Reef Islands Research Articles

A satellite-derived bathymetry method combining depth invariant index and adaptive logarithmic ratio: A case study in the Xisha Islands without in-situ measurements

Accurate bathymetric data is crucial for various aspects such as marine resource exploitation and marine ecological conservation. Currently, satellite-derived bathymetry (SDB) based on empirical and physical models has been widely utilized in constructing underwater terrain in shallow seas. However, the application of such SDB models is limited in remote island reef areas lacking in-situ measurement data. To overcome this issue, the manuscript proposes an unconstrained SDB optimization method without in-situ measurement data, utilizing satellite multispectral imagery (Geoeye-1) and spaceborne LiDAR data (ICESat-2). By classifying the seafloor substrate in coral reef areas into sandy and coral, based on the depth invariant index (DII), we employ an adaptive logarithmic ratio model for unconstrained SDB. The ICESat-2 LiDAR data are then used to correct the SDB results, achieving bathymetry optimization in the coral reef area of the Xisha Islands. Additionally, the proposed method is applied to Yuanzhi Island of the Xisha Islands, and the accuracy of the bathymetric results is evaluated against ALB (Airborne LiDAR Bathymetry) data. The findings demonstrate that compared to conventional methods, our method can improve the accuracy of SDB results with good adaptability. In the Yuanzhi Island area, the proposed method yields SDB results with an R2 of 0.93, an MAE (Mean Absolute Error) of 0.94, and an RMSE (Root Mean Square Error) of 1.12 m, compared to ALB data. The average error is less than 10 % of the maximum depth, essentially meeting the requirements of the International Hydrographic Organization (IHO) standards for depth measurement error when depth is <20 m. This study can offer a novel approach for enhancing bathymetric accuracy around offshore and remote islands, where gathering underwater terrain data is challenging.

Integrated physiological response by four species of Rhodophyta to submarine groundwater discharge reveals complex patterns among closely-related species

Algal physiological ecology on submarine groundwater discharge (SGD) influenced reefs is likely shaped by intermittent, tidally-driven estuarine conditions that occur with SGD fluxes of fresh-to-brackish groundwater from the subterranean estuary to reef ecosystems. SGD is a common inconspicuous feature worldwide on reefs of basaltic high islands and continental margins. Yet, SGD-driven dynamics of algal physiology are not well understood. To understand how invasive species have physiologically outcompeted native species on many SGD-influenced reefs, physiology in tissue water potential (TWP) regulation, photosynthesis, nitrogen storage, and cellular anatomy were measured across a gradient of SGD-influence, for four Rhodophyte species. Compared with non-SGD conditions, SGD was associated with higher TWP, larger medulla cells with thinner walls, and thinner cortical cell walls for two invasives, Gracilaria salicornia and Acanthophora spicifera, higher photosynthetic rates in G. salicornia, greater nitrogen concentration for A. spicifera and G. salicornia, and increased δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\delta$$\\end{document}15N ratios for A. spicifera, G. salicornia, and native Laurencia dendroidea. Distinct physiological strategies were measured for the two invasive species across the gradient of SGD-influence, and for L. dendroidea and Gracilaria perplexa offshore. This study illuminates species-specific physiological response, and how introduced opportunistic species may outcompete native species under conditions of SGD.

APPLICATION OF AMINO ACID RACEMIZATION TO THE DATING OF FORAMINIFERS AND THE IDENTIFICATION OF REMANIÉ FOSSILS AND REWORKED SEDIMENTS

ABSTRACT The reworking of sediments and fossils is a pervasive element of the stratigraphical record and is frustrated by the fact that the taphonomic nature of fossils may not always reliably indicate the presence of remanié fossils. In this work, the extent of racemization of amino acids, a measure of fossil age based on an increasing ratio of D- to L-amino acids, is used to evaluate the integrity of the sedimentary record in selected case studies, based on the analysis of single foraminifers or samples containing fewer than 10 individuals. The short lifespan of foraminifers (c. 2–24 months), enhances their application for defining the age of sedimentary events, as they more closely relate to the age of the depositional event. The selected case studies range in spatial scale from defining the taphonomically active zone in geologically recent shallow marine sediments, to the application of foraminifers in reef island sediment budgets in Tarawa, to the reworking of foraminifers from Late Pleistocene calcarenites into modern beach sediments. The species Elphidium macelliforme, Amphistegina sp., and Lamellodiscorbis dimidiatus are shown to be particularly suited for amino acid racemization investigations of sediment reworking.

