Search

Many ecosystems are greatly affected by ecosystem engineering, such as coastal salt marshes, where macrophytes trap sediment by reducing hydrodynamic energy. Nevertheless, little is known about the costs and benefits that are imposed on engineering species by the traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing ecosystem engineering efficiency defined as the benefit--cost ratio per unit of biomass investment for two species from the intertidal habitat: the stiff grass Spartina anglica and the flexible grass Puccinellia maritima. These species were selected for their ability to modify their habitat by trapping large quantities of sediment despite their contrasting growth form. On a biomass basis, dissipation of hydrodynamic energy from waves (a proxy for benefits associated with ecosystem engineering capability as it relates to the sediment trapping capability) was strikingly similar for both salt marsh species, indicating that both species are equally effective in modifying their habitat. The drag forces per unit biomass (a proxy for costs associated with ecosystem engineering ability as it relates to the requirements on tissue construction and shoot anchoring to prevent breaking and/or washing away) were slightly higher in the species with flexible shoots. As a result, stiff Spartina vegetation had slightly higher ecosystem engineering efficiency, due to lower engineering costs rather than to a higher engineering effect. Thus, Spartina is a slightly more efficient rather than a more effective ecosystem engineer. Ecosystem engineering efficiency was found to be a species-specific characteristic, independent of vegetation density and relatively constant in space. Analyzing ecosystem engineering by quantifying trade-offs offers a useful way toward developing a better understanding of different engineering strategies. Key words: drag; extended phenotype; niche construction; sediment accretion; trade-offs; waves.

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1600-0706.2008.16892.x Byline: T. J. Bouma, M. Friedrichs, B. K. van Wesenbeeck, S. Temmerman, G. Graf, P. M. J. Herman Abstract: Spartina anglica is an autogenic ecosystem engineer. At a local (within-vegetation) scale, it improves plant growth by enhancing sediment accretion through attenuation of hydrodynamic energy with its shoots. This constitutes a short-range positive feedback. The vegetation also shows a long-distance negative feedback through formation of erosion troughs around the tussock, which are restricting lateral expansion. There is a growing recognition of the important role of such scale-dependent feedbacks for landscape structuring and ecosystem stability. By a series of flume studies, we provide direct experimental evidence that 1) both the local positive and the long-distance negative feedback are strongly density-dependent with clear thresholds, and 2) that both feedbacks are inherently linked via their density dependence, partly as the result of conservation laws. The observed thresholds occurred at vegetation densities commonly found in the field. We demonstrate that threshold densities are affected by external hydrodynamic forcing for the long-distance negative feedback. We expect the same to be true for the local positive feedback (sediment accretion), so that under mild hydrodynamic forcing, this local positive feedback may occur in the absence of a long-distance negative feedback (formation of erosion troughs). The density dependence of nearby positive and the long-distance negative feedback and how external conditions affect the linkage of both feedbacks are important factors shaping the vegetated salt-marsh landscape. Article History: Manuscript Accepted 28 July 2008 Article note: T. J. Bouma (t.bouma@nioo.knaw.nl), B. K. van Wesenbeeck and P. M. J. Herman, Centre for Estuarine and Marine Ecology, Netherlands Inst. of Ecology (NIOO-KNAW), PO Box 140, NL-4400 AC, the Netherlands. - M. Friedrichs and G. Graf, Inst. for Biological Sciences-Marine Biology, Univ. of Rostock, Albert Einstein street 3, DE-18051 Rostock, Germany. - S. Temmerman, Dept of Biology, Univ. Antwerpen, Universiteitsplein 1, BE-2610 Wilrijk, Belgium.

Vegetation is traditionally regarded to reduce the erosion of channels in both fluvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, flat substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the flow, leading to flow concentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our findings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, flow, and landform have an important control on landscape evolution. Keywords: salt marsh, tidal creek, gully erosion, biogeomorphology.

