Presence Of Seagrass Research Articles

Influence of the seagrass Thalassia hemprichii on coral reef mesocosms exposed to ocean acidification and experimentally elevated temperatures

Ocean acidification (OA) and warming currently threaten coastal ecosystems across the globe. However, it is possible that the former process could actually benefit marine plants, such as seagrasses. The purpose of this study was to examine whether the effects of the seagrass Thalassia hemprichii can increase the resilience of OA-challenged coral reef mesocosms whose temperatures were gradually elevated. It was found that seagrass shoot density, photosynthetic efficiency, and leaf growth rate actually increased with rising temperatures under OA. Macroalgal growth rates were higher in the seagrass-free mesocosms, but the calcification rate of the model reef coral Pocillopora damicornis was higher in coral reef mesocosms featuring seagrasses under OA at 25 and 28 °C. Both the macroalgal growth rate and the coral calcification rate decreased in all mesocosms when the temperature was raised to 31 °C under OA. However, the variation in gross primary production, ecosystem respiration, and net ecosystem production in the seagrass mesocosms was lower than in seagrass-free controls, suggesting that the presence of seagrass in the mesocosms helped to stabilize the metabolism of the system in response to simulated climate change.

Assessing behavioural and social responses to an eco-mooring trial for Zostera marina conservation management in Torbay, Southwest England

Seagrass beds are critically important habitats delivering a range of ecosystem service benefits that support human wellbeing. They are, however, declining globally at an unprecedented rate. The damaging effects on Zostera marina (Common Eelgrass) seagrass beds in UK waters from mechanical recreational boating activities, namely anchoring and traditional swing mooring scour, suggest that measures of implementing management to mitigate human impacts need prioritising. Eco-moorings, a design that reduces the abrasion pressure of anchoring and mooring on the seabed, are such a mitigation method. In the UK, limited test sites and a lack of social acceptance of the technology compounds the limited uptake of eco-moorings to date. To advance the evidence base, this study focusses on a bespoke eco-mooring design deployed in 2017 to protect seagrass beds within a popular anchorage in Southwest England. We assess the behavioural and social responses of recreational boaters to the trial eco-mooring through 1) mapping of boating activity pre- and post-deployment and 2) structured questionnaires both on-site and online to local and national audiences respectively. Results of mapped boating activity demonstrated a shoreward distribution shift of anchoring away from the deployed eco-mooring, with 45.4% of all anchoring vessels directly within or within a range of potential influence of the seagrass bed. Through the deployment of the eco-mooring (where no mooring previously existed), an estimated additional 20% of anchoring pressure was alleviated from the seagrass bed. A total of 89.6% of on-site respondents reacted positively to the prospect of further eco-moorings being deployed locally. When questioned, 74.6% of on-site and 82.8% of online respondents demonstrated a preference for the physical action of mooring a vessel over anchoring. This suggests that implementation of eco-moorings in seagrass-dense regions could successfully alleviate anchor damage, especially where over half of respondents indicated an awareness of seagrass presence prior to anchoring their vessel and/or hesitance to cease visiting preferential anchoring sites (sheltered bays). This study recommends targeting conservation guidance at the powerboat community, the dominant group anchoring/mooring in the study area and identified as not currently engaged in seagrass education campaigns. It is proposed that management introduce incentives such as free of charge eco-moorings and/or anchoring charges in Marine Protected Areas/Voluntary No Anchor Zones to conserve and recover seagrass and other sensitive seabed habitats nationally.

Application of deep learning techniques for determining the spatial extent and classification of seagrass beds, Trang, Thailand

Abstract Few studies have investigated the long-term temporal dynamics of seagrass beds, especially in Southeast Asia. Remote sensing is one of the best methods for observing these dynamic patterns, and the advent of deep learning technology has led to recent advances in this method. This study examined the feasibility of applying image classification methods to supervised classification and deep learning methods for monitoring seagrass beds. The study site was a relatively natural seagrass bed in Hat Chao Mai National Park, Trang Province, Thailand, for which aerial photographs from the 1970s were available. Although we achieved low accuracy in differentiating among various densities of vegetation coverage, classification related to the presence of seagrass was possible with an accuracy of 80% or more using both classification methods. Automatic classification of benthic cover using deep learning provided similar or better accuracy than that of the other methods even when grayscale images were used. The results also demonstrate that it is possible to monitor the temporal dynamics of an entire seagrass area, as well as variations within sub-regions, located in close proximity to a river mouth.

