rhizome length, shoot density and the presence of an apical meristem. Each shoot of H. ovalis is capable of acting as an apical meristem and therefore any fragment of this species was considered a potentially viable fragment. 2.5.5. Influence of settling velocities on species distribution To determine whether the observed differences in settling velocities showed any relationship with species distribution within Port Curtis, a species distribution index for each species was calculated using data from a habitat prediction model for each species (Jackson et al., in prep). The model identified areas of suitable habitat for each species by overlaying variables of current velocity, depth, wave exposure and slope to determine the probability of the presence of each species. A species distribution index was calculated as the percentage of area actually inhabited by each species, out of the total area of modelled potential habitat in Port Curtis with p  0.5 of seagrass growth (i.e., realised niche as a proportion of fundamental niche). The relationship between the mean settling velocities of the seagrass species and their respective distribution indices was explored by linear correlation. Prior to analysis the data was transformed to Z scores to standardise for variables measured on two different scales. 3. Results 3.1. Differences in settling velocities between species Upon initial introduction to the tank, 91% of all Z. muelleri fragments floated, 62% of H. ovalis, 24% of the H. uninervis and only 2% of H. decipiens. Floating fragments primarily displayed positive, rather than neutral buoyancy. On average floating Z. muelleri fragments remained buoyant for 21 days (±6.0 s.d.), with 18 days (±6.8 s.d.) at positive buoyancy and 3 days (±4.9 s.d.) at neutral buoyancy. Z. muelleri fragments which floated started to lose colour and rigour at approximately 14 days, which may have been a sign of loss of viability. Those fragments of H. ovalis which floated, remained buoyant for an average of 4 .5 days (±2.6 s.d.), 3.9 days (±2 s.d.) positively buoyant and 0.6 days (±0.8 s.d.) neutrally bouyant. H. uninervis fragments which floated upon initial introduction to the tank remained positively buoyant for 3 days (±0.8 s.d.), with none showing neutral buoyancy. The only fragment of H. decipiens which floated remained neutrally buoyant for 0.5 days before sinking. None of the 40 fragments of H. spinulosa floated. There was no significant difference in mean settling velocities between the species (F[4, 199] ¼ 3.17, p ¼ 0.015; Fig. 2). Because of the borderline result, an a posteriori StudenteNewmaneKeuls test was performed, which determined that H. spinulosa fragments settled faster than the remaining species, particularly H. decipiens and Z. muelleri. Mean Reynold's number differed significantly between species (F[4, 199] ¼ 7.528 (p < 0.001; Fig. 3). An a posteriori SNK test revealed that the mean Reynold's number of H. spinulosa fragments was significantly different to all the remaining species. Z. muelleri fragments had the lowest mean Reynold's number. 3.2. Influence of morphometric variables on settling velocities Shoot number was a significant predictor of settling velocity for all four species (Table 1), noting that Z. muelleri was excluded from the model due to the low number of initial sinking fragments. For H. spinulosa, H. uninervis and H. decipiens there was a significant negative contribution of the number of shoots in predicting settling velocity (B ¼ 0.97, 0.36 and 0.44 respectively; Table 1). 211 E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 was not available for H. decipiens due to constraints of the predictive model. There was a significant negative correlation between species distribution and mean fragment settling velocities (R2 ¼ 0.958, p ¼ <0.05). 3.4. Influence of temperature stress on settling velocity Fig. 2. The mean fragment settling velocity of each species of seagrass. Bars indicate ± one standard error of the mean. Within species, temperature did not significantly influence the settling velocities of H. uninervis, however, both 5  C and 10  C above ambient temperature saw a significant increase in settling velocity of H. ovalis and Z. muelleri fragments (Fig. 5). There was a statistically significant interaction between examined species (Z. muelleri, H. ovalis and H. uninervis) and temperature treatment (F[4,45] ¼ 4.82, p < 0.01; Fig. 5). There was no significant difference between species at ambient temperatures. At 5  C greater than ambient, H. uninervis had a significantly slower mean settling velocity (0.0024 ms 1, ±0.0016 SE) than Z. muelleri (0.0214 ms 1, ±0.0043 S E.) and H. ovalis (0.0355 ms 1, ±0.0076 S E.). Halophila ovalis had significantly faster settling rates than Z. muelleri. At 10  C above ambient H. ovalis had significantly faster mean settling velocity (0.0374 ms 1, ±0.0076 S E.) than Z. muelleri (0.0206 ms 1, ±0.0046 S E.) and H. uninervis (0.0120 ms 1, ±0.0047 S E.). Mean settling velocities of Z. muelleri and H. uninervis were not significantly different from each other at 10  C above ambient. 