Beyond hinges and spires: A critical examination of archaeomalacological quantification methodologies using coral reef molluscan assemblages from Jiigurru (Lizard Island Group), northern Great Barrier Reef

Quantification, as one of the pillars of the zooarchaeological subdiscipline, is an invaluable component of the toolkit researchers use to study past people-animal interactions. Despite being the subject of rigorous (zoo)archaeological debate, the calculation of Minimum Number of Individuals (MNI) values remains one of the main methods of quantifying the relative abundance of taxa within faunal assemblages. Choosing the appropriate quantification protocol to calculate the MNI of archaeological invertebrate assemblages can be challenging due to regional taxonomic considerations and the myriad of quantification methodologies and frameworks available in the global archaeomalacological literature. In an Australian context, methodologies for quantifying coral reef molluscan assemblages have not been explicitly evaluated. Using archaeological molluscan assemblages from two midden sites (Freshwater Bay Midden and Mangrove Beach Headland Midden) on the northern Great Barrier Reef island group of Jiigurru (the Lizard Island Group), we critically examine two commonly adopted archaeomalacological quantification methodologies: the NRE MNI and tMNI protocols. The NRE MNI methodology uses one to two non-repetitive elements (NREs) of molluscs, whilst the tMNI protocol includes a wider range of elements akin to vertebrate MNI quantification methodologies. Through a comparison of taxa abundances and statistical analyses, results show that the tMNI protocol, with some modification, is best suited for the Jiigurru assemblages. Higher MNI values and an increased assemblage diversity, evenness, and richness were recorded for the molluscan assemblages at both midden sites when the tMNI protocol was applied. This study foregrounds the importance of data transparency when reporting quantification protocols and outcomes to ensure the highest degree of data quality, replicability, and usability.

Insights from tensile fracture properties and full-field strain evolution of deep coral reef limestone under dynamic loads

Coral reef limestone (CRL) commonly undergoes dynamic tension when underground structures of island reefs encounter impacts, explosions, or seismic activities. Given the complexity of biological pores, the dynamic tensile fracture characteristics of CRL are poorly understood. Therefore, the dynamic tensile fracture behaviors of deep CRL were systematically observed by Split Hopkinson Pressure Bar tests and digital image techniques. Comparing to traditional rocks, the macro-pores near failure band would significantly change cracking path. The failure patterns are dominated by loading rate. The strongly dependence of dynamic tensile strength and dynamic crack initiation toughness on loading rate suggests the two indices overcome the effect of CRL macro-pores under dynamic impacts. At low loading rates, tensile fractures predominantly follow intergranular cracks, whereas transgranular cracks dominate at higher rates. The fractal dimension of fracture surface decreases with increasing crack propagation velocity, loading rate, and dynamic crack initiation toughness. Due to the unique marine sedimentary environment, the mechanical heterogeneity in multiple scales distinguishes CRL from terrestrial rock materials. The insights into underlying mechanisms of dynamic tension provide support to optimization of blasting scheme and stability assessments for island underground engineering.

Microeukaryotic communities varied along the gradient in the seaweed habitats: A case study in a typical reef island in the East China Sea

Seaweeds are ecologically important primary producers, forming unique habitats. Microeukaryotes are pivotal in ecological functioning of seaweeds, but their ecological role and processes in seaweed habitats remain poorly understood due to a lack of field data. Here we presented an innovative insights into the microeukaryotic communities along the seaweed habitat gradients using 18S rDNA sequencing around a typical reef island, Gouqi Island. We set and verified habitat gradients of brown (HB, mainly Sargassum) and green algae (HG, mainly Ulva), and the control site between them (M), with detritus samples collected from collectors by SCUBA (Self-Contained Underwater Breathing Apparatus). Along the seaweed habitats, microeukaryotic biodiversity decreased from the detritus close to the seaweed habitats to surface water with increasing distance from detritus. For the dominant ecological function, clear distinction was also found in detritus and water samples along the seaweed habitat gradient, where phototrophs, mixotrophs and phagotrophs dominated in the detritus of brown algae dominated site, medium site, and green algae dominated site, respectively, responding differently to the environmental factors and seaweeds by Mantel analysis. Furthermore, we found that stochastic processes dominated in microeukaryotic community assembly in the brown seaweed habitat whereas deterministic processes dominated in green algae habitat. Highly varied occupancy was revealed in the green algae habitat by Specificity-Occupancy (SPEC-OCCU) analysis. Our result contributed to a better understanding of the ecological functioning of microeukaryotic communities in seaweed habitats, and further provided data for the bioassessment and conservation of seaweed habitats within marine ecosystems.