Propagule risk in a marine foundation species: Seascape effects on Zostera marina seed predation

Abstract Both biotic and abiotic factors can affect whether seagrass meadows establish as large, continuous features or as a mosaic of smaller patches, and these processes act across multiple spatial scales. Of these processes, sexual recruitment is considered vital for the recovery and resilience of seagrass meadows. Because numerous estuarine fauna can impact sexual recruitment by consuming seeds, it is important to understand variation in seed consumption throughout the seascape. We sought to determine how cross‐scale processes affected seed consumption of Zostera marina, a widely distributed seagrass species. We deployed seed predation units (SPU) within patches ranging in size from 0.28 to 28.48 m2, and on unvegetated, sandy bottom. Seed consumption was analysed across patch sizes, at patch centres and edges and within the context of total seagrass cover within buffer zones surrounding SPU locations. Using structural equation modelling, we found that measured seascape parameters explained 34% of the observed variation in seed consumption. Specifically, we found that seagrass presence had the strongest effect on seed predation followed by a negative effect of patch size on seed consumption. Although there was no direct effect of percent seagrass cover (within a 4‐m buffer of the SPU) on seed predation, percent seagrass cover had a negative indirect effect through its positive correlation with patch size. However, we found that once variation due to patch size and areal cover were accounted for, seeds placed at patch interiors actually experienced greater consumption rates. We did not find similar relationships with our potential seed consumers, although our snapshot sampling likely obscured transient effects. We argue that seagrass recruitment could be affected by seed predation, which is influenced by multiple processes, including predator dispersal, behaviour and aggregation. Since seed‐based restoration is becoming more common, and the role of sexual recruitment in meadow maintenance appears to be increasingly important for seagrass resilience, our results demonstrate that for current management strategies to be successful, there is a need to better understand seed predation in seagrass‐dominated seascapes. Synthesis. Factors that influence propagule supply and recruitment success of seagrass operate and interact across multiple spatial scales, which can have implications for management.

Seagrass community at tawang coast pacitan in rainy and dry season

Ecologically, the presence of seagrass contributes to an ecosystem offering for human need. This research was conducted to study the seagrass appearance at Tawang Coast Pacitan, East Java, Indonesia during rainy and dry season using a percentage of the covered area and physico chemical factors. The method used was transect-plot. The physical and chemical factors measured included temperature, light penetration, rapidity flow, high water rate, salinity, wave level, sediment texture, pH, dissolved oxygen, and nutrients such as ammonium, nitrate, C-organic and phosphorus pentoxide. The result showed that seagrass bed in Tawang Coast was formed by three seagrass: Cymodocea rotundata, Thalassia hemprichii, and Halophila ovalis. The average percentage of seagrass cover was 37.66 % in rainy season and 32.49% in dry season. According to decree of the Minister of Environment No. 200 in 2004, the seagrass bed conditions in Tawang Coast in the rainy season and dry season were categorized into less rich/less healthy. The decline in the seagrass covering percentage in the dry season was caused by light and temperature factors. Seagrasses in Tawang Coast were associated with different types of organisms.

Seascape context modifies how fish respond to restored oyster reef structures

Abstract The seascape context of coastal ecosystems plays a pivotal role in shaping patterns in fish recruitment, abundance, and diversity. It might also be a principal determinant in structuring the recruitment of fish assemblages to restored habitats, but the trajectories of these relationships require further testing. In this study, we surveyed fish assemblages from 14 restored oyster reefs and 14 control sites in the Noosa River, Queensland, Australia, that differed in the presence or absence of seagrass within 500 m, over four periods using baited cameras. Fish assemblages at oyster reefs differed from those at control sites, with higher species richness (1.4 times) and more individuals of taxa that are harvested by fishers (1.8 times). The presence or absence of seagrass nearby affected the abundance of a key harvestable fish species (yellowfin bream Acanthopagrus australis) on oyster reefs, but not the overall composition of fish assemblages, species richness, or the total abundance of harvestable fishes overall. These findings highlight the importance of considering species-specific patterns in seascape utilization when selecting restoration sites and setting restoration goals, and suggest that the effects of restoration on fish assemblages might be optimized by focusing efforts in prime positions in coastal seascapes.