3.5. Composition of in-situ floating material Fig. 3. The mean Reynold's numbers of each species of seagrass fragments. Bars indicate ± one standard error of the mean. Superscripts show means that are not significantly different when compared using an a posteriori SNK test. However for H. ovalis there was a strong positive relationship between shoot number and settling velocity (B ¼ 0.51; Table 1). Rhizome length did not contribute to predicting settling velocity for any of the examined species. However, weight of the fragments contributed to settling velocities of H. uninervis and H. decipiens, with an increase in weight resulting in increased settling velocities (Table 1). Surface area showed a significant positive contribution to the settling velocity of H. uninervis. H. spinulosa settling velocities were significantly lower for fragments with a meristem (t ¼ 3.77, p < 0.01). H. uninervis and H. ovalis settling rate between fragments with and without a meristem showed no difference. The densities of in situ floating material varied between methods of collection (see Table 2). The drag net caught the largest numbers of Z. muelleri and H. ovalis fragments (0.084 m 3 and 0.05 m 3, respectively). The plankton net was the only method to collect H. uninervis. The plankton net collected 0.058 fragments m 3 of Z. muelleri, 0.022 fragments m 3 of H. ovalis and 0.014 fragments m 3 of H. uninervis. This compares to the dip netting which recorded 0.06 fragments of Z. muelleri m 3, 0.035 fragments of H. ovalis m 3 and 0 fragments of H. uninervis. No fragments of 3.3. Relationship between settling velocities on species distribution The species most widely distributed in Port Curtis is Z. muelleri (38.28%), followed by H. ovalis (30.42%), H. uninervis (18.16%) then H. spinulosa (3.23%) (Fig. 4). The proportional distribution index Fig. 4. The negative correlation between species distribution and fragment settling velocities. Species distribution decreases with increasing fragment settling velocity. Table 1 Results of Ridge Regression General Linear Model on the settling velocities of seagrass fragments from four subtropical species (n ¼ 40). Significant B values are highlighted in bold. Negative signs indicate a negative relationship. Predictors of settling velocity for Halophila spinulosa Halophila ovalis Halodule uninervis Halophila decipiens B Number of shoots Rhizome length Weight ¡0.97 0.51 ¡0.36 ¡0.44 0.23 0.21 0.07 0.09 0.06 0.01 0.20 0.51 F p value Adjusted R2 24.55 9.28 94.80 111.24 <0.001 <0.001 <0.001 <0.001 0.70 0.50 0.90 0.92 Surface area 0.21 0.16 0.48 0.02 212 E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 Fig. 5. Mean settling velocities of Halophila ovalis, Halodule uninervis and Zostera muelleri following 28 days exposure to ambient temperature and 5 and 10  C above ambient temperature. Table 2 Composition of fragments (with rhizome and root) collected via multiple methods (low and high tide), s.d. is the standard deviation. Quantitative Plankton net tow (fragments per m3) Dip net (fragments per m3) Drag net (fragments per m3) Qualitative Total fragments collected % of fragments with apical meristem Mean rhizome length (s.d.) Mean number of shoots (s.d.) Zostera muelleri Halophila ovalis Halodule uninervis 0.06 0.06 0.08 0.02 0.04 0.05 0.01 0 0 148 87.15 2.31 (1.76) 1.66 (0.92) 57 49.12 4.76 (1.54) 1.14(0.35) 4 75 4.1 (0.67) 1.75 (0.95) H. decipiens or H. spinulosa were collected. Eighty seven percent of the Z. muelleri fragments collected had an apical meristem, compared to 49% of the H. ovalis fragments and 75% of H. uninervis. Mean number of shoots was similar across species. 