Seagrass Canopy Research Articles

Small-scale variability in geomorphological settings influences mangrove-derived organic matter export in a tropical bay

Abstract. Organic matter (OM) exchanges between adjacent habitats affect the dynamics and functioning of coastal systems, as well as the role of the different primary producers as energy and nutrient sources in food webs. Elemental (C, N, C : N) and isotope (δ13C) signatures and fatty acid (FA) profiles were used to assess the influence of geomorphological setting in two climatic seasons on the export and fate of mangrove OM across a tidally influenced tropical area, Gazi Bay (Kenya). The main results indicate that tidal transport, along with riverine runoff, plays a significant role in the distribution of mangrove organic matter. In particular, a marked spatial variability in the export of organic matter from mangroves to adjacent habitats was due to the different settings of the creeks flowing into the bay. Kinondo Creek acted as a mangrove retention site, where export of mangrove material was limited to the contiguous intertidal area, while Kidogoweni Creek acted as a flow-through system, from which mangrove material spreads into the bay, especially in the rainy season. This pattern was evident from the isotopic signature of primary producers, which were more 13C-depleted in the Kinondo Creek and nearby, due to the lower dilution of the dissolved inorganic carbon (DIC) pool, typically depleted as an effect of intense mangrove mineralisation. Despite the trapping efficiency of the seagrass canopy, suspended particulate OM showed the important contribution of mangroves across the whole bay, up to the coral reef, as an effect of the strong ebb tide. Overall, mixing model outcomes indicated a widespread mixed contribution of both allochthonous and autochthonous OM sources across Gazi Bay. Moreover, FAs indicated a notable contribution of brown macroalgae and bacteria in both sediment and suspended pools. These results suggest that ecological connectivity in Gazi Bay is strongly influenced by geomorphological setting, which may have far-reaching consequences for the functioning of the whole ecosystem and the local food webs.

Effect of habitat fragmentation on abundance and diversity of seagrass-associated crustacean assemblage at Hurghada, Red Sea

Seagrass beds in Red Sea are increasingly subject to both natural and anthropogenic disturbance and fragmentation, resulting in an increase in the edge habitats. Consequently, the present study was designed to study the effect of habitat fragmentation of seagrass on the diversity and abundance of epiphytic crustaceans inhabiting seagrass canopies. For this purpose, two sites, containing different seagrass patch sizes, were selected for seasonal sampling during the period from mid-April 2015 to mid-January 2016 around Hurghada, Red Sea. Evaluations of the differences in crustacean assemblage structure, abundance and diversity between fragmented and continuous seagrass beds as well as the edge and center of the same seagrass patches were figured out. Overall, 33 crustacean species belong to 30 genera, 25 families, and 7 orders were recorded, from all different seagrass patches with a comparable temporal and spatial distribution; whereas all recorded crustacean species belong to class Malacostraca. Results indicated that large seagrass patches harbor higher crustacean’s densities than medium and small seagrass patches, which promote the assumption of the negative effect of seagrass fragmentation. The abundance of crustacean groups are also markedly increase in the large patches except decapods in one site. Reduction of habitat size led to reducing the number of crustacean species in both sites. However, Evenness values (J') showed that habitat fragmentation had no effect on diversity. THE Total crustaceans abundance is relatively increase toward the patch edge with a percentage of increase up to 18 %. However, crustacean species richness was insensitive to differences in patch edge-center microhabitat. Relation between seagrass shoot density/biomass and the total crustacean abundance also figured out in this study.