In-situ CT scan-based analysis of damage evolution of coral reef limestone under cyclic loads

The subtropical hurricane, ocean circulation, and excavation disturbance by deep engineering of island reefs bring many challenges for the insight into the mechanical behaviors of coral reef limestone (CRL) under cyclic loads. This paper investigated the damage characteristics of coral gravel limestone (CGL) under triaxial cyclic loads based on the in-situ computed tomography (CT) technique and digital volume correlation method (DVC). The results suggested that the plastic strain of CGL still existed after unloading in the stage of linear elastic deformation. Crack initiation within the specimen occurred at the stress level of about 0.72, and accelerated damage occurred when the stress level exceeded 0.86. Subsequently, we utilized DVC technique to visualize the evolution process of 3-D strain field under cyclic loads. The strain field of the specimen generated strain release in the locality of crack initiation, and the heterogeneous degree of strain field firstly increase and then decrease with the increase of upper limit pressure. Moreover, the secant modulus exhibits a trend consistent with the heterogeneous degree of strain field. Therefore, we introduced the concept of self-information entropy to characterized the heterogeneous degree of strain field, attempting to describe the process of compression and damage of sample. The evolving relationship between secant modulus and axial strain of CGL under cyclic loads was established based on the heterogeneity of strain field. The proposed model provides promising way to predict secant modulus of porous rock materials based on-site CT testing and in-situ measurement in practical engineering.

Investigation of mechanical characterization and damage evolution of coral reef sand concrete using in-situ CT and digital volume correlation techniques

Coral reef sand concrete (CRSC) plays an indispensable role in island reef projects, but limited knowledge of its damage poses safety and stability concerns for coral reef sand concrete structures. Therefore, the in-situ computed tomography (CT) is employed to investigate the strength and deformation, and pore structure evolution of coral reef sand concrete under uniaxial load. Additionally, the crack propagation and three-dimensional strain field evolution under different stress states are revealed by the digital volume correlation (DVC) techniques. The results demonstrate that when the stress is below 35.79 MPa, the porosity gradually decreases as the stress increases. However, when the stress exceeds 35.79 MPa, the local porosity increases with the rising stress, and the porosity at 39.89 MPa is higher than the initial porosity. Moreover, stresses evidently concentrate at areas of local maximum porosity, resulting in the initiation and development of cracks in those regions. During the failure process, CRSC experiences splitting damage firstly caused by longitudinal cracks, followed by the extension of these cracks into oblique cracks, resulting in shear damage. Coral aggregate is the weak phase in CRSC, exhibiting transgranular fractures. Additionally, CRSC exhibits significant strain in the pores and coral aggregates, leading to crack development along the direction of the pores or coral aggregates. Therefore, this study offers a deeper understanding of the damage mechanisms of CRSC, providing theoretical support for its further research and practical application in far-sea projects.

Computational study on complex wave hydrodynamics of multidirectional extreme waves at fringing reef

Due to the special topography of coral reefs, most of the energy of incoming waves can be reduced through the wave breakings at the reef edge, thus playing a vital role in protecting the coastal regions behind coral reefs. In severe weather conditions, wind-generated waves tend to propagate in multiple directions in real ocean environment. The abrupt multidirectional wave-focusing mechanisms may result in the formation of huge waves in both deep and shallow waters, threatening the safety of coastal infrastructures and well-being of coastal communities at the reef islands. Therefore, it is crucial to thoroughly examine the complex hydrodynamics of multidirectional extreme waves at the fringing reefs. Previous studies on reef wave hydrodynamics predominantly focus on unidirectional waves. Only a small number of studies have considered the wave hydrodynamics of fringing reefs under multidirectional waves, let alone multidirectional extreme waves. To address the knowledge gap in previous studies, this study analyzes the complex hydrodynamic processes of multidirectional extreme waves at the fringing reef by applying a nonhydrostatic numerical wave model (NHWAVE). Influences of several key factors are analyzed, such as significant wave height, peak wave period, water depth, incident wave angle, and reef topography. It is expected that the research findings of this study will contribute to a deeper understanding of the hydrodynamics of multidirectional extreme waves at the fringing reef.

On the mechanical behaviour of a coral silt from the South China Sea

During land reclamation on the reef islands, large amounts of silt-sized coral soils were created by segregation and degradation. The accumulation of silt-sized coral soils is fairy uncommon while the research on the geotechnical properties of the coral silt is very limited. In this study, a systematic experimental investigation on the mechanical behaviour of a coral silt obtained from a reclaimed reef island in the South China Sea has been performed, with comparisons to the coral sand collected from the same area. Similar to the coral sand, the coral silt particles also exhibit irregular particle shape and intra-particle pore due to their nature origin. According to the limiting water contents, the coral silt is classified as a low-plasticity clayey silt. Under one-dimensional compression, the coral silt exhibits a much quicker convergence compared to coral sand. Prior to convergence, the compressibility of coral silt is higher. After yielding, the compressibility of coral sand becomes higher due to significant particle breakage. The loose coral silt subjected to undrained shearing at low confining pressures exhibits obvious strain softening behaviour, indicating static liquefaction or flow failure. The peak and critical state friction angles of coral silt are lower than those of coral sand but much higher than those of the other terrigenous clayey silts. A curved critical state line with well-defined horizontal asymptote in the deviatoric stress – mean effective stress plane is identified for coral silt, again indicating higher potential to static liquefaction.

