Seagrass Habitats Research Articles

Seagrass Structural Traits Drive Fish Assemblages in Small-Scale Fisheries

Seagrasses – a group of foundation species in coastal ecosystems – provide key habitat for diverse and abundant faunal assemblages and support numerous ecosystem functions and services. However, whether the habitat role of seagrasses is influenced by seagrass diversity, by dominant species or both, remains unclear. To that end, we sought to investigate the specific seagrass characteristics (e.g., species diversity, seagrass traits) that influence tropical fish assemblages, and place this in the context of small-scale fishery use. We surveyed seagrass variables at 55 plots, nested within 12 sites around Zanzibar (Tanzania) in the Western Indian Ocean, and used Baited Remote Underwater Video (BRUV) systems to assess fish assemblages across plots. Using linear mixed models, we reveal that seagrass structural complexity and depth were the best predictors of fish abundance, with higher abundance occurring in deeper meadows or meadows with high canopy, leaf length and number of leaves per shoot. Moreover, an interaction between seagrass cover and land-use was the best predictor of fish species richness, where sites closer to human impacts were less affected by cover than sites with lower human impact. Overall, models with seagrass species richness or functional diversity as predictors poorly explained fish assemblages. Fish taxa that were important for small-scale fishery sectors (e.g., emperors, snappers, rabbitfish, and parrotfish) were primarily driven by seagrass structural complexity. Our results provide a unique analysis of the relationship between seagrass habitat and its associated fish assemblages in that we show that seagrass species diversity had little effect on seagrass fish assemblages, which instead appear driven by specific seagrass traits and seagrass cover. If conserving high value species that support adjacent fisheries is the priority for protecting seagrass meadows, then seagrass areas should be chosen with high cover and structural complexity that are in deeper waters. Any conservation measures also need to balance the needs of fishers that use the resources supported by seagrasses.

Analysis of Seagrass Condition Thalassia hemprichii in Kabung Island Waters, Bengkayang Regency, West Kalimantan

Seagrass is one of the important ecosystems and habitats for marine organisms. Thalassia hemprichii is one of the seagrass species in Kabung Island Waters. Studies on seagrass ecosystem conditions must be carried out as initial information for integrated aquatic management. Analysis of seagrass ecosystem conditions includes density and coverage percentage. Seagrass data was collected using the line transect method with a length of 50 cm and quadrants transect of 1 m x 1 m perpendicular to the coastline toward the sea. The density of T. hemprichii at Stations I and II were 50.83 and 68.27 ind/m 2 , respectively. The average coverage percentage at Stations I and II were 5.05% and 5.85%, respectively. These ecosystem conditions could be categorized as rare density. Physico-chemical parameters of the waters of Kabung Island including temperature, salinity, pH, dissolved oxygen (DO), water visibility, depth, and current have been carried out. The Kabung Island waters have a salinity of 32-33‰, temperature 29-30 o C, and pH 8.0-8.4. The depth and water visibility of the seagrass habitat was 0.6 m. The current velocity was 0.07-0.29 m/s. These parameters were optimum conditions for seagrass life. However, the dissolved oxygen in Kabung Island was low at 2.2-4 mg/L.

Biodiversity consequences of Caulerpa prolifera takeover of a coastal lagoon

The genus Caulerpa has attracted much attention because many of its species were introduced into non-native regions and became notoriously invasive. This is the case of Caulerpa prolifera that has been rapidly expanding in Ria Formosa lagoon, taking over the deeper unvegetated soft bottoms and competing with seagrass meadows in the shallower areas. Here we address how C. prolifera invasion may affect the support of biodiversity, and specifically, the provision of habitat and nursery for commercial species by the native habitats of this coastal lagoon. Even though no significant differences in total species richness, diversity and evenness were found between C. prolifera and the native unvegetated habitat, the dissimilarity between these two habitats was highest, mostly driven by the extreme reduction of the gastropod Bittium reticulatum and of the tanaid Apseudopsis formosus. This may implicate changes in the trophic interactions of the ecosystem, for example decreasing the tanaid food source for seahorses, which are presently endangered in the lagoon. On the other hand, the fauna species richness, diversity and evenness were significantly higher in the native seagrass habitat than in C. prolifera. Juveniles of valuable flat and sparid fish were only observed in unvegetated sediments and seagrass meadows, respectively. The aggressive spread of C. prolifera in Ria Formosa may alter the structure of native faunal communities, with likely negative implications on fisheries. Nevertheless, the global biodiversity of the lagoon will not be likely drastically affected unless the seaweed takes over the seagrass meadows.