The exchange of dissolved nutrients between the water column and substrate pore-water due to hydrodynamic adjustment at seagrass meadow edges: A flume study

Seagrasses need dissolved nutrients to maintain their productivity through uptake processes, from substrate pore-water via their roots and/or from the water column via their leaves. Here, we present the first study of exchanges of dissolved nutrients between pore water and the water column in the vicinity of seagrass canopies. We address the following research questions, using a laboratory flume experiment: (1) How does solute exchange between the water column and substrate pore water vary spatially within seagrass patches? (2) How does seagrass leaf length affect this solute exchange? (3) How does the measured rate of solute exchange compare with seagrasses' rates of uptake of dissolved nutrients? Our results indicate that solute intrusion from the water column into the substrate pore water is highest in the area around seagrass patches' leading edges, where flow deceleration is strongest, and decreases approximately linearly with downstream distance into the patch. The decrease in measured flow speed in the canopy fits well the predictions of previously reported models of arrays of rigid obstacles. The length of the region in which the concentration of solute that has infiltrated into the substrate at the upstream end of the seagrass patches is similar to the length scale predicted from model estimates of infiltration rate (based on the substrate permeability) and the length of time over 24-h runs. We conclude that the mechanism we identify only pertains near canopy edges, and therefore that other mechanisms must govern nutrient supply in the interior of seagrass meadows.

Interactions between Fragmented Seagrass Canopies and the Local Hydrodynamics.

The systematic creation of gaps within canopies results in fragmentation and the architecture of fragmented canopies differs substantially from non-fragmented canopies. Canopy fragmentation leads to spatial heterogeneity in hydrodynamics and therefore heterogeneity in the sheltering of canopy communities. Identifying the level of instability due to canopy fragmentation is important for canopies in coastal areas impacted by human activities and indeed, climate change. The gap orientation relative to the wave direction is expected to play an important role in determining wave attenuation and sheltering. Initially we investigated the effect of a single transversal gap within a canopy (i.e. a gap oriented perpendicular to the wave direction) on hydrodynamics, which was compared to fully vegetated canopies (i.e. no gaps) and also to bare sediment. The wave velocity increased with gap width for the two canopy densities studied (2.5% and 10% solid plant fraction) reaching wave velocities found over bare sediments. The turbulent kinetic energy (TKE) within the gap also increased, but was more attenuated by the adjacent vegetation than the wave velocity. As expected, denser canopies produced a greater attenuation of both the wave velocity and the turbulent kinetic energy within an adjacent gap, compared to sparse canopies. Using non-dimensional analysis and our experimental data, a parameterization for predicting TKE in a canopy gap was formulated, as a function of easily measured variables. Based on the experimental results, a fragmented canopy model was then developed to determine the overall mixing level in such canopies. The model revealed that canopies with large gaps present more mixing than canopies with small gaps despite having the same total gap area in the canopy. Furthermore, for the same total gap area, dense fragmented canopies provide more shelter than sparse fragmented canopies.

Organic carbon in seagrass sediments is influenced by seagrass canopy complexity, turbidity, wave height, and water depth

AbstractSeagrass meadows are important marine carbon sinks, yet they are threatened and declining worldwide. Seagrass management and conservation requires adequate understanding of the physical and biological factors determining carbon content in seagrass sediments. Here, we identified key factors that influence carbon content in seagrass meadows across several environmental gradients in Moreton Bay, SE Queensland. Sampling was conducted in two regions: (1) Canopy Complexity, 98 sites on the Eastern Banks, where seagrass canopy structure and species composition varied while turbidity was consistently low; and (2) Turbidity Gradient, 11 locations across the entire bay, where turbidity varied among sampling locations. Sediment organic carbon content and seagrass structural complexity (shoot density, leaf area, and species specific characteristics) were measured from shallow sediment and seagrass biomass cores at each location, respectively. Environmental data were obtained from empirical measurements (water quality) and models (wave height). The key factors influencing carbon content in seagrass sediments were seagrass structural complexity, turbidity, water depth, and wave height. In the Canopy Complexity region, carbon content was higher for shallower sites and those with higher seagrass structural complexity. When turbidity varied along the Turbidity Gradient, carbon content was higher at sites with high turbidity. In both regions carbon content was consistently higher in sheltered areas with lower wave height. Seagrass canopy structure, water depth, turbidity, and hydrodynamic setting of seagrass meadows should therefore be considered in conservation and management strategies that aim to maximize sediment carbon content.