Experimental study on impact performance of seawater sea-sand concrete with recycled aggregates

On islands distant from the mainland, obtaining raw materials for concrete production is often more challenging. To achieve sustainable development in island reef engineering, using discarded marine concrete and coral waste generated during island construction as recycled aggregates are of considerable significance. The preparation of Recycled Coral Aggregate Concrete (RCAC) for island reef engineering thus holds substantial importance. In this study, RCAC and Natural Aggregate Concrete (NAC), both designed with a compressive strength of C60, were prepared. Initially, the fundamental physical properties of the recycled coarse aggregate, such as apparent density, water absorption, and crushing index, were determined. Subsequently, a comparative analysis of the quasi-static mechanical properties of RCAC with varying proportions of recycled coral coarse aggregate (RCCA) was conducted. Furthermore, the impact compression mechanical properties of different RCAC specimens under various strain rates were examined using the Ф100mm Split Hopkinson Pressure Bar (SHPB) apparatus. The microstructure and long-term drying shrinkage performance of RCAC were also analyzed using Scanning Electron Microscopy (SEM) and a drying shrinkage apparatus. The finding indicated that the 28-day compressive strength of RCAC specimens with 100% coarse aggregate replacement reached a maximum of 62.4 MPa. The quasi-static compressive strength of RCAC specimens with 50% and 100% RCCA replacement was only 11.5% and 14.2% lower than that of NAC, respectively. Under impact loading, the dynamic compressive strength of RCAC specimens increased with the strain rate, with peak stress exhibiting an approximately linear relationship with the strain rate. The energy dissipation of RCAC specimens generally occurred in three stages, with the reflected and absorbed energies of the specimens increasing linearly with strain rate. At the same strain rate, the transmitted energy of RCAC specimens was higher than that of NAC specimens. Microstructural analysis revealed that the morphology of recycled coral aggregate is characterized by its porous and rough surface. The interfacial transition zone between the recycled coral aggregate and the cement mortar was relatively dense. Incorporating recycled coarse aggregate significantly affected the drying shrinkage properties of the concrete, with higher contents of RCCA leading to greater drying shrinkage rates.

Evolution characteristics of calcareous sand force chain based on particle breakage

Abstract Calcareous sand is easily broken under external force, which brings great difficulties to island reef engineering. Based on the particle flow program, a discrete element model that can reproduce the results of laboratory tests is established, the large principal stress method is introduced to identify the particle force chain, and the bond strength between particles is increased to obtain an unbreakable model with the same initial conditions, and different confining pressures are compared and analyzed. The evolution law of the force chain of the following two models establishes a macro-meso cross-scale analysis in the deformation process of calcareous sand, explores the internal mechanism of the crushing of calcareous sand particles. The results show that particle breakage plays an important role in the evolution of the force chain. Particle breakage will reduce the probability of the force chain on both sides of the axis, forcing the probability of the axial force chain to rise steadily. The macroscopic deviatoric stress is the external manifestation of the probability of the axial force chain on the meso level. The faster the probability of the force chain in the direction of the potential shear band increases, the more obvious the shear band is.

Feeding biology of crown-of-thorns seastars across sites differing in Acropora availability

Crown-of-thorns seastars (COTS, Acanthaster spp.) are a major contributor to coral mortality across the Indo-Pacific and can cause extensive reef degradation. The diet preferences of COTS can influence coral community structure by predation on fast-growing genera such as Acropora and avoidance of rare coral genera. In non-outbreaking populations, this preference can increase species diversity. The feeding biology of Acanthaster cf. solaris was compared at two sites (Shark Alley and Second Lagoon) on One Tree Island reef, located in the southern Great Barrier Reef, to determine whether the availability of Acropora influences differences in COTS movement, feeding preference and feeding rates within the same reef system. Acanthaster cf. solaris were tracked daily for five days across both sites, with measurements of movement, feeding scars and coral composition recorded over this time. While Shark Alley and Second Lagoon have similar live coral cover (40 and 44 % respectively), Shark Alley has significantly lower Acropora availability than Second Lagoon (2 vs 32 %). The feeding rate of COTS was significantly different between Shark Alley and Second Lagoon (259.8 and 733.8 cm2 of coral per day, respectively), but did not differ between seastar size (25–40 cm and >40 cm). Acanthaster cf. solaris showed preference for Pocillopora, Seriatopora, Acropora and Isopora and an avoidance of Porites at both sites. The results suggest that for coral reef sites where Acropora is not dominant, COTS outbreaks may be less likely to initiate, with comparatively low feeding rates found in comparison to coral reefs where Acropora is dominant.