Seagrass can mitigate negative ocean acidification effects on calcifying algae

The ultimate effect that ocean acidification (OA) and warming will have on the physiology of calcifying algae is still largely uncertain. Responses depend on the complex interactions between seawater chemistry, global/local stressors and species-specific physiologies. There is a significant gap regarding the effect that metabolic interactions between coexisting species may have on local seawater chemistry and the concurrent effect of OA. Here, we manipulated CO2 and temperature to evaluate the physiological responses of two common photoautotrophs from shallow tropical marine coastal ecosystems in Brazil: the calcifying alga Halimeda cuneata, and the seagrass Halodule wrightii. We tested whether or not seagrass presence can influence the calcification rate of a widespread and abundant species of Halimeda under OA and warming. Our results demonstrate that under elevated CO2, the high photosynthetic rates of H. wrightii contribute to raise H. cuneata calcification more than two-fold and thus we suggest that H. cuneata populations coexisting with H. wrightii may have a higher resilience to OA conditions. This conclusion supports the more general hypothesis that, in coastal and shallow reef environments, the metabolic interactions between calcifying and non-calcifying organisms are instrumental in providing refuge against OA effects and increasing the resilience of the more OA-susceptible species.

Seagrass Meadow Impacts on Universiti Malaysia Sabah (UMS) Beach, Kota Kinabalu Sabah (Malaysia)

Seagrass meadow is known for its contribution as a habitat, ability to attenuate the wave’s energy and as natural protection for the beach. The coastline of Universiti Malaysia Sabah (UMS) consists of important coral reef and seagrass ecosystems. Seagrass meadow is found at the northern part of the UMS beach at an area known as the Outdoor Development Centre (ODEC) while patchy coral reefs are found at the deeper part of the shoreline. The objectives of this study are to determine the beach profile and sediment characteristics of UMS-ODEC beach during the Southwest and Northeast monsoons and to compare the beach profile at areas where seagrass meadow is present and exposed beach area to the open sea. Beach profiling measurement was done at the three transects placed perpendicular to the UMS-ODEC beach. Transects 1 was with presence of seagrass and transects 2 and 3 were without seagrass. Sediment samples were taken at each transect for sediment characteristics identification. The assessment was done at an interval of every two month starting from May 2016 to March 2017. Beach profile elevation of UMS-ODEC Beach is higher during the Northeast monsoon than during the Southwest monsoon. It was also identified that large accretion happened at transects 1 and 2 between the month of July 2016 and September 2016 and between the month of January 2017 and March 2017. The erosion process happened at all transects between the month of September 2016 and November 2016 during the peak of the Southwest monsoon and continued to erode between the month of November 2016 and January 2017 during the Northeast monsoon. Sediment characteristics of mean, sorting, skewness at all transects were categorized under fine sand, moderately sorted and coarsely skewed, respectively during the Northeast monsoon and vary sediment characteristics except skewness of at all transects during the southwest monsoon. Based on this one year measurement of beach profile and sediment characteristics, there is no clear impact of the seagrass meadow as a natural coastal protection for the UMS-ODEC beach.

Seagrass detection in the mediterranean: A supervised learning approach

We deal with the problem of detecting seagrass presence/absence and distinguishing seagrass families in the Mediterranean via supervised learning methods. By merging datasets about seagrass presence and other external environmental variables, we develop suitable training data, enhanced by seagrass absence data algorithmically produced based on certain hypotheses. Experiments comparing several popular classification algorithms yield up to 93.4% accuracy in detecting seagrass presence. In a feature strength analysis, the most important variables determining presence–absence are found to be Chlorophyll-α levels and Distance-to-Coast. For determining family, variables cannot be easily singled out; several different variables seem to be of importance, with Chlorophyll-α surpassing all others. In both problems, tree-based classification algorithms perform better than others, with Random Forest being the most effective. Hidden preferences reveal that Cymodocea and Posidonia favor the low, limited-range chlorophyll-α levels (<0.5 mg/m3), Halophila tolerates higher salinities (>39), while Ruppia prefers euryhaline conditions (37.5–39).