Biologically mediated modifications of the abiotic environment, also called ecosystem engineering, can significantly affect a broad range of ecosystems. Nevertheless, remarkably little work has focused on the costs and benefits that ecosystem engineers obtain from traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing two autogenic engineers, which vary in the degree in which they affect their abiotic environment via their physical structure. That is, we compared two plant species from the intertidal coastal zone (Spartina anglica and Zostera noltii), whose shoots are exposed to similar currents and waves, but differ in the extent that they modify their environment via reduction of hydrodynamic energy. Our results indicate that there can be trade-offs related to the traits that underlies autogenic ecosystem engineering capacity. Dissipation of hydrodynamic forces from waves was roughly a factor of three higher in vegetation with stiff leaves compared to those with flexible leaves. Drag was highest and most sensitive to hydrodynamic forces in stiff vegetation that does not bend with the flow. Thus, shoot stiffness determines both the capacity to reduce hydrodynamic energy (i.e., proxy for ecosystem engineering capacity) and the drag that needs to be resisted (i.e., proxy for associated costs). Our study underlines the importance of insight in the trade-offs involved in ecosystem engineering as a first step toward understanding the adaptive nature of ecosystem engineering. Key words: autogenic ecosystem engineers: current velocity; drag; extended phenotype; niche construction; sediment accretion; Spartina anglica; trade-offs; waves; Zostera noltii.

Physiological adaptations for inhabiting transitional environments with strongly variable abiotic conditions can sometimes be displayed as behavioural shifts. A striking example might be found in bivalve species that inhabit estuaries characterised by fluctuations in environment. The opening and closing of their valves, so called gaping activity, represents behaviour that is required for two key physiological functions: food intake and respiration. Linking valve-gaping behaviour to environmental drivers can greatly improve our understanding and modelling of bivalve bioenergetics. Nowadays large data sets on gaping activity can be collected with automated sensors, but interpretation is difficult due to the large amount of environmental drivers and the intra-individual variability. This study aims to understand whether an unsupervised machine learning method (k-means clustering) can be used to identify patterns in gaping activity. Two commercially important congener mussels, Mytilus galloprovincialis and Mytilus edulis inhabiting two transitional coastal areas, the Venice Lagoon and the Wadden Sea, were fitted with sensors to monitor valve-gaping, while a comprehensive set of environmental parameters was also monitored. Data were analysed by applying three times a k-mean algorithm to the gaping time series. In the 1st analyses, the algorithm was applied to the overall gaping time series, including daily variations. We identified at both sites three clusters that were characterised by different average daily gaping aperture. The algorithm was subsequently reapplied to relate daily means of gaping to environmental conditions, being temperatures, oxygen saturation and chlorophyll levels. This 2nd analyses revealed that mean gaping aperture was mainly linked to food availability. A 3rd follow-up analysis aimed at exploring daily patterns. This third analysis again revealed consistent patterns amongst the two sites, where two clusters emerged that showed different degrees of oscillatory behaviour. There was however no obvious relationship between this fine scale oscillatory behaviours and environmental variables, but in the Venice Lagoon there was a site effect. Overall, we show that clustering algorithms can disentangle behavioural patterns within complex series of big data. The latter offers new opportunities to improve site-specific bioenergetic bivalve models by rephrasing the clearance and respiration terms based on the mean gaping aperture, provided that further laboratory experimentations are conducted to extrapolate parameters linking aperture with energy inputs and outputs. • Bivalve gaping regulates two key physiological functions: food intake and respiration. • Two congener mussels were studied in two estuaries with unsupervised learning. • Similar patterns relating to average daily aperture were identified for both species. • Clustering of gaping and environment linked wider apertures with food availability. • Patterns of oscillatory gaping behaviour could also identified with clustering method. [ABSTRACT FROM AUTHOR]

Abstract: There is a growing recognition of the important role of scale-dependent feedback for biogeomorphological landscape formation, where organisms locally improve survival and growth but at the same time negatively affect organisms at larger distance. However, little is known on how scale-dependent bio-geomorphic feedback is influenced by organism traits in combination with abiotic forcing. This was studied by measuring in a flume, the flow patterns around patches of three contrasting marsh species (Spartina anglica, Puccinellia maritima and Salicornia procumbens), using the flow acceleration around vegetation patches and deceleration within vegetation patches as quantitative proxy for the negative and positive feedback to the vegetation performance. The importance of external forcing was assessed by comparing three realistic current velocities: 0.1, 0.2 and 0.3ms−1. Our results showed that the dense clonal growth of stiff Spartina anglica shoots caused strongest flow deviations, irrespective of the applied current velocity. In contrast, the more sparsely growing, shorter stiff shoots of Salicornia procumbens induced much less flow deviation, allowing more water to pass through and over the vegetation canopy. The dense but highly flexible shoots of Puccinellia maritima caused strong flow deviations at low velocities, which diminished at higher velocities due to bending of the vegetation. Overall, these hydrodynamic results demonstrate that plant species traits interact with environmental conditions in creating scale-dependent feedbacks explaining why the effects of vegetation on landscape formation in saltmarshes are species specific. [Copyright &y& Elsevier]