Comparison of bathymetry and seagrass mapping with hyperspectral imagery and airborne bathymetric lidar in a shallow estuarine environment

ABSTRACTWe compared hyperspectral imagery and single-wavelength airborne bathymetric light detection and ranging (lidar) for shallow water (<2 m) bathymetry and seagrass mapping. Both the bathymetric results from hyperspectral imagery and airborne bathymetric lidar reveal that the presence of a strongly reflecting benthic layer under seagrass affects the elevation estimates towards the bottom depth instead of the top of seagrass canopy. Full waveform lidar was also investigated for bathymetry and similar performance to discrete lidar was observed. A provisional classification was performed with limited ground reference samples and four supervised classifiers were applied in the study to investigate the capability of airborne bathymetric lidar and hyperspectral imagery to identify seagrass genera. The overall classification accuracy is highly variable and strongly dependent on the classification strategy used. Features from bathymetric lidar alone are not sufficient for substrate classification, while hyperspectral imagery alone showed significant capability for substrate classification with over 95% overall accuracy. The fusion of hyperspectral imagery and bathymetric lidar only marginally improved the overall accuracy of seagrass classification.

Population structure of the pearly razorfish, &lt;em&gt;Xyrichtys novacula&lt;/em&gt; (Actinopterygii: Labridae), in sand-seagrass mosaics: spatial variation according to habitat features and sampling techniques

Habitat structure affects the distribution of fishes, particularly across reef-dominated habitats, but few studies have connected patterns in the abundance of soft-bottom fishes with the structure of the habitat. The spatial and temporal patterns of variation in the abundance, biomass and population structure of the pearly razorfish, Xyrichtys novacula, inhabiting sand-Cymodocea nodosa seagrass mosaics were described through two complementary techniques: underwater visual counts and seine nets. We sought to analyse whether biotic (seagrass shoot density, leaf length and meadow cover) and abiotic (sediment composition and particle size) structural elements explained variation in patterns of abundance and biomass. Underwater visual counts registered a larger abundance of individuals and proved significant variation in fish abundance and biomass at the scale of locations, which was otherwise not detected through seine nets. Seasonal variation in fish abundance and biomass was, in all cases, minor. Habitat structural elements helped to explain patterns in fish abundance and biomass. This fish species was particularly abundant in sediments dominated by coarse sands in continuous meadows of C. nodosa ( > 90% seagrass cover) with intermediate densities of 500 to 1000 shoots m–2, followed by large-sized seagrass patches with >1000 shoots m–2. A trade-off between protection provided by seagrass canopies and protection derived from its burial behaviour, limited under high seagrass shoot densities, may explain spatial variation patterns.

Sub-tropical seagrass ecosystem metabolism measured by eddy covariance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 529:75-90 (2015) - DOI: https://doi.org/10.3354/meps11314 Sub-tropical seagrass ecosystem metabolism measured by eddy covariance Matthew H. Long1,2,*, Peter Berg1, Karen J. McGlathery1, Joseph C. Zieman1 1Department of Environmental Sciences, University of Virginia, 291 McCormick Rd, Charlottesville, Virginia 22904, USA 2Present address: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA *Corresponding author: mlong@whoi.edu ABSTRACT: The metabolism of seagrass ecosystems was examined at 4 sites in south Florida, USA, using the eddy covariance technique under in situ conditions. Three sites were located across a phosphorus-driven productivity gradient to examine the combined effects of dynamic variables (irradiance, flow velocity) and state variables (sediment phosphorus and organic content, seagrass biomass) on ecosystem metabolism and trophic status. Gross primary production and respiration rates varied significantly across Florida Bay in the summer of 2012 with the lowest rates (64 and –53 mmol O2 m–2 d–1, respectively) in low-phosphorus sediments in the northeast and the highest (287 and –212 mmol O2 m–2 d–1, respectively) in the southwest where sediment phosphorus, organic matter, and seagrass biomass are higher. Seagrass ecosystems offshore of the Florida Keys had similar large daily production and respiration rates (397 and –17 mmol O2 m–2 d–1, respectively) and were influenced by flow through the permeable offshore sediments. Across all sites, net ecosystem metabolism rates indicated that the seagrass ecosystems were autotrophic in the summertime. Substantial day-to-day variability in metabolic rates was found due to variations in irradiance and flow velocity. At all sites the relationship between photosynthesis and irradiance was linear and did not show any sign of saturation over the entire irradiance range (up to 1400 µmol photons m–2 s–1). This was likely due to the efficient use of light by the large photosynthetic surface area of the seagrass canopy, an effect which can only be examined by in situ measurements that integrate across all autotrophs in the seagrass ecosystem. KEY WORDS: Seagrass · Eddy covariance · Metabolism · Carbon cycling Full text in pdf format PreviousNextCite this article as: Long MH, Berg P, McGlathery KJ, Zieman JC (2015) Sub-tropical seagrass ecosystem metabolism measured by eddy covariance. Mar Ecol Prog Ser 529:75-90. https://doi.org/10.3354/meps11314 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 529. Online publication date: June 08, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Trophic implications and faunal resilience following one-off and successive disturbances to an Amphibolis griffithii seagrass system