A dynamic view of seagrass meadows in the wake of successful green turtle conservation.

Concerted conservation efforts have led to a remarkable recovery of multiple green turtle (Chelonia mydas) populations worldwide. The voracious feeding of these returning populations is radically transforming tropical seagrass habitats in ways that prompt a re-think of the reference state and management plans for seagrass meadows.

Variable contributions of seafloor communities to ecosystem metabolism across a gradient of habitat-forming species

The contributions of habitat-forming species to the biodiversity and ecosystem processes of marine and terrestrial ecosystems are widely recognized. Aquatic plants are considered foundation species in shallow ecosystems, as they maintain biodiversity and sustain many ecosystem functions such as primary production and respiration. Despite the increasing amount of biodiversity-ecosystem functioning experiments in seagrass habitats, the effects of benthic variability on ecosystem functioning are rarely investigated across spatially variable aquatic plant habitats. Here, we quantitatively link seasonal variability in seafloor metabolism (i.e. gross primary production and community respiration) with major benthic community components (i.e. microphytobenthos, aquatic plants and macrofauna) across a structural complexity gradient of habitat-forming species (in terms of shoot density and biomass), ranging from bare sand, to a sparse mixture of plants to a dense monospecific seagrass meadow. The increasing complexity gradient enhanced the magnitude of the relationships between benthic community and seafloor metabolism. The daily average seafloor metabolism per season at the bare site was similar to the sparse site, highlighting the role of microphytobenthos for seafloor metabolism in shallow unvegetated sediments. The contribution of the associated macrofauna to the seafloor respiration was similar to the aquatic plant community contribution. Infauna was the main macrofaunal component significantly explaining the seasonal variability of seafloor respiration. However, benthic community-metabolism relationships were stronger within the plant community than within the macrofauna community (i.e. steepest slopes and lowest p-values). Understanding these relationships are a priority since climate change and biodiversity loss are reducing habitat complexity around the world, jeopardizing valuable ecosystem functions and services.

The first nation-wide assessment identifies valuable blue‑carbon seagrass habitat in Indonesia is in moderate condition

Seagrasses help mitigate climate change by capturing and storing carbon for a very long time. Despite their significance, multiple threats are putting pressure on this valuable habitat. Seagrass conservation and restoration is essential to maintain the carbon sequestration service, along with the other ecosystem services (e.g., fisheries production). Information on the status of seagrass habitat is required to enable proactive management strategies. In this study, we assessed the ecological status of seagrass meadows in Indonesia, a globally significant hotspot of blue carbon habitat, using five resilience parameters: seagrass species richness, seagrass cover, macroalgal cover, epiphyte cover, and water transparency. We collected data on these parameters from 18 locations across Indonesia and developed the Seagrass Ecological Quality Index (SEQI, range 0–1) that summarises the parameters into a standard seagrass assessment. We recorded that Thalassia hemprichii and Enhalus acoroides were the most common species in the sampling locations. Seagrass cover was on average 39 ± 3.6% (mean ± SE) which was lower than measured 3 years before (46%). Macroalgal cover and epiphyte cover were relatively low with mean values 10 ± 1.8% and 22 ± 4.4% respectively. Most locations had clear water. Our analysis showed that Indonesia's seagrass meadows are generally in moderate condition, indicated by the mean value of SEQI 0.68 ± 0.02. These findings provide a baseline for seagrass conservation and management. It enables setting up of specific goals for management action plans, at both the local, national and international level, and where appropriate, track the progress of management actions based on threat mitigation or restoration over time. The SEQI is a useful tool to summarise the condition of tropical seagrass meadows in Indonesia and could be applied across Southeast Asia or the Tropical Indo-Pacific bioregion.