Abstract: The objectives of the present study were twofold: (1) to identify spatial sedimentation and erosion patterns developing within patches of epibenthic structures (i.e. physical structures that protrude from the sediments, originating either from animals or plants) as a consequence of biophysical interactions; and (2) to assess the relevance of hydrodynamic flume studies for the long-term sediment dynamics in the field. We addressed these objectives by using patches of well-defined artificial structures (bamboo canes) for which we could easily monitor the long-term sediment dynamics in the field, measure the hydrodynamic effects in detail in the flume, and simulate the field and flume set-up with a commercially available hydrodynamic model. Two-year monitoring in the field showed that sedimentation was much larger in the high-density patches than the low-density ones. Within the high-density patches, comparable spatial patterns emerged at different field sites: erosion at the front and the side of the patches, and sedimentation more down-stream within the patches. The low-density patches showed no such patterns, and were generally characterised by some small-scale erosion directly around individual bamboo canes. Sedimentation and erosion in the field was well explained by the patterns in bed shear stress that were derived from our flume measurements. The 3D hydrodynamic modelling facilitated up-scaling of the flume results to the field, but failed to simulate accurately the effects at the leading edge. We conclude that: (A) field observations on sedimentation revealed interesting spatial patterns, but could not elucidate underlying processes; (B) detailed hydrodynamic measurements in a flume can elucidate these underlying processes, provided that appropriate scaling is being used; (C) flume studies are by definition not able to capture all spatial scales that are relevant for estuarine landscape formation and will always cause some flow artefacts; (D) hydrodynamic modelling offers a valuable tool to upscale flume observations, even though present models are not yet capable of fully reproducing all detailed spatial patterns; and (E) spatial heterogeneity is very important when looking at small-scale patches. There is a need for more spatially explicit and scale-dependent knowledge on bio-physical interactions. [Copyright &y& Elsevier]

Seagrasses comprise 78 species and are rarely invasive. But the seagrass Halophila stipulacea , firstly recorded in the Caribbean in the year 2002, has spread quickly throughout the region. Previous works have described this species as invasive in the Caribbean, forming dense mats that exclude native seagrass species. During a reconnaissance field survey of Caribbean seagrass meadows at the islands of Bonaire and Sint Maarten in 2013, we observed that this species was only extremely dense at 5 out of 10 studied meadows. Compared to areas with sparse growth of H. stipulacea , these dense meadows showed consistently higher nutrient concentrations, as indicated by higher leaf tissue N contents of the seagrass Thalassia testudinum (dense when C:N < 22.5) and sediments (dense when %N > 11.3). Thus, the potential invasiveness of this non-native seagrass most likely depends on the environmental conditions, especially the nutrient concentrations. [ABSTRACT FROM AUTHOR]

Vegetation is increasingly recognized as an important control on flow and landform patterns in many landscape types. Field studies on the landscape-scale effect of vegetation in fluvial and tidal floodplains are relatively scarce while insights are especially based on flume and numerical models. Large-scale flow patterns and landforms were measured on a vegetated tidal marsh and unvegetated tidal flat, in particular dynamic changes in two-dimensional water surface slopes, flow velocities and directions on an extensive network of locations. It was found that during flooding and drainage of the vegetated tidal marsh, the flow was concentrated in and routed through bare tidal channels, whereas on the unvegetated tidal flat more homogeneous sheet flow occurred. On the marsh the peak flow velocities were lower on the vegetated platform (< 0.1 m/s) compared to their adjacent tidal channels (0.3− 1 m/s), while on the bare tidal flat the channel and platform flow velocities were not significantly different from each other (0.1 − 0.4 m/s). These differences in flow patterns have important implications for the landform differences between vegetated and bare tidal floodplains: (1) for tidal channels with channel widths smaller than 10 m, width-to-depth ratios (= β) were smaller for the tidal marsh channels (β ~ 3) than for the tidal flat channels (β ~ 9) (due to flow concentration towards the channels in the marsh); and (2) the vegetated marsh platform exhibits a clear levee-basin topography (due to vegetation-induced platform flooding from the channels), while there is no levee-basin topography on the bare flat (due to sheet flow). This study not only emphasizes previous findings on the direct effect of vegetation, acting as frictional constraints on flow in tidal marshes, it also suggests that the indirect effect of vegetation, namely its induced micro-topography, influences the flooding and drainage behavior of marsh systems potentially altering feedbacks caused by direct vegetation effects. [ABSTRACT FROM AUTHOR]