Exploring the Impacts of Seagrass on Coupled Marsh-Tidal Flat Morphodynamics

1 Intertidal coastal environments are prone to changes induced by sea level rise, increases in storminess, temperature, and anthropogenic disturbances. It is unclear how changes in external drivers may affect the dynamics of low energy coastal environments because their response is non-linear, and characterized by many thresholds and discontinuities. As such, process-based modeling of the ecogeomorphic processes underlying the dynamics of these ecosystems is useful, not only to predict their change through time, but also to generate new hypotheses and research questions. Here, we used a three-point dynamic model to investigate how seagrass might affect the behavior of coupled marsh-tidal flat systems. The model directly incorporates ecogeomorphological feedbacks among wind waves, salt marsh vegetation, allochthonous sediment loading, seagrasses and sea level rise. The model was applied to examine potential behaviors of salt marsh systems in the Virginia coastal bays. Differences due to the presence or absence of seagrass and stochastic versus constant drivers lead to the emergence of complex behaviors in the coupled salt marsh-tidal flat system. In intertidal areas without seagrass, small tidal flats are unlikely to expand and provide enough sediment to the salt marshes to combat sea level rise. However, as the tidal flat expands, the concurrent increase in sediment supply due to wave-induced processes allows for the salt marsh to maintain pace with sea level at the expense of salt marsh extent. The presence of seagrass has two effects: 1) it decreases near bed shear stresses thus reducing the sediment flux to the salt marsh platform; 2) it reduces the wave energy acting on the salt marsh scarp, thus reducing boundary erosion. Model results indicate that the reductions in wave power and near bed shear stresses when seagrass is present provide an overall stabilizing effect on the coupled marsh-tidal flat system; but as water depth increases due to sea level rise or as external sediment supply increases, light conditions decline and the system reverts to that of a bare tidal flat.

Recent (post-1930) changes in the extent of subtidal seagrass (Zostera muelleri) beds of the eastern Bay of Islands, New Zealand

ABSTRACTThe surface areas of 12 subtidal seagrass Zostera muelleri beds at the islands and adjacent mainland shores of the eastern Bay of Islands (35° 12′ S, 174° 10′ E), New Zealand were estimated from aerial images. It appears that little subtidal seagrass existed until after the early-1950s, so significant beds here may be a relatively recent biome. Possible explanations for recent emergence of subtidal seagrass include that debilitating periods of turbid water and pulses of sedimentation associated with the land clearances of the late 1800s had worked through the marine ecosystem. An overall peak in seagrass-cover in the 1960s to 1980s was followed by declines in several beds in the 1990s to 2000s, with recovery since. The temporal presence of seagrass was well-correlated among the three mainland beds, and moderately well among the south-facing beds on the islands, consistent with mechanisms driving seagrass establishment and persistence operating at reasonably broad scales.

New sighting of seagrasses in the Eastern Tropical Pacific (Bahía Potrero, Costa Rica)