Low altitude spatial assessment and monitoring of intertidal seagrass meadows beyond the visible spectrum using a remotely piloted aircraft system

Seagrass meadows contribute to multiple ecosystem services in the marine environment. Despite their ecological importance and the services they provide, seagrass habitats globally have shown a major decline. To understand this loss, in this study, we demonstrate the potential of a multi-rotor and a multispectral sensor for spatial assessment and monitoring for seagrass meadows in Waitemata Harbour, Auckland, New Zealand. Initially, we used structure from motion photogrammetry to generate a high-spatial-resolution orthomosaic of 3.5 cm/pixel. Next, we compared seagrass reflectance from a handheld field spectroradiometer to the reflectance extracted from the RPAS multispectral orthomosaic. This reflectance information was then used to create a spectral index. After that, we used an object-based image analysis (OBIA) technique to segment the orthomosaic and perform a supervised classification. Consequently, an overall accuracy of 95% and a Kappa Coefficient of 0.81 was achieved. This high accuracy is attributed to the sensor's capability to capture high-resolution imagery beyond the visible spectrum as the ability to distinguish meadows from other land cover features was improved. Similarly, researchers elsewhere can gain valuable insights to observe local changes and identify drivers of change. The technique used in this research can directly improve our understanding of seagrass spatial dynamics for conservation and planning.

Benthic and coral reef community field data for Heron Reef, Southern Great Barrier Reef, Australia, 2002\u20132018

This paper describes benthic coral reef community composition point-based field data sets derived from georeferenced photoquadrats using machine learning. Annually over a 17 year period (2002–2018), data were collected using downward-looking photoquadrats that capture an approximately 1 m2 footprint along 100 m–1500 m transect surveys distributed along the reef slope and across the reef flat of Heron Reef (28 km2), Southern Great Barrier Reef, Australia. Benthic community composition for the photoquadrats was automatically interpreted through deep learning, following initial manual calibration of the algorithm. The resulting data sets support understanding of coral reef biology, ecology, mapping and dynamics. Similar methods to derive the benthic data have been published for seagrass habitats, however here we have adapted the methods for application to coral reef habitats, with the integration of automatic photoquadrat analysis. The approach presented is globally applicable for various submerged and benthic community ecological applications, and provides the basis for further studies at this site, regional to global comparative studies, and for the design of similar monitoring programs elsewhere.

Elevated Calf Mortality and Long-Term Responses of Wild Bottlenose Dolphins to Extreme Climate Events: Impacts of Foraging Specialization and Provisioning

As demands for wildlife tourism increase, provisioning has become a popular means of providing up-close viewing to the public. At Monkey Mia, Shark Bay, Australia, up to five adult female Indo-Pacific bottlenose dolphins (Tursiops aduncus) visit a 100 m stretch of beach daily to receive fish handouts. In 2011, a severe marine heatwave (MHW) devastated seagrass and fish populations in Shark Bay. Offspring survival declined precipitously among seagrass specialists (dolphins that forage disproportionately in seagrass habitat). As all provisioned dolphins at the site are seagrass specialists, we examined how provisioned and non-provisioned seagrass specialists responded to the MHW. Using 27 years of data we compare habitat use, home range size, calf mortality, and predation risk between provisioned and non-provisioned females and their offspring before and after the MHW. Our results show that provisioned females have extremely small home ranges compared to non-provisioned females, a pattern attributable to their efforts to remain near the site of fish handouts. However, weaned offspring (juveniles) born to provisioned females who are not provisioned themselves also had much smaller home ranges, suggesting a persistent maternal effect on their behavior. After the MHW, adult females increased their use of seagrass habitats, but not their home range size. Provisioned females had significantlylowercalf mortality than non-provisioned females, a pattern most evident pre-MHW, and, in the first 5 years after the MHW (peri-MHW, 2011–2015), calf mortality did not significantly increase for either group. However, the ecosystem did not recover, and post-MHW (2016–2020), calf mortality was substantially higher, regardless of provisioning status. With few survivors, the impact of the MHW on juvenile mortality post-weaning is not known. However, over three decades, juvenile mortality among offspring of provisioned vs. non-provisioned females did not statistically differ. Thus, the survival benefits accrued to calves in the provisioned group likely cease after weaning. Finally, although shark attack rates on seagrass specialists did not change over time, elevated predation on calves cannot be ruled out as a cause of death post-MHW. We discuss our results as they relate to anthropogenic influences on dolphin behavioral plasticity and responses to extreme climate events.