Tidal-scale biophysical interactions establish particular flow routing and sedimentation patterns in coastal mangroves. Sluggish water flows through the mangrove vegetation and enhanced sediment deposition are essential to maintain these valuable ecosystems, thereby enabling their contribution to coastal protection and stabilization. Spatially explicit field observations of tidal-scale flow routing and sediment deposition were obtained in an elevated mangrove stand dissected by tidal creeks, located in the Trang river estuary at the Thai Andaman coast. An accurate and efficient depth-averaged process-based numerical model of this field site was developed in Delft3D to study the contributions of various biogeophysical mangrove settings to the observed tidal dynamics and to study the impacts of changes of these environmental conditions. The creeks are found to form the major pathway for the tidal inflow during the lower tides, while the sheltered interior of the forest is an effective sediment sink during the higher tides. A numerical sensitivity analysis of the initial response—or adaptive capacity—of the studied mangrove system to instantaneous environmental changes reveals the stable state of the study site: deposition rates are largely imposed by the topography and relative elevation, while they are rather independent of the vegetation density. Deeper inundations of the mangroves favor sheet flows through the forest and spatially averaged deposition rates decrease, particularly when this coincides with decreasing vegetation densities. Moreover, the sediment trapping efficiency is found to reduce significantly with diminishing sediment inputs and with mangrove area losses. These results clearly indicate the sensitivity of mangroves' ecosystem engineering ability—in terms of sedimentation—to climate change and anthropogenic threats. [ABSTRACT FROM AUTHOR]

Abstract: Seagrasses have declined at a global scale due to light reduction and toxicity events, caused by eutrophication and increased sediment loading. Although several studies have tested effects of light reduction and toxicants on seagrasses, there is at present no information available on their interacting effects. In a full-factorial 5-day laboratory experiment, we studied short-term interactive effects of light conditions, pH and reduced nitrogen (NH x ) in the water layer, mimicking pulses of river discharge, on the tropical early successional species Halodule uninervis and the late successional species Thalassia hemprichii. In contrast to recent results reported for the temperate species Zostera marina, increased NH x supply did not affect leaf mortality or photochemical efficiency in H. uninervis and in 7 out of 8 treatments for T. hemprichii. However, both tropical species demonstrated striking differences in nitrogen accumulation, free amino acid composition and free NH3 accumulation. The increase in tissue nitrogen content was two times higher for H. uninervis than for T. hemprichii. Nitrogen stored as free amino acids (especially asparagine) only increased in H. uninervis. High pH only affected T. hemprichii, but only when not shaded, by doubling its free NH3 concentrations, concomitantly decreasing its photosynthetic efficiency. Our results indicate that the early successional H. uninervis has higher tolerance to high NH x loads as compared to the late successional T. hemprichii. H. uninervis was better able to avoid toxic internal NH x levels by further assimilating glutamine into asparagine in contrast to T. hemprichii. Moreover, both tropical species seem to cope much better with high NH x than the temperate Z. marina. The implications for the distribution and succession of seagrass species under high nutrient loads are discussed. [Copyright &y& Elsevier]

Abstract: The coastal environment shows a wide range of bed patterns, for which sandwaves and sandbanks are among the most common. Less known in this context is the high benthic diversity in the coastal environment, which gives rise to the question to what extend the benthos interacts with the shape of the seabed. This paper reviews field and flume experiments on bio-geomorphological influences between benthos and sediment and tests the hypothesis that both the occurrence and the dimensions of sandwaves are dependent on the benthic diversity in the North Sea. Mathematical inclusions to account for biological activity in idealized models reveal that biota is able to influence the wavelength of sandwaves significantly, compared to the default case. More importantly, the models indicate that biota is able to induce bed patterns under conditions when the physical parameters suggest a stable flat bed and vice versa. Present model explorations indicate that future research should focus on the parameterization of subtidal biological activity on sediment dynamics and thereby on seabed patterns. Such knowledge will enable process-based modeling of the spatial and temporal variation in biological activity on seabed morphodynamics and validate the proposed modeling approach with field measurements. [Copyright &y& Elsevier]