4. Discussion An understanding of the dispersal potential of seagrasses requires knowledge of key traits that are often lacking for many species (Short, 1987). With a rise in studies assessing the connectivity between seagrass populations, often utilizing particle dispersal modelling (Erftemeijer et al., 2008; Harwell and Orth, 2002; Thomson et al., 2014), such information is increasingly sought. This study investigated the settling velocities of vegetative fragments from five subtropical seagrass species to indicate their relative dispersal potential. The five objectives were to determine whether there is a difference in settling velocities between species, whether morphometric variables are related to settling velocity and whether settling velocity is related to species distribution. Due to the temperature stress floating fragments would likely experience with time in subtropical environments, the influence of increased temperature on fragment settling velocity was also examined. The current study demonstrated a large variation in the settling velocities of seagrass fragments and, hence, dispersal potential of different seagrass species. Analysis of fragments suggests that H. spinulosa settled significantly faster than the remaining intertidal species, with Z. muelleri settling the slowest (Fig. 2). In Port Curtis, H. spinulosa is predominantly found in very-low intertidal (ephemeral intertidal species in this locale) and sub-tidal habitats (Thomas et al., 2010). The rapid settling of their vegetative fragments is perhaps an adaptation to limit dispersal to habitats outside of this optimum habitat, reducing the risk of being carried by tidal currents to shallower intertidal areas. Hall et al. (2006) also highlight that seagrass fragments with short viability periods must settle quickly to ensure successful reestablishment; which may explain the observed higher settlement velocities of H. spinulosa observed in this study. Consistent with this hypothesis, species with slower settling velocities, including Z. muelleri, may have a longer viability period; however the current study did not measure realised viability of fragments. In a recent study, StaffordBell et al. (2015) examined the buoyancy and longevity of vegetative fragments of Z. muelleri from the beach wrack, and determined that 50% of fragments were buoyant, compared to the 91% of fresh fragments observed in this study. They noted only a marginal decline in Z. muelleri fragment viability after 5 weeks (96% still viable, Stafford-Bell et al., 2015). In the current study, Z. muelleri fragments which floated started to lose colour and rigour at approximately 14 days. In fact many fragments did not become negatively buoyant until many of the leaves had discoloured and in some cases detached from the rhizome. A shorter potential viability period for the Z. muelleri in this study may have been due to higher temperatures and greater UV radiation found in Queensland compared to Victoria (conditions which were replicated in the E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 laboratory). Comparative studies across biogeographical gradients are needed to ascertain whether dispersal potential differs at these scales. Hall et al. (2006) and Touchette and Burkholder (2000) examined floating fragment viability for subtropical populations of Halodule wrightii, Halophila johnsonii (potentially the introduced species Halophila ovalis, that is found in temperate and tropical locales) and temperate Zostera marina (respectively). Ewanchuck and Williams (1996) noted that fragments of Z. marina remained viable for up to 6 weeks, potentially allowing dispersal over great distances dependent on ocean currents. The dispersal potential of seagrass fragments can alter substantially depending on their buoyancy and position in the water column. Fragments that are positively buoyant are subject to dispersal via both wind and tidal currents, whilst neutrally buoyant fragments are, in general, only subjected to tidal currents and therefore may have lower dispersal potentials. In the current study differences in the proportion of fragments floating were observed with the majority of fragments of Z. muelleri and H. ovalis floating upon initial testing and showing primarily positive buoyancy. It should also be noted that once negatively buoyant and settled on the seabed fragments may still move along the sea floor however dispersal will be subject to a drag coefficients related to the structural complexity of the seabed (McMahon et al., 2014). Further research is required to test the viability of Port Curtis seagrass species fragments. Whilst buoyancy and viability are important in connecting seagrass meadows over larger distances, fragment settling velocities are likely to contribute to the connectivity of a local metapopulation and the dynamic distributions of different seagrass species (Reynolds et al., 2013; Pulliam, 