Performance Assessment of Posidonia oceanica (L.) Delile Restoration Experiment on Dead matte Twelve Years after Planting—Structural and Functional Meadow Features

Following the restoration of natural conditions by reducing human pressures, reforestation is currently considered a possible option to accelerate the recovery of seagrass habitats. Long-term monitoring programs theoretically represent an ideal solution to assess whether a reforestation plan has produced the desired results. Here, we report on the performance of a 20 m2 patch of Posidonia oceanica transplanted on dead matte twelve years after transplantation in the Gulf of Palermo, northwestern Sicily. Photo mosaic performed in the area allowed us to detect 23 transplanted patches of both regular and irregular shape, ranging from 0.1 to 2.7 m2 and an overall surface close to 19 m2. Meadow density was 331.6 ± 17.7 shoot m−2 (currently five times higher than the initial value of 66 shoots m−2), and it did not show statistical differences from a close by natural meadow (331.2 ± 14.9). Total primary production, estimated by lepidochronology, varied from 333.0 to 332.7 g dw m2/year, at the transplanted and natural stand, respectively. These results suggest that complete restoration of P. oceanica on dead matte is possible in a relatively short time (a decade), thus representing a good starting point for upscaling the recovery of the degraded meadows in the area.

Annotated Video Footage for Automated Identification and Counting of Fish in Unconstrained Seagrass Habitats

Technological advances are improving the collection, processing and analysis of ecological data. One of these technologies that has been adopted in recent studies by ecologists is computer vision (CV). CV is a rapidly developing area of machine learning that aims to infer image content at the same level humans can by extracting information from pixels (LeCun et al., 2015; Weinstein, 2018). CV in ecology has gained much attention as it can quickly and accurately process image from remote video imagery while allowing scientists to monitor both individuals and populations at unprecedented spatial and temporal scales. Automated analysis of imagery through CV has also become more accurate and streamlined with the implementation of deep learning (a subset of machine learning) models that have improved the capacity to processes raw images compared to traditional machine learning methods (LeCun et al., 2015; Villon et al., 2016). As the use of camera systems for monitoring fish abundances is common practice in conservation ecology (Gilby et al., 2017; Whitmarsh et al., 2017; Langlois et al., 2020), deep learning allows for the automated processing of big data from video or images, a step which usually creates a bottleneck when these data must be analyzed manually.

Comparing five methods for quantifying abundance and diversity of fish assemblages in seagrass habitat

Seagrass fishes perform key ecological roles and are a critical component of many of the world’s fisheries. A sound understanding of seagrass fish communities, based on robust methods, is therefore integral for their successful environmental management and conservation. Here we quantitatively compare taxa and size class of seagrass fish assemblages collected using; baited and unbaited remote underwater video (BRUV, URUV), diver operated video (DOV), underwater visual census (UVC) and trawling. We assess comparisons of these methods in terms of biodiversity indicators and power to detect change. All five methods were deployed in coastal embayments dominated by Posidonia spp. and within three marine parks along the south-western coast of Western Australia. Fish assemblages recorded using the different methods separated into two distinct groups; those dominated by smaller and less motile species, that are typically present beneath the canopy among seagrass leaves; and those dominated by larger, faster swimming species present above the seagrass canopy. Trawling provided the most effective method for assessing species within the canopy, including those of conservation concern. Conversely, BRUV followed by URUV recorded the greatest number of individuals, successfully detecting the larger supra-canopy and highly motile species typically omitted by trawling. Both trawl and BRUV methods had a higher power to detect change in species richness of seagrass fishes compared to other methods, even though each method recorded different components of the fish assemblage. Fish assemblages recorded using UVC detected both inter and supra-canopy species, but fish abundances recorded by this method were low compared to trawl and BRUV surveys, resulting in reduced power to detect change in richness. DOV was a comparatively less effective method, recording few fish, as well as low richness and diversity values. The combination of trawling and stationary video techniques, particularly BRUV, provided a two-method combination able to holistically measure commonly used indicators of fish assemblage composition in seagrass habitat. These results demonstrate that careful consideration of the methodology is essential when assessing ichthyofauna in seagrass habitat, particularly when only a single method can be employed. As the extent of methodological influence on recording fish assemblages also varied among the three survey areas, geographical differences in the structure of fish and seagrass communities should be considered when selecting an appropriate sampling method.