Disturbances in seagrass systems often lead to considerable loss of seagrass fauna. We examined the capacity for seagrass fauna, across multiple trophic levels, to recover from disturbances, using empirical and modelling techniques. Model outputs, using Ecosim with Ecopath (EwE), were consistent with the results of field investigations, highlighting the models robustness. Modelled outcomes suggest second and third order consumers are likely to be negatively effected by disturbances in the seagrass canopy. Particularly piscivores, which once disturbed, appear unlikely to recover following severe declines in primary productivity. EwE also revealed the complex interaction between the duration and intensity of disturbances on seagrass fauna, which may differentially affect higher order consumers. Further, modelling predicted a variable capacity of higher order consumers to recover from successive disturbances, suggesting taxa with comparatively fast reproductive cycles and short generation terms would be more resilient than taxa with comparatively long generation terms and slow reproductive cycles.

Let it flow: how does an underlying current affect wave propagation over a natural seagrass meadow?

Temperate seagrass beds can be found within intertidal and tidal areas of variable hydrodynamic forcing. To investigate the interaction between hydrodynamics and seagrass plants, Zostera noltii meadows were exposed to a range of combinations of waves and flow in a flume. Velocity profiles were obtained to observe the evolution of hydrodynamic forcing due to the presence of a seagrass canopy. Observations under flow without waves agreed with those of previous studies and showed that turbulent kinetic energy (TKE) at the top of the canopy increased with increasing flow velocity as well as with distance from the leading edge. The highest TKE values were achieved for the highest flow velocity without waves, and the presence of waves generally led to reduced TKE values. Profiles of time-averaged downstream velocity were not affected by the presence of waves. However, flow reduction near the bed increased with higher seagrass density, resulting in an S-shaped profile for the highest density and flow velocity. Underlying currents affected wave orbital velocities, causing a reduction of orbital diameters in the free-stream region. In the canopy region, this process was superimposed by the damping effect of the seagrass meadow, but only positive flow components were reduced by seagrass presence. Streamlining and oscillating motion of seagrass leaves were observed under flow and waves, respectively. These processes did not affect the wave/flow attenuation capacity of the meadow, which indicates that the ecosystem-engineering traits of seagrasses are not compromised by increased flow velocity or in the presence of waves.

Consumer depletion alters seagrass resistance to an invasive macroalga.