2000). This theory is supported by the significant negative correlation between fragment settling velocity and seagrass distribution index (i.e., the proportion of occupied suitable patch). Although seagrass seeds and spathes may also have a role at such scales, with the exception of Z. muelleri most seagrass seeds in this region are highly negatively buoyant (Short, 1987). These results indicate that H. spinulosa is likely to be less resilient to disturbance events than Z. muelleri when considering their existing distribution, potential for long-distance dispersal, and strength of metapopulation connectivity to aid in natural recovery. The rapid settlement of vegetative fragments of H. spinulosa has implications for this species’ distribution in Port Curtis Bay. The fragments have limited long-distance dispersal potential and despite habitat modelling showing an abundance of potentially suitable habitat available for this species in Port Curtis, it is currently unable to exploit it. Connectivity between geographically isolated metapopulations is likely to be limited and hence natural recruitment and recovery following disturbance events may be limited. This result supports the postulation that in Port Curtis, H. spinulosa is most at threat of local extinction due to restricted connectivity of settlement opportunities. This is also reflected in the loss of 66% of H. spinulosa monitored meadows in Port Curtis between 2009 and 2013 (Bryant et al., 2014). In terms of protecting and enhancing the remaining meadows, H. spinulosa may benefit from effective transplantation methods (Zhou et al., 2014) that utilise a decision-based framework to select optimal sites, species and transplant units (Campbell, 2002; van Katwijk et al., 2009; Fonseca, 2011). Some morphometric variables (shoot density, weight and surface area) had a significant influence on settling velocity, but few relationships were interpretable. For H. spinulosa, H. uninervis and H. decipiens as the number of shoots increased settling velocity decreased, but this was not a function of rhizome length, weight (with the exception of H. decipiens) or surface area, suggesting that the internal anatomy of the leaves (such as lacunae size and density) may be influencing buoyancy. For H. ovalis number of shoots 213 was shown to positively contribute to the settling velocity. This result and the fact that H. ovalis seeds are negatively buoyant (Inglis 2001) has implications for this species dispersal, since greater dispersal distances would be obtained with smaller sized fragments. It would be expected that as the size of fragments decreases, so would photosynthetic capability and the amount of stored starch in rhizomes. However, since each shoot of H. ovalis is a meristem capable of clonal growth, the potential for fragments of this species to colonise once settled is greater than other local species. In addition to morphometrics of the fragments, the environmental conditions that the fragments are exposed to once detached may also influence viability and settlement velocity or buoyancy. Stafford-Bell et al. (2015) observed a potentially confounding negative influence of increased temperature on the buoyancy and viability of the fragments. In the current study, an examination of the influence of a 5 and 10  C increase in ambient temperature (between 26  C and 28  C degrees during the study) revealed that after 28 days there was no difference in settling velocities between species at ambient temperatures, but at 5 and 10  C above ambient temperature significant differences were observed. In both Z. muelleri and H. ovalis settling velocities were significantly greater at higher temperatures (although there was no significant difference between 5 and 10  C). Such a change in settling velocity may confer an advantage in terms of recruiting to suitable habitat, whereby plants trapped in shallow water pools on intertidal mudflats (where temperatures can reach up to 30  C, personal observations) lose buoyancy and settle out. However, 30  C is around the upper thermal limit of Z. muelleri (Collier et al., 2011; York et al., 2013), and therefore these fragments may also lose viability as a result of thermal stress. H. uninervis was not significantly influenced by temperature. The current study represents an extreme, chronic heating event, since under normal circumstances temperatures would reduce at night and then warm up again during the day. However, given climate change predictions for sea temperature increases in this area (2  C), the negative