Abstract Many organisms rely on chemical signals and cues to determine habitat suitability and safety. Chemical signals can mediate many interactions, including those between predators and their prey. Altering prey behaviour, these non-consumptive effects (NCEs) can influence population and community dynamics. Understanding how NCEs influence early life history stages, such as 'decisions' of benthic species with planktonic larvae about where to settle, can provide useful information on the ecological functioning of these systems as well as the management for commercial usage, although most studies have so far focused on intertidal systems which are already subject to a set of stressful conditions. With a shallow subtidal field experiment we investigated NCEs of the common starfish Asterias rubens on one of its main preys, the blue mussels Mytilus edulis. We tested the hypotheses that (1) the presence of starfish reduces mussels settlement and that (2) the mussels that settle will invest more energy towards induced defences than to growth, and will thus remain smaller than mussels settling in an area without starfish. Two independent trials revealed a significant reduction of mussel spat on the collectors in the presence of starfish after a two-week deployment period. There was however no effect of starfish on the size distribution of the mussel spat. The delayed observation of effects of starfish, absent after the first week but evident afterwards, suggests a time dependency of NCE's on spat settlement. Harnessing this ecologically important information has the potential to increase yield of mussel seeds available for fisheries by either removing starfish from the ground-based settling areas at the onset and for the duration of spatfall or by using floating substrates that are away from the bottom-bound starfish. Moreover, these results also underlines the potential of using predator cues in the application for sustainable natural antifouling compounds in situations with low recruitment pressures. Highlights • Most studies of predator cues on recruitment focus on single time points. • We used a well-studied predator-prey system to identify indirect effects. • Recruitment of mussels was significantly reduced by starfish cues. • This reduction was magnified over time. [ABSTRACT FROM AUTHOR]

The Berau Continental Shelf (BCS) in East Kalimantan, Indonesia, harbours various tropical marine ecosystems, including mangroves, seagrass meadows and coral reefs. These ecosystem are located partly within reach of the Berau River plume, which may affect ecosystem health through exposure to land-derived sediments, nutrients and pollutants carried by the plume. This study aims (1) to assess the exposure risk of the BCS coastal ecosystems to river plume water, measured as exposure time to three different salinity levels, (2) to identify the relationships between these salinity levels and the abundance and diversity of coral and seagrass ecosystems, and (3) to determine a suitable indicator for the impacts of salinity on coral reef and seagrass health. We analysed hydrodynamic models, classified salinity levels, and quantified the correlations between the salinity model parameters and ecological metrics for the BCS systems. An Empirical Orthogonal Functions (EOF) analysis revealed three modes of river plume dispersal patterns, which strongly reflect monsoon seasonality. The first mode, explaining 39% of the variability, was associated with the southward movement of the plume due to northerly winds, while the second and third modes (explaining 29% and 26% of the variability, respectively) were associated with the northeastward migration of the plume related to southwesterly and southerly winds. Exposure to low salinity showed higher correlations with biological indicators than mean salinity, indicating that low salinity is a more suitable indicator for coastal ecosystem health. Significant correlations ( R 2 ) were found between exposure time to low salinity (days with salinity values below 25 PSU) with coral cover, coral species richness, seagrass cover, the number of seagrass species, seagrass leaf phosphorus, nitrogen, C:N ratio and iron content. By comparing the correlation coefficients and the slopes of the regression lines, our study suggests that coral reefs are more susceptible to low salinity levels exposure than seagrass meadows. Regarding the risk of corals being exposed to low salinity, nearshore and northern barrier reefs were classified as “high risk”, the middle barrier reef as “medium to high risk” and southern barrier reefs as “medium risk”. Further offshore, the oceanic reefs were classified as “low risk”. Regarding the seagrass meadows, the nearshore region was categorized as “high risk”, the barrier reef as “medium to low risk” and oceanic reefs as “low risk”. This study contributes to assessing the potential impacts of salinity on the BCS ecosystems, and further provides a knowledge base for marine conservation planning. [ABSTRACT FROM AUTHOR]

Vegetation-wave interactions are critical in determining the capacity of coastal salt marshes to reduce wave energy (wave dissipation), enhance sedimentation and protect the shoreline from erosion. While vegetation-induced wave dissipation is increasingly recognized in low wave energy environments, little is known about: (i) the effect of vegetation on wave dissipation during storms when wave heights and water levels are highest; and (ii) the ability of different plant species to dissipate waves and to maintain their integrity under storm surge conditions. Experiments undertaken in one of the world’s largest wave flumes allowed, for the first time, the study of vegetation-wave interactions at near-field scale, under wave heights ranging from 0.1–0.9 m (corresponding to orbital velocities of 2–91 cm s −1 ) and water depths up to 2 m, in canopies of two typical NW European salt marsh grasses: Puccinellia maritima ( Puccinellia ) and Elymus athericus ( Elymus ). Results indicate that plant flexibility and height, as well as wave conditions and water depth, play an important role in determining how salt marsh vegetation interacts with waves. Under medium conditions (orbital velocity 42–63 cm s −1 ), the effect of Puccinellia and Elymus on wave orbital velocities varied with water depth and wave period. Under high water levels (2 m) and long wave periods (4.1 s), within the flexible, low-growing Puccinellia canopy orbital velocity was reduced by 35% while in the more rigid, tall Elymus canopy deflection and folding of stems occurred and no significant effect on orbital velocity was found. Under low water levels (1 m) and short wave periods (2.9 s) by contrast, Elymus reduced near-bed velocity more than Puccinellia . Under high orbital velocities (≥74 cm s −1 ), flattening of the canopy and an increase of orbital velocity was observed for both Puccinellia and Elymus . Stem folding and breakage in Elymus at a threshold orbital velocity ≥ 42 cm s −1 coincided with a levelling-off in the marsh wave dissipation capacity, while Puccinellia survived even extreme wave forces without physical damage. These findings suggest a species-specific control of wave dissipation by salt marshes which can potentially inform predictions of the wave dissipation capacity of marshes and their resilience to storm surge conditions. [ABSTRACT FROM AUTHOR]