Few field studies have investigated how changes at one trophic level can affect the invasibility of other trophic levels. We examined the hypothesis that the spread of an introduced alga in disturbed seagrass beds with degraded canopies depends on the depletion of large consumers. We mimicked the degradation of seagrass canopies by clipping shoot density and reducing leaf length, simulating natural and anthropogenic stressors such as fish overgrazing and water quality. Caulerpa racemosa was transplanted into each plot and large consumers were excluded from half of them using cages. Potential cage artifacts were assessed by measuring irradiance, scouring by leaf movement, water flow, and sedimentation. Algal invasion of the seagrass bed differed based on the size of consumers. The alga had higher cover and size under the cages, where the seagrass was characterized by reduced shoot density and canopy height. Furthermore, canopy height had a significant effect depending on canopy density. The alteration of seagrass canopies increased the spread of C. racemosa only when large consumers were absent. Our results suggest that protecting declining habitats and/or restoring fish populations will limit the expansion of C. racemosa. Because MPAs also enhance the abundance and size of fish consuming seagrass they can indirectly promote algal invasion. The effects of MPAs on invasive species are context dependent and require balancing opposing forces, such as the conservation of seagrass canopy structure and the protection of fish grazing the seagrass.

Resistance of Posidonia oceanica seagrass meadows to the spread of the introduced green alga Caulerpa cylindracea: assessment of the role of light

Posidonia oceanica seagrass meadows are one of most resistant Mediterranean habitats to invasion by the green alga Caulerpa cylindracea. We evaluated the hypothesis that light reduction caused by the seagrass canopy can limit algal photosynthesis and growth and hence potentially explain this resistance. To this end, we analysed light regimes and C. cylindracea biomass and photoacclimative variables measured outside and within P. oceanica meadows at different sites and during contrasting times. The success of photoacclimatory responses was assessed using an ecophysiological, carbon balance approach. C. cylindracea abundance significantly varied depending on the sampling site and time, but its biomass was always 10- to 50-fold higher outside the meadow. Outside the canopy, C. cylindracea showed characteristic morphological and photosynthetic plasticity closely related to the spatio-temporal variation in light regimes, which varied as expected with depth and season. Under these conditions, the alga was able to perform successful photoacclimation, although some degree of light limitation was observed at the deepest sites and in winter conditions, as indicated by near-zero carbon balance and lower algal abundances. Within the P. oceanica canopy, light was reduced by 60–89 % relative to that outside and was at its lowest levels recorded (1–7 % of the sub-surface irradiance), close to the minimum light requirements for growth. Light limitation was evident inside the canopy in the winter sampling, when the photosynthetic plasticity of the alga appears to be exceeded and when carbon balances were clearly negative. Therefore, light appears to play a key role in the apparent incapacity of C. cylindracea to penetrate within P. oceanica meadow edges.

Leaf and canopy scale characterization of the photoprotective response to high-light stress of the seagrassThalassia testudinum

The survival of tropical seagrasses in shallow and highly illuminated reef environments relies on the leaf photoacclimatory high-light response. In this study, we characterized this response for the tropical seagrass Thalassia testudinum and analysed the role of seagrass canopy in the regulation of leaf photoacclimation. We determined maximum capacity for non-photochemical quenching (NPQmax); xanthophyll cycle pool size and its de-epoxidation state; and the support of photosystem II (PSII) repair to maintain PSII photochemical efficiency, and compared their variation along the leaf against changes in leaf pigmentation, absorptance, and PSII maximum photochemical efficiency (Fv : Fm). Significant trends were documented: NPQmax increases toward the tip whereas pigmentation, absorptance, Fv : Fm and the support of PSII-repair decline. Such heterogeneous leaf condition allows minimizing photodamage and leaf maintenance costs in the upper part of the canopy while maintaining high productivity at the lowest-shaded canopy levels. NPQmax for laboratory and field determinations were as high (4–5) as values reported for sun-acclimated land leaves and supported by similar mechanisms. According to this, the high NPQmax capacity of T. testudinum leaves and the ability of canopy self-shading to reduce light stress can explain the success of this climax seagrass in the reef environment along the Caribbean. This large habitat-builder may have acquired key ecological and evolutionary advantages through its capacity to regulate canopy light field and leaf ability to respond to highly heterogeneous illumination. Our study provides a physiological basis to support the important role of seagrass canopy for large species in photoprotection and plant productivity optimization.