Seagrass meadows provide a multitude of ecosystem services, yet they are currently threatened and declining worldwide due to anthropogenic impacts. Current knowledge of seagrass presence in the Eastern Tropical Pacific (ETP) is scarce, in part due to challenges in finding the small seagrass species found and their dynamic and possibly ephemeral nature, with only very limited reports of currently extant meadows. Here, we characterize seagrasses at a new location, Bahía Potrero, on the northern Pacific coast of Costa Rica. Seagrasses were sighted at this location on four occasions between 2015 and 2017. Two seagrass species were found, Halophila baillonis and Halodule beaudettei. Seagrasses were present at ∼ 3–6 m depth at mean sea level (local tidal range ∼ 3 m). Biomass and sediment cores were collected in 2017, total biomass was 3.9 ± 4.0 g DW m−2 and density 569 ± 493 shoots m−2. Leaf length was 1.1 ± 0.29 cm, width 0.5 ± 0.08 cm, and area 1.4 ± 0.67 cm2. Above ground biomass δ13C was -11.7 ± 0.8‰ and δ15N 5.4 ± 0.8‰, while below ground was -11.7 ± 1.3‰ and 5.3 ± 0.8‰. Sediment δ13C was -21.2 ± 0.6‰ and δ15N was 8.6 ± 0.4‰. Sediments were dominated by very fine sand, with 1.6 ± 0.3% organic carbon content and 23.0 ± 7.9% inorganic carbon. Carbon content standardized to 10 cm sediment depth was 20.5 ± 5.1 Mg OC ha-1 and 295.1 ± 109.8 Mg IC ha-1. Given the numerous threats that seagrasses are under and the lack of information on seagrasses in the ETP it is critical to increase our knowledge on seagrasses in this region for adequate management and conservation initiatives.

Seagrass Impact on Sediment Exchange Between Tidal Flats and Salt Marsh, and The Sediment Budget of Shallow Bays

AbstractSeagrasses are marine flowering plants that strongly impact their physical and biological surroundings and are therefore frequently referred to as ecological engineers. The effect of seagrasses on coastal bay resilience and sediment transport dynamics is understudied. Here we use six historical maps of seagrass distribution in Barnegat Bay, USA, to investigate the role of these vegetated surfaces on the sediment storage capacity of shallow bays. Analyses are carried out by means of the Coupled‐Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) numerical modeling framework. Results show that a decline in the extent of seagrass meadows reduces the sediment mass potentially stored within bay systems. The presence of seagrass reduces shear stress values across the entire bay, including unvegetated areas, and promotes sediment deposition on tidal flats. On the other hand, the presence of seagrasses decreases suspended sediment concentrations, which in turn reduces the delivery of sediment to marsh platforms. Results highlight the relevance of seagrasses for the long‐term survival of coastal ecosystems, and the complex dynamics regulating the interaction between subtidal and intertidal landscapes.

Water residence time controls the feedback between seagrass, sediment and light: Implications for restoration

Feedbacks between seagrass and the local environmental conditions may hinder attempts to restore seagrass by inducing alternative stable states. A one-dimensional physical-biological model was used to identify the conditions under which a feedback between seagrass, sediment and light can yield alternative stable states of seagrass presence and absence (bistability). Based on our model results, a prediction of whether a given seagrass meadow is large enough to promote seagrass growth can now be made. If the water residence time within the spatial area of the meadow is similar to or greater than the sediment settling time, which is calculated from the ratio of water depth to sediment vertical settling velocity, the meadow is large enough for the feedback to potentially reduce the local suspended sediment concentration. This has important implications for seagrass restoration: for a proposed restoration plot, if the water residence time is similar to or greater than the sediment settling time, the scale of restoration is large enough for the feedback between seagrass, sediment and light to locally improve water clarity. More generally, this calculation can be used to identify areas where this feedback is likely to generate bistability, and to estimate the minimum suitable meadow size in such locations.

Habitat–fishery linkages in two major south-eastern Australian estuaries show that the C4 saltmarsh plant Sporobolus virginicus is a significant contributor to fisheries productivity

Estuarine fisheries productivity is dependent upon numerous factors, including the productivity of primary producers supporting the food web and the transport of organic matter derived from those primary producers. In this study, we use stable isotope ratios in a Bayesian mixing model to estimate the contribution of primary producers to fully recruited commercial species in two important estuarine commercial fisheries in south-eastern Australia; the Hunter and Clarence estuaries. The C4 saltmarsh plant Sporobolus virginicus had the greatest contribution to consumer diet among almost all sites and times (25–95%), though for prawns the presence of seagrass may be exerting some influence on this calculated contribution in the Clarence estuary. Particulate organic matter (POM; 30%) and fine benthic organic matter (FBOM; 39–41%) also contributed significantly to consumer diet. Mangroves and other C3 sources generally had the lowest contribution to consumers (1–31%). While the exact contributions of each source are uncertain within our Bayesian framework, these results highlight the relatively large role of saltmarsh habitat as a contributor to fishery productivity, especially in estuaries with no seagrasses. Given the anthropogenic threats to saltmarsh habitat, there is potential for loss of fishery productivity with further loss of saltmarsh areal extent.