An important ecosystem service of tropical coastal vegetation including seagrass beds and mangrove forests is their ability to accumulate carbon. Here we attempt to establish the driving forces for the accumulation of surface organic carbon in southern Thailand coastal systems. Across 12 sites we found that in line with expectations, seagrass beds (0.6 ± 0.09%) and mangrove forests (0.9 ± 0.3%) had higher organic carbon in the surface (top 5 cm) sediment than un-vegetated mudflats (0.4 ± 0.04%). Unexpectedly, however, mangrove forests in this region retained organic carbon, rather than outwell it, under normal tidal conditions. No relationship was found between organic carbon and substrate grain size. The most interesting finding of our study was that climax and pioneer seagrass species retained more carbon than mixed-species meadows, suggesting that plant morphology and meadow characteristics can be important factors in organic carbon accumulation. Insights such as these are important in developing carbon management strategies involving coastal ecosystems such as offsetting of carbon emissions. The ability of tropical coastal vegetation to sequester carbon is an important aspect for valuing the ecosystems. Our results provide some initial insight into the factors affecting carbon sequestration in these ecosystems, but also highlight the need for further research on a global scale. [ABSTRACT FROM AUTHOR]

While attention in coastal ecosystem restoration has increased over the last two decades, the success rate of efforts remains relatively low. To increase success rates, physical restoration techniques often utilize supporting or protective materials to provide a stable surface for transplantation, and in some cases reduce herbivory and hydrodynamic disturbances. In this study, we evaluated the effectiveness of traditional (staples, burlap) and novel (BESE- elements, a biodegradable potato starch lattice) physical restoration techniques on the growth of transplanted Halodule wrightii seagrass. A first experiment revealed that seagrass planted in both two-stacked BESE structure without planting holes and four-stacked BESE with holes had significantly higher shoot count and blade length than four-stacked BESE without holes, with the latter design losing all seagrass shortly after deployment as shoots could not float through the structure. In a second experiment, the BESE lattice treatment (four-stacked with holes) had three times the shoot count and equal to greater blade length compared to traditional methods of physical restoration (staples and burlap), likely due to BESE providing some protection from hydrodynamic activity. However, disturbances, possibly including herbivory and hydrodynamic activity (culminating with Hurricane Irma), prevented long term study, illustrating the importance of stochastic abiotic factors in seagrass planting success. Overall our study demonstrates the effectiveness of using BESE lattice designs and similar physical techniques in the restoration of seagrass beds. • Seagrass restoration is expensive and often unsuccessful. • Traditional and novel physical restoration techniques were compared. • Biodegradable starch (BESE) lattices yielded better growth. • BESE may improve restoration in hydrodynamically active environments. [ABSTRACT FROM AUTHOR]

Abstract: Sediment dynamics have an important influence on the morphological evolution of tidal wetlands, which consist of mudflats and salt marshes. To understand the nature of sediment behavior under combined current–wave action at an exposed tidal wetland, we measured the waves, currents, water depths, bed-level changes, and sediment properties at a mudflat–salt marsh transition on the Yangtze Delta, China, during five consecutive tides under onshore winds of ~8m/s, and calculated the bed shear stresses due to currents (τ c ), waves (τ w ), combined current–wave action (τ cw ), and the critical shear stress for erosion of the bottom sediment (τ ce ). The bed shear stresses under combined current–wave action (τ cw ) were approximately five times higher on the mudflat (up to 1.11N/m2; average 0.27N/m2) than on the salt marsh (up to 0.14N/m2; average, 0.06N/m2). On the mudflat, τ cw was larger than the critical erosion shear stress (τ ce =0.103N/m2) for 70% of the period of submergence, whereas τ cw was always lower than τ ce at the salt marsh site (τ ce =0.116N/m2). This result indicates that the sediment dynamics on the mudflat were dominated by erosion, whereas at the salt marsh they were governed by deposition, which is in agreement with the observed bed-level change during the study period (−3.3mm/tide on the mudflat and 3.0mm/tide on the salt marsh). A comparison of τ cw values calculated using the models for bed shear stresses under combined current–wave action indicates that both models are applicable to the present case and effectively predict the bottom shear stress under combined current–wave action. Overall, we conclude that τ cw in combination with τ ce is useful in assessing the hydrodynamic mechanisms that underlie the morphological evolution of exposed tidal wetlands. [Copyright &y& Elsevier]