Ethnobotany of three sea grass species from Port Blair, a step towards its conservation in Andaman Islands

Ethno-phycology is the study of the relationships of human society to flora and its ecosystem. In the marine world, ethno-biology is a thriving area of research which have yielded valuable ethno-biological knowledge. The main goal of the present study was to understand the effect of abiotic factors as well as the natural and anthropogenic disturbances that shape seagrass community in Andaman Island and to reveal the knowledge of the local people, about the significance of seagrass habitat and its conservation. The qualitative study on distribution of seagrasses in Chidiyatapu (11° 29' 30" to 11° 30' 34" N and 92° 35' 10" to 92° 42' 30" E) was carried out during December2012 to February 2013. A total of three seagrass species such as Thalassia hempirichi (Ehrenberg) Ascherson, 1871, Halodule uninervis (R.Brown) J.D.Hooker 1858, Halophila ovalis (Forsskål) Ascherson 1882, where identified. Among them, T. hempirichi, and H. ovalis found in this study was reported to have the ethno-medicinal value from west coast of India. As a part of the study, the semi-structured survey was carried out among the local coastal people to analyse the Traditional Ecological Knowledge (TEK). The survey has revealed the unawareness of coastal residents about the medicinal, nutritional as well as conservational values of seagrasses. An effective implementation has to be taken to make them aware that seagrasses are fundamental components of healthy marine ecosystems and the local livelihoods that rely on them. The present findings provide the first report on the ethno-phycology of seagrasses from South Andaman Island.

Spatial Connectivity and Drivers of Shark Habitat Use Within a Large Marine Protected Area in the Caribbean, The Bahamas Shark Sanctuary

Marine protected areas (MPAs) have emerged as potentially important conservation tools for the conservation of biodiversity and mitigation of climate impacts. Among MPAs, a large percentage has been created with the implicit goal of protecting shark populations, including 17 shark sanctuaries which fully protect sharks throughout their jurisdiction. The Commonwealth of the Bahamas represents a long-term MPA for sharks, following the banning of commercial longlining in 1993 and subsequent designation as a shark sanctuary in 2011. Little is known, however, about the long-term behavior and space use of sharks within this protected area, particularly among reef-associated sharks for which the sanctuary presumably offers the most benefit. We used acoustic telemetry to advance our understanding of the ecology of such sharks, namely Caribbean reef sharks (Carcharhinus perezi) and tiger sharks (Galeocerdo cuvier), over two discrete islands (New Providence and Great Exuma) varying in human activity level, over 2 years. We evaluated which factors influenced the likelihood of detection of individuals, analyzed patterns of movement and occurrence, and identified variability in habitat selection among species and regions, using a dataset of 23 Caribbean reef sharks and 15 tiger sharks which were passively monitored in two arrays with a combined total of 13 acoustic receivers. Caribbean reef sharks had lower detection probabilities than tiger sharks, and exhibited relatively low habitat connectivity and high residency, while tiger sharks demonstrated wider roaming behavior across much greater space. Tiger sharks were associated with shallow seagrass habitats where available, but frequently transited between and connected different habitat types. Our data support the notion that large MPAs afford greater degrees of protection for highly resident species such as Caribbean reef sharks, yet still may provide substantial benefits for more migratory species such as tiger sharks. We discuss these findings within the context of species-habitat linkages, ecosystem services, and the establishment of future MPAs.