Increased sediment loads cause non-linear decreases in seagrass suitable habitat extent.

Land-based activities, including deforestation, agriculture, and urbanisation, cause increased erosion, reduced inland and coastal water quality, and subsequent loss or degradation of downstream coastal marine ecosystems. Quantitative approaches to link sediment loads from catchments to metrics of downstream marine ecosystem state are required to calculate the cost effectiveness of taking conservation actions on land to benefits accrued in the ocean. Here we quantify the relationship between sediment loads derived from landscapes to habitat suitability of seagrass meadows in Moreton Bay, Queensland, Australia. We use the following approach: (1) a catchment hydrological model generates sediment loads; (2) a statistical model links sediment loads to water clarity at monthly time-steps; (3) a species distribution model (SDM) factors in water clarity, bathymetry, wave height, and substrate suitability to predict seagrass habitat suitability at monthly time-steps; and (4) a statistical model quantifies the effect of sediment loads on area of seagrass suitable habitat in a given year. The relationship between sediment loads and seagrass suitable habitat is non-linear: large increases in sediment have a disproportionately large negative impact on availability of seagrass suitable habitat. Varying the temporal scale of analysis (monthly vs. yearly), or varying the threshold value used to delineate predicted seagrass presence vs. absence, both affect the magnitude, but not the overall shape, of the relationship between sediment loads and seagrass suitable habitat area. Quantifying the link between sediment produced from catchments and extent of downstream marine ecosystems allows assessment of the relative costs and benefits of taking conservation actions on land or in the ocean, respectively, to marine ecosystems.

Measuring the role of seagrasses in regulating sediment surface elevation

Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human welfare as well as ecological integrity. In common with the other ‘blue carbon’ habitats (mangroves and tidal marshes) seagrasses are thought to provide coastal defence and encourage sediment stabilisation and surface elevation. A sophisticated understanding of sediment elevation dynamics in mangroves and tidal marshes has been gained by monitoring a wide range of different sites, located in varying hydrogeomorphological conditions over long periods. In contrast, similar evidence for seagrasses is sparse; the present study is a contribution towards filling this gap. Surface elevation change pins were deployed in four locations, Scotland, Kenya, Tanzania and Saudi Arabia, in both seagrass and unvegetated control plots in the low intertidal and shallow subtidal zone. The presence of seagrass had a highly significant, positive impact on surface elevation at all sites. Combined data from the current work and the literature show an average difference of 31 mm per year in elevation rates between vegetated and unvegetated areas, which emphasizes the important contribution of seagrass in facilitating sediment surface elevation and reducing erosion. This paper presents the first multi-site study for sediment surface elevation in seagrasses in different settings and species.

Carbon storage in the seagrass meadows of Gazi Bay, Kenya.

Vegetated marine habitats are globally important carbon sinks, making a significant contribution towards mitigating climate change, and they provide a wide range of other ecosystem services. However, large gaps in knowledge remain, particularly for seagrass meadows in Africa. The present study estimated biomass and sediment organic carbon (Corg) stocks of four dominant seagrass species in Gazi Bay, Kenya. It compared sediment Corg between seagrass areas in vegetated and un-vegetated ‘controls’, using the naturally patchy occurence of seagrass at this site to test the impacts of seagrass growth on sediment Corg. It also explored relationships between the sediment and above-ground Corg, as well as between the total biomass and above-ground parameters. Sediment Corg was significantly different between species, range: 160.7–233.8 Mg C ha-1 (compared to the global range of 115.3 to 829.2 Mg C ha-1). Vegetated areas in all species had significantly higher sediment Corg compared with un-vegetated controls; the presence of seagrass increased Corg by 4–6 times. Biomass carbon differed significantly between species with means ranging between 4.8–7.1 Mg C ha-1 compared to the global range of 2.5–7.3 Mg C ha-1. To our knowledge, these are among the first results on seagrass sediment Corg to be reported from African seagrass beds; and contribute towards our understanding of the role of seagrass in global carbon dynamics.