Seagrass canopy photosynthetic response is a function of canopy density and light environment: a model for Amphibolis griffithii.

A three-dimensional computer model of canopies of the seagrass Amphibolis griffithii was used to investigate the consequences of variations in canopy structure and benthic light environment on leaf-level photosynthetic saturation state. The model was constructed using empirical data of plant morphometrics from a previously conducted shading experiment and validated well to in-situ data on light attenuation in canopies of different densities. Using published values of the leaf-level saturating irradiance for photosynthesis, results show that the interaction of canopy density and canopy-scale photosynthetic response is complex and non-linear, due to the combination of self-shading and the non-linearity of photosynthesis versus irradiance (P-I) curves near saturating irradiance. Therefore studies of light limitation in seagrasses should consider variation in canopy structure and density. Based on empirical work, we propose a number of possible measures for canopy scale photosynthetic response that can be plotted to yield isoclines in the space of canopy density and light environment. These plots can be used to interpret the significance of canopy changes induced as a response to decreases in the benthic light environment: in some cases canopy thinning can lead to an equivalent leaf level light environment, in others physiological changes may also be required but these alone may be inadequate for canopy survival. By providing insight to these processes the methods developed here could be a valuable management tool for seagrass conservation during dredging or other coastal developments.

Interactions between seagrass complexity, hydrodynamic flow and biomixing alter food availability for associated filter-feeding organisms.

Seagrass shoots interact with hydrodynamic forces and thereby a positively or negatively influence the survival of associated species. The modification of these forces indirectly alters the physical transport and flux of edible particles within seagrass meadows, which will influence the growth and survivorship of associated filter-feeding organisms. The present work contributes to gaining insight into the mechanisms controlling the availability of resources for filter feeders inhabiting seagrass canopies, both from physical (influenced by seagrass density and patchiness) and biological (regulated by filter feeder density) perspectives. A factorial experiment was conducted in a large racetrack flume, which combined changes in hydrodynamic conditions, chlorophyll a concentration in the water and food intake rate (FIR) in a model active filter-feeding organism (the cockle). Results showed that seagrass density and patchiness modified both hydrodynamic forces and availability of resources for filter feeders. Chlorophyll a water content decreased to 50% of the initial value when densities of both seagrass shoots and cockles were high. Also, filter feeder density controlled resource availability within seagrass patches, depending on its spatial position within the racetrack flume. Under high density of filter-feeding organisms, chlorophyll a levels were lower between patches. This suggests that the pumping activity of cockles (i.e. biomixing) is an emergent key factor affecting both resource availability and FIR for filter feeders in dense canopies. Applying our results to natural conditions, we suggest the existence of a direct correlation between habitat complexity (i.e. shoot density and degree of patchiness) and filter feeders density. Fragmented and low-density patches seem to offer both greater protection from hydrodynamic forces and higher resource availability. In denser patches, however, resources are allocated mostly within the canopy, which would benefit filter feeders if they occurred at low densities, but would be limiting when filter feeder were at high densities.

Evidence of season-dependency in vegetation effects on macrofauna in temperate seagrass meadows (Baltic Sea).

Seagrasses and associated macrophytes are important components of coastal systems as ecosystem engineers, habitat formers, and providers of food and shelter for other organisms. The positive impacts of seagrass vegetation on zoobenthic abundance and diversity (as compared to bare sands) are well documented, but only in surveys performed in summer, which is the season of maximum canopy development. Here we present the results of the first study of the relationship between the seasonal variability of seagrass vegetation and persistence and magnitude of contrasts in faunal communities between vegetated and bare sediments. The composition, abundance, biomass, and diversity of macrozoobenthos in both habitats were compared five times throughout the year in temperate eelgrass meadows in the southern Baltic Sea. Significant positive effects of macrophyte cover on invertebrate density and biomass were recorded only in June, July, and October when the seagrass canopy was relatively well developed. The effects of vegetation cover on faunal species richness, diversity, and composition persisted throughout the year, but the magnitude of these effects varied seasonally and followed changes in macrophyte biomass. The strongest effects were observed in July and coincided with maximums in seagrass biomass and the diversity and biomass of other macrophytes. These observations indicate that in temperate, clearly seasonal systems the assessment of macrophyte impact cannot be based solely on observations performed in just one season, especially when that season is the one in which macrophyte growth is at its maximum. The widely held belief that macrophyte cover strongly influences benthic fauna in marine coastal habitats, which is based on summer surveys, should be revisited and complemented with information obtained in other seasons.