Practice of Territorial Use Rights in Fisheries in Coastal Fishery Management in China: A Case Study of the Island Reefs Fishery Lease Policy from Shengsi County in Zhejiang Province

Coastal fisheries are vital for food supply, employment, and social stability. However, overfishing is a common problem, often attributed to open access. Territorial Use Rights in Fisheries (TURFs) have gained popularity as a tool to improve the sustainability of coastal fisheries, but research on TURFs in China is limited. This paper examines the island reef fishery lease policy implemented in Shengsi County, Zhejiang Province, within the Ma’an Islands National Marine Special Reserve. The policy leases the use of island reefs to a collective, aiming to address overfishing and resource degradation by establishing fishing area boundaries and limiting the number of users and usage time. Technically, it is an application of TURFs, and it represents a shift from traditional fishing licenses to collective compliance. This experience has disrupted the existing top–down fishery governance structure and signifies a transition to a rights-based fishery management system in China. Through a comprehensive investigation and analysis of the policy’s implementation, this research identifies the factors contributing to its flaws. These include the inadequate provision of national and local government policies, insufficient support for policy implementation from fishery management authorities, and the limited involvement of fishermen’s organizations. To enhance coastal fishery management, we propose that the Chinese government should build upon existing foundations by clarifying fishing rights through central and local laws and policies. Additionally, there is a need to strengthen data monitoring of coastal fisheries, conduct multidisciplinary research to improve the allocation mechanism of fishing rights, adopt diverse fishery management approaches to enhance supervision capabilities, establish a collaborative governance mechanism, and foster coordination and cooperation between grassroots fishermen’s organizations and government departments.

Effects of Environmental and Climatic Changes on Coral Reef Islands.

Coral reef islands are low-lying, wave-deposited sedimentary landforms. Using an eco-morphodynamic framework, this review examines the sensitivity of islands to climatic and environmental change. Reef island formation and morphological dynamics are directly controlled by nearshore wave processes and ecologically mediated sediment supply. The review highlights that reef islands are intrinsically dynamic landforms, able to adjust their morphology (size, shape, and location) on reef surfaces in response to changes in these processes. A suite of ecological and oceanographic processes also indirectly impact hydrodynamic and sediment processes and thereby regulate morphological change, though the temporal scales and magnitudes of impacts on islands vary, leading to divergent morphodynamic outcomes. Climatic change will modify the direct and indirect processes, causing complex positive and negative outcomes on islands. Understanding this complexity is critical to improve predictive capabilities for island physical change and resolve the timescales of change and lag times for impacts to be expressed in island systems.

Acoustic monitoring of artificial reefs reveals Atlantic cod and weakfish spawning and presence of individual bottlenose dolphins.

The artificial reefs in New York's waters provide structure in areas that are typically flat and sandy, creating habitat for a multitude of species as an area to spawn, forage, and reside. Passive acoustic data collected on the Fire Island and Shinnecock artificial reefs between 2018 and 2022 detected spawning-associated calls of weakfish (Cynoscion regalis) and Atlantic cod (Gadus morhua), as well as the presence of individual bottlenose dolphins (Tursiops truncatus) through their signature whistles. Weakfish and Atlantic cod were more vocally active on the Fire Island reef, where Atlantic cod grunts peaked during a new moon phase in December, and weakfish spawning experienced variable peaks between mid-July and mid-August on both reefs. Fifty-seven individual bottlenose dolphins were identified, with whistle repeats ranging from seconds to years apart. Passive acoustic monitoring allows for simultaneous collection of information on multiple species at different trophic levels as well as behavioral information that helps managers understand how these animals utilize these habitats, which can lead to improved conservation measures.

Operational characteristics of an integrated island energy system based on multi-energy complementarity

This study addresses the intermittent renewable energy supply and the large footprint of battery storage on an island reef in China by proposing an integrated energy system that incorporates hydrogen production, storage, and utilisation. Mathematical models for wind and photovoltaic power generation, energy storage, hydrogen production and utilisation, diesel generators, and energy management systems are established. Additionally, an integrated energy system is constructed using