Processes affecting the spatial distribution of seagrass meadow sedimentary material on Yao Yai Island, Thailand

Many islands throughout SE Asia are experiencing rapid development and land-cover conversion that potentially threaten sensitive coastal ecosystems, such as seagrasses, through increased loading of sediment and nutrients originating from disturbed catchments draining to the sea. To evaluate this threat for one such island in Southern Thailand (Yao Yai), we perform sediment source tracing via end-member mixing analysis using stable isotopes δ13C and δ15N in organic matter to explore sediment loading in a seagrass meadow. The analysis indicates that sedimentary material in the meadow originates mostly from ocean-associated sources (∼62% from seagrass detritus, seston, and ocean sediments). Terrestrial material comprises ∼19% of the organic material found in the seagrass meadow, with another 20% originating from an adjacent mangrove forest. Approximately one-fourth of the seagrass meadow material (24%) is detritus that has been (re)deposited internally. The high contribution of terrestrial-derived organic matter deposited near the river mouth demonstrates that substantial quantities of sediment are being transferred from upslope erosion sources into the seagrass meadow. However, only a small amount of this material is deposited throughout the entire bay because much of the terrestrial- and mangrove-derived sediment is transferred to the open ocean via channels that are periodically dredged to allow boat access to two small inland harbours. This positive affect of dredging has not received very much attention in existing literature. River water flowing to the channels during falling tide delivers sediment to these efficient pathways, where much of it bypasses the seagrass meadow at periods of time when sediment deposition would normally be the greatest. There is growing concern that ongoing land-cover changes and planned urbanization related to tourism and agriculture on the island may boost sediment/nutrients above a critical threshold, beyond that revealed in our baseline survey. Our tracer-based sediment source approach did not corroborate our observations of substantial erosion and land degradation in the upper catchment—but this could be a result of sediment flushing through the dredged channels. We encourage others to combine such methods with sediment budgeting approaches to triangulate results for consistency. Finally, from an ecological perspective, the high presence of seagrass detritus we found in bay sediments suggests seagrass is potentially a key source of nutrients for the meadow itself, as well as other connected ecosystems.

Spatial patterns of seagrass dispersal and settlement

AbstractAimThe movement of propagules among plant populations affects their ability to replenish and recover after a disturbance. Quantitative data on recovery strategies, including the effectiveness of population connectivity, are often lacking at broad spatial and temporal scales. We use numerical modelling to predict seagrass propagule dispersal and settlement to provide an approach for circumstances where direct, or even indirect, measures of population dynamics are difficult to establish.LocationGreat Barrier Reef, Australia.MethodsWe used the finite‐element Second‐generation Louvain‐la‐Neuve Ice‐ocean Model (SLIM) to resolve the hydrodynamics of the central Great Barrier Reef and to simulate the dispersal of seagrass. We predicted dispersal and settlement patterns by releasing 10.6 million passive particles representing seagrass propagules at known sites of seagrass presence. We considered two fractions when modelling seagrass dispersal: floating and suspended propagules. Both fractions were modelled using 34 simulations run for a maximum of 8 weeks during the peak seagrass reproductive period, capturing variability in winds, tides and currents.ResultsThe ‘virtual’ seagrass propagules moved on average between 30 and 60 km, but distances of over 900 km also occurred. Most particle movement was to the north‐west. The season (month) of release and source locations of the particles correlated with their dispersal distance, particularly for particles released offshore, with the complex coastal topography impeding movements close to the coast. The replenishment and recovery potential of the northernmost meadows was influenced by southern meadows. Protected north‐facing bays were less likely to receive particles.Main conclusionsOur approach advances the conservation and management of marine biodiversity by predicting a key component of ecosystem resilience at a spatial scale that informs marine planning. We show a complex interaction among time, wind, water movement and topography that can guide a management response to improving replenishment and recovery after disturbance events.