implications for dispersal ability of seagrass vegetative fragments indicated here, warrant further investigation, particularly of acute effects (i.e. changes over short time periods, accounting for tidal and diel cycles). It would also be useful to identify thresholds of temperature increase that have a significant influence on seagrass fragment buoyancy and viability. Dispersal ability and viability will only successfully contribute to establishment of seagrass at new sites if fragments are able to reattach (McMahon et al., 2014). Successful reattachment is likely to be a result of a combination of landscape and microtopographical scale environmental factors. For example, DI Carlo et al. (2005) found that cobbles and boulders create crevices aiding the entanglement and anchorage of vegetative fragments of Posidonia oceania. Hall et al. (2006) also report that the macroalgae Enteromorpha (now accepted as Ulva) facilitates seagrass attachment by trapping fragments at the sediment level. In Port Curtis, other factors, suggested by Harwell and Orth (2002), worth investigating include variations in shear bed stresses, substrate types, current ripples and contributions of biotic communities including existing seagrass beds, macroalgae, oyster reefs and polychaete worm tubes. 5. Conclusion Understanding the relative connectivity of different seagrass meadows provides an important knowledge base for answering questions about why seagrass meadows may be declining or recovering, where restoration may make the biggest contribution to wider metapopulation survival and management decisions on seagrass conservation. The current study has created knowledge 214 E.J. Weatherall et al. / Estuarine, Coastal and Shelf Science 169 (2016) 207e215 that can be utilised to better understand and improve the management of seagrasses, especially in anthropogenically impacted coastal environments, such as Port Curtis Bay, Queensland. The buoyancy and settlement velocity of five species of subtropical seagrasses and the traits that may influence dispersal were investigated. The data clearly shows that the H. spinulosa (which inhabits the extreme lower intertidal in this locality) sinks faster than the other intertidal species that were examined, resulting in a limited capacity to spread. Alternatively, Z. muelleri floated for longer, which was reflected in the species realised and potential niche occupancy. Across species of seagrass the study found a significant negative correlation between fragment settling velocity and local seagrass distribution (i.e., the proportion of occupied suitable patch). Such information is valuable when making decisions about the species composition of seagrass transplants. For H. spinulosa, H. uninervis and H. decipiens as the number of shoots increased settling velocity decreased, but this was not a function of rhizome length, weight (with the exception of H. decipiens) or surface area, suggesting that the internal anatomy of the leaves (such as lacunae size and density) may be influencing buoyancy. Threats which dislodge smaller fragments of seagrass (for example grazing) may therefore promote dispersal compared to those dislodging larger fragments (e.g. propeller scarring), but this requires more investigation. Exposure to increased temperature (based on a chronic heating event) increased settling of Z. muelleri and H. ovalis, which may aid settlement in intertidal pools, but may also indicate reduced dispersal potential with increasing seawater temperatures resulting from climate change, although an assessment of more acute effects are needed. Conflict of interest The authors declare that they have no conflict of interest. Author contributions EJ and MC originally formulated the idea; EW, EJ and MC designed the experiment and RH, EW and EJ developed the methodology; EW, EJ, MC, RH all conducted fieldwork; EW conducted laboratory work; EW and EJ performed the statistical analysis; EW and EJ wrote the manuscript (in order of contribution); MC and RH provided editorial advice. Acknowledgements Special thanks are extended to William Debois (Photopia Studios Ltd) for image analysis and staff at the CQUniversity Gladstone Environmental Science Centre for supporting this study. This work was supported by a CQUniversity Summer Scholarship (Weatherall), CQUniversity RDI Merit Grant (Jackson and Campbell), Queensland Government Smart Futures Mid-Career Fellowship (Campbell) and a CQUniversity Research Advancement Award Scheme (Campbell). 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