A multidisciplinary approach to identify priority areas for the monitoring of a vulnerable family of fishes in Spanish Marine National Parks

BackgroundSyngnathid fishes (Actinopterygii, Syngnathidae) are flagship species strongly associated with seaweed and seagrass habitats. Seahorses and pipefishes are highly vulnerable to anthropogenic and environmental disturbances, but most species are currently Data Deficient according to the IUCN (2019), requiring more biological and ecological research. This study provides the first insights into syngnathid populations in the two marine Spanish National Parks (PNIA—Atlantic- and PNAC—Mediterranean). Fishes were collected periodically, marked, morphologically identified, analysed for size, weight, sex and sexual maturity, and sampled for stable isotope and genetic identification. Due the scarcity of previous information, habitat characteristics were also assessed in PNIA.ResultsSyngnathid diversity and abundance were low, with two species identified in PNIA (Hippocampus guttulatus and Syngnathus acus) and four in PNAC (S. abaster, S. acus, S. typhle and Nerophis maculatus). Syngnathids from both National Parks (NP) differed isotopically, with much lower δ15N in PNAC than in PNIA. The dominant species were S. abaster in PNAC and S. acus in PNIA. Syngnathids preferred less exposed sites in macroalgal assemblages in PNIA and Cymodocea meadows in PNAC. The occurrence of very large specimens, the absence of small-medium sizes and the isotopic comparison with a nearby population suggest that the population of Syngnathus acus (the dominant syngnathid in PNIA) mainly comprised breeders that migrate seasonally. Mitochondrial cytochrome b sequence variants were detected for H. guttulatus, S. acus, and S. abaster, and a novel 16S rDNA haplotype was obtained in N. maculatus. Our data suggest the presence of a cryptic divergent mitochondrial lineage of Syngnathus abaster species in PNAC.ConclusionsThis is the first multidisciplinary approach to the study of syngnathids in Spanish marine NPs. Habitat preferences and population characteristics in both NPs differed. Further studies are needed to assess the occurrence of a species complex for S. abaster, discarding potential misidentifications of genus Syngnathus in PNAC, and evaluate migratory events in PNIA. We propose several preferential sites in both NPs for future monitoring of syngnathid populations and some recommendations for their conservation.

Exposure of coastal environments to river-sourced plastic pollution

Marine litter is a global problem which poses an increasing threat to ecosystem services, human health, safety and sustainable livelihoods. In order to better plan plastic pollution monitoring and clean-up activities, and to develop policies and programmes to deter and mitigate plastic pollution, information is urgently needed on the different types of coastal ecosystem that are impacted by land-sourced plastic inputs, especially those located in proximity to river mouths where plastic waste is discharged into the ocean. We overlayed the most current existing information on the input of plastic to the sea from land-based sources with maps of coastal environments and ecosystems. We found an inverse relationship exists between coastal geomorphic type, plastic trapping efficiency and the mass of plastic received. River-dominated coasts comprise only 0.87% of the global coast and yet they receive 52% of plastic pollution delivered by fluvial systems. Tide-dominated coasts receive 29.9% of river-borne plastic pollution and this is also where mangrove and salt marsh habitats are most common. Wave-dominated coasts receive 11.6% of river-borne plastic pollution and this is where seagrass habitat is most common. Finally, rocky shores comprise 72.5% of the global coast, containing fjords and coral reefs, while only receiving 6.4% of river-borne plastic pollution. Mangroves are the most proximal to river-borne plastic pollution point sources of the four habitat types studied here; 54.0% of mangrove habitat is within 20 km of a river that discharges more than 1 t/yr of plastic pollution into the ocean. For seagrass, salt marsh and coral reefs the figures are 24.1%, 22.7% and 16.5%, respectively. The findings allow us to better understand the environmental fate of plastic pollution, to advance numerical models and to guide managers and decision-makers on the most appropriate responses and actions needed to monitor and reduce plastic pollution.

Variation in Species Composition, Size and Fitness of Two Multi-Species Sea Turtle Assemblages Using Different Neritic Habitats

The neritic environment is rich in resources and as such plays a crucial role as foraging habitat for multi-species marine assemblages, including sea turtles. However, this habitat also experiences a wide array of anthropogenic threats. To prioritize conservation funds, targeting areas that support multi-species assemblages is ideal. This is particularly important in the Gulf of Mexico where restoration actions are currently ongoing following the Deepwater Horizon oil spill. To better understand these areas in the Gulf of Mexico, we characterized two multi-species aggregations of sea turtles captured in different neritic habitats. We described species composition and size classes of turtles, and calculated body condition index for 642 individuals of three species captured from 2011 to 2019: 13.6% loggerheads (Caretta caretta), 44.9% Kemp’s ridleys (Lepidochelys kempii) and 41.4% green turtles (Chelonia mydas). Species composition differed between the two sites with more loggerheads captured in seagrass and a greater proportion of green turtles captured in sand bottom. Turtles in sand bottom were smaller and weighed less than those captured in seagrass. Although small and large turtles were captured at both sites, the proportions differed between sites. Body condition index of green turtles was lower in sand habitat than seagrass habitat; there was no difference for Kemp’s ridleys or loggerheads. In general, smaller green turtles had a higher body condition index than larger green turtles. We have identified another habitat type used by juvenile sea turtle species in the northern Gulf of Mexico. In addition, we highlight the importance of habitat selection by immature turtles recruiting from the oceanic to the neritic environment, particularly for green turtles.