Canopy-scale modifications of the seagrass Amphibolis griffithii in response to and recovery from light reduction

A manipulative experiment using Amphibolis griffithii seagrass, a clonal plant with a complex canopy, tested within-canopy responses to and recovery from light reduction. There were consistent patterns in the distribution and arrangement of leaves, growth rates and resources within the seagrass canopy, and they were modified under reduced light and following recovery from this impact. Under light reduction, plants responded by: reducing leaves in the mid-canopy where the maximum biomass was located; maintaining meristems throughout the canopy; and re-allocating nitrogen to the top of the canopy. During recovery plants increased the number of leaves throughout the canopy and enhanced growth in the part of the canopy that had lost most biomass. These responses have the potential to enhance light capture and recovery of the meadow. There is a clear evolutionary advantage for these submerged plants to be able to modify traits within the canopy, which increase the chance of survival under light stress.

Low-Canopy Seagrass Beds Still Provide Important Coastal Protection Services

One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.

Meiofauna and harpacticoid copepods in different habitats of a Mediterranean seagrass meadow

This study investigated whether associated meiobenthic communities, especially harpacticoid copepods, differed amongst habitats. Five pre-defined habitats within and next to thePosidonia oceanicaseagrass meadow were sampled: living seagrass canopy leaves (LL), small (SMF) and large (LMF) macrophytodetritus fragment accumulations and sand, bare (BS) and covered (CS). The highest meiofauna abundances were recorded in the BS for the core sampled habitats (BS, CS, SMF and LMF) and in the LMF for seagrass material habitats (SMF, LMF and LL). Harpacticoid copepods were the most abundant taxon in all habitats. The assemblage composition at copepod family level showed two distinct habitats clusters: a leaf (LMF and LL) and a sediment cluster (BS, CS and SMF). Subsequently, stable isotope analyses were conducted to analyse the relationship between copepods and their potential food sources in seagrass material habitats. Based on δ13C isotopic analyses and SIAR mixing model, harpacticoid copepods relied for 70% on epiphytes and for 30% onP. oceanicaleaf material in the LMF and LL habitats.

The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community

Lay AbstractAquatic macrophytes (such as macroalgae and sea grasses) are able to take up nutrients directly from the surrounding water in which they inhabit. How fast nutrients are transported into their structures (such as leaves or fronds) can be influenced by water flow. However, macrophytes tend to form canopies made up of many individual structures, and the presence of these canopies has large effects on local water flow. This implies that in shallow regions the architecture of different canopies may well be a key factor in setting rates of nutrient transport to the sea floor. Predicting these effects is complicated because in many natural systems macrophyte communities often form patchy distributions consisting of bare and vegetated areas, as well as patches of different species with different canopy types. Here we use a new application of stable isotope labeling to directly measure the in situ uptake of ammonium at the transition between short, dense Caulerpa prolifera (a macroalga) and tall, sparse Cymodocea nodosa (a sea grass) canopies. We found that as water flowed over the dense macroalgae into the sparse sea grass canopy, it accelerated, resulting in horizontal and vertical variations in water velocity and mixing that appeared to explain variations in the ammonium uptake of the different types of organisms living on and within the canopies. These results highlight how the interplay between the physical structure of organisms and flow may determine the nutrient transport of individual species within macrophyte communities, creating nutrient transport microhabitats that may influence biodiversity and ecosystem functioning.