Abstract: Although several recent studies point at the importance of seagrass roots for nutrient acquisition in oligotrophic tropical ecosystems, remarkably little is known about the root architecture of tropical seagrasses. The present study provides a detailed description of the root architecture of six seagrass species that were extracted from three sites differing in sediment type and nutrient availability (i.e., except for one species that was not present at one of the sites). Number of roots per node, order of root branching, length and diameter per root order, root hair density and length were determined and used to calculate a topological index for the different species and habitats. Root architecture differed strongly between species. The relatively long-lived and slowly-growing species Thalassia hemprichii and Enhalus acoroides were characterised by short internodes with relatively few unbranched roots per node and a high root hair density. More fast growing species such as Cymodocea rotundata, Cymodocea serrulata and Syringodium isoetifolium had many roots per node, the majority of which were branched. Surprisingly, differences in sediment type and nutrient availability, as present between locations, had little effect on root architecture. We expect this to be due to a relatively homogeneous nutrient distribution at the small scale within water saturated sediments. Overall, all seagrass species had relatively simple branching (topological index > 0.7), comparable to angiosperms of the low temperate salt marsh. We speculate that relatively simple root architecture of plants in flooded systems reflects the need for a minimal path length for oxygen transport from shoots to roots. [Copyright &y& Elsevier]

Erosion of tidal flats is to a large degree determined by the erosion threshold of their cohesive sediments, i.e., the critical bottom shear stress identifying the onset of erosion. Given that the erodibility of tidal flats can vary strongly over both space and time, rapid in situ measuring methods for quantifying the critical bottom shear stress are needed. As the largest changes in tidal flat elevation are generally observed during storms when shear stress is wave dominated, we developed a mobile Oscillatory-Channel Resuspension flume, OsCaR, as a rapid method to measure the critical wave-generated shear stress on sediment cores directly taken from the field. We assessed the performance of the OsCaR-flume by conducting erodibility measurements on a range of artificial sand – mud mixtures, and along a natural longshore estuarine sediment type and benthic assemblage gradient. Measured erodibility of the artificial sediments matched well with sediment transport theory, indicating that the OsCaR-flume is able to reproduce wave-generated shear stress accurately. Moreover, the patterns observed in the influence of total benthic metabolic rate divided over functional groups on sediment erodibility show that the OsCaR-flume is able to capture the main benthic processes, as they corresponded well to previous laboratory based studies. • A new method for measuring natural sediment erodibility under waves is introduced. • Sediment resuspension rates can be used to quantify critical bed shear stress. • Benthic metabolic rate may explain erodibility patterns in a field context. [ABSTRACT FROM AUTHOR]

Transitional coastal zones are subject to high degrees of temporal fluctuation in environmental conditions, with these patterns varying in space. Gaining an in depth understanding of how sessile organisms cope with and respond to such environmental changes at multiple scales is needed to i) advance fundamental knowledge, ii) predict how organisms may react to stressors and iii) support the management of halieutic resources in transitional coastal areas. We addressed this question using mussels (Mytilus galloprovincialis) as model system. Valve-gaping sensor were deployed at multiple sites within the southern Venice Lagoon over a period of 6 months, to investigate the existence of periodicity in valve-gaping and its relationship with environmental variables, such as temperature and chlorophyll-a. Gaping behaviour was found to have periodic rhythms, of ~12 h and ~ 24 h, which were most pronounced in the inner part of lagoon part and were strongest during summer months. In autumn, the dual periodicity became weaker and mostly the 12 h remained. Gaping was closely linked with tide, but the relationship in terms of phasing varied upon location. Surprisingly, no clear direct relationships were found with chlorophyll-a, but food delivery may be mediated by tide itself. The results highlight the heterogeneity of behaviour and the endogenic nature of circadian rhythms in space and time. These findings have important implications for management of transitional areas where tidal alteration may have impacts on key behaviours, and emphasize the importance of characterizing their rhythms before using these as stress indicator. Moreover, the described tidal relationships should be included in growth models of bivalves in these systems. Unlabelled Image • It is necessary to understand how organisms respond to environmental changes. • Transitional coastal areas are great model systems. • Mussels behaviour was monitored long-term and in continuous in three sites. • It followed the tidal rhythm particularly in more internal site and in summer. • Responses to small scale changes are important for management and for predictions. [ABSTRACT FROM AUTHOR]