A STUDY ON THE LIGHT ATTENUATION CAUSED BY PERIPHYTON IN THE BURGAS BAY, THE BLACK SEA

Quantifying PAR attenuation caused by periphyton is a crucial step in setting good management criteria for seagrass habitats protection. To help forward the process of its implementation in the Bulgarian coastal waters we set out to verify experimentally this relationship within several perennial, shallow, sublittoral seagrass meadows. Four types of functional relationships between the dry weight and the PAR quantity have been tested: exponential rise to a maximum, Michaelis-Menten, natural logarithm, negative exponential function (both constrained and unconstrained form). The exponential rise to a maximum proved to be the most appropriate curve for a description of the data under the current experiment. The coefficients obtained: 73.22 (71.02 ÷ 75.48, 95% confidence interval) and 0.8299 (0.7507 ÷ 0.9300, 95% confidence interval) are in the range reported on other experimental studies. The application of a variable for evaluation of the effect of the species composition, especially within the higher loads interval, would improve the curve precision. Nevertheless, it can be useful to assess the seagrasses habitat suitability and the risk of light stress in the Burgas Bay (the Black Sea).

Thresholds in catchment nitrogen load for shifts from seagrass to nuisance macroalgae in shallow intertidal estuaries

AbstractEutrophication of shallow coastal ecosystems often manifests as dense mats of opportunistic macroalgae, degraded underlying sediments and displacement of ecologically important primary producers including seagrass beds. Ecological thresholds of drivers that cause these shifts in ecosystem state are needed to manage eutrophication symptoms before tipping points are crossed. This study quantifies total nitrogen (TN) load thresholds for abrupt changes in ecologically important seagrass habitat and the appearance of nuisance macroalgae as a function of estimated catchment TN load to shallow, intertidal estuaries in New Zealand. Results indicate that the extents of both macrophyte types are strongly governed by increasing TN load to the estuaries, while total suspended sediment load was not significant alone or as an interaction term. Seagrass extent was unpredictable below a critical TN load envelope (< 50 mg N m−2 d−1), likely because there are additional factors than TN that influence seagrass cover, but once surpassed, seagrass became restricted to less than 10% of intertidal area or absent. Conversely, nuisance macroalgae was predominantly absent unless TN load exceeded 41.15 ± 21.1 mg N m−2 d−1, expanding in extent by 2.7% of intertidal area for every 50 mg N m−2 d−1 increase above threshold levels. This research underscores the importance of managing diffuse nitrogen inputs to safeguard the structure and function of shallow estuarine ecosystems before tipping points in dominant macrophyte type are crossed.

Sparse Gaussian process for online seagrass semantic mapping

Over recent decades, we have witnessed a widescale deterioration in seagrass habitats in the Mediterranean Sea. Although the causes underlying this regression are not yet clear, there is a broad consensus that new methods are required to generate high-quality data on seagrass meadows. Specifically, there is a lack of available data in order to assess changes over time in the spatial distribution of seagrass. This article introduces a new methodology to ease data-gathering operations in Posidonia oceanica meadows for benthic mapping, by using the latest developments in lightweight autonomous vehicles and image processing. The proposed methodology has been designed to build a series of online maps, by employing an input data feed based on an encoder–decoder convolutional neural network (CNN) that automatically segments the images recorded by the vehicle. In turn, it uses a sparse Gaussian Process (GP) to map uncertainty linked to the spatial distribution of seagrass. This article presents the most suitable CNN and GP, as well as performance validation results for the method, based on data acquired in three field tests.