Shellfish aquaculture is considered a sustainable way to help meet rising protein demands worldwide. In shallow coastal dynamic ecosystems mussels can be cultivated directly on the seabed, however this method has a low return as mussels exposed to natural environments risk dislodgment, high predation rates, sedimentation and competition. The formation of spatial patterns in natural mussel beds, that result in 'organized patchiness', is thought to be an adaptive mechanism to reduce population losses. The driver and effects of this patterning need to be disentangled at multiple spatial scales in which patterns are observed. With a field experiment we aimed to understand how small-scale density (actual cover) and patterning (perimeter: area ratio of clumps and number of mussel layers) can be altered by manipulating large scale density (re-laying biomass), that farmers could control during seeding activity. Within this study we considered the interplay between environmental conditions (manipulating flow rate with the use of large mesh cages) and density for pattern development and persistence, and the repercussions of this on mussel productivity (growth and condition). We further investigated local scale processes, such as the role of within-clump biological activity (biodeposition), that may be a predictor for the larger scale observations of losses and persistence relative to density. We found that manipulating density by controlling seeding biomass from boats is not an accurate predictor of actual seabed density and resulting patterning. The growth and condition of the mussels was only influenced by local scale effects, resulting in high 'within clumps' variation. Aiming for an intermediate density to avoid both excessive fragmentation and excessive layering may be viewed as an optimal strategy to maximise returns, but we encourage the incorporation of the hierarchy of multiple scales of density in future studies of patterning that will allow the inclusion of these effects in a model of growth and productivity. • Separate environment and biomass effects on Mytilus on-bottom production. • Seeding biomass is not a predictor of actual seabed densities and patterning. • Mussels health parameters showed high 'within clumps' variation. • Mussel layering should be minimised in calm environments. • Mussel fragmentation should be minimised in highly dynamic habitats. [ABSTRACT FROM AUTHOR]

Shallow tropical bays in the Caribbean, like Orient Bay and Galion Bay in Saint Martin, are often sheltered by coral reefs. In the relatively calm environment behind the reefs, seagrass meadows grow. Together, these ecosystems provide valuable ecosystem services like coastal protection, biodiversity hotspots, nursery grounds for animals and enhancing tourism and fisheries. However, sea-level rise imperils these ecosystems and the services they provide because of changing hydrodynamic conditions, with potential effects on the interdependencies between these ecosystems. By means of a hydrodynamic model that accounts for the interaction with vegetation (Delft3D Flexible Mesh), the impact of sea-level rise (0.87 m in 2100) is investigated for three scenarios of future reef development (i.e. keep-up, give-up and catch-up). If coral reefs cannot keep up with sea-level rise, the wave height and flow velocity increase significantly within associated bays, with the wave height doubling locally in case of eroding reefs in our model simulations. Since the presence of seagrass strongly depends on the hydrodynamic conditions, the response of seagrass to the future hydrodynamic conditions is projected using a habitat suitability model that is based on a logistic regression. The spatial character of the bays determines the response of seagrass. In Orient Bay, which is deeper and partly exposed to higher waves, the seagrass will likely migrate from the deeper parts to shallow areas that become suitable for seagrass because of the surf zone moving landward. In contrast, the conditions for seagrass worsen in Galion Bay for the catch-up and give-up scenario; due to the shallowness of this bay, the seagrass cannot escape to more suitable areas, resulting in significant seagrass loss. It is shown that healthy coastal ecosystems are able to limit the change in hydrodynamic conditions due to sea-level rise. Therefore, preserving these ecosystems is key for ensuring the resilience of shallow tropical bays to sea-level rise and maintaining their ecosystem services. • Sea-level rise intensifies hydrodynamic conditions in shallow tropical bays. • Wave dissipation by coral reefs controls the impact of sea-level rise. • The spatial character of the bay determines the response of seagrass. • Tropical marine ecosystems can mitigate the impact of sea-level rise. [ABSTRACT FROM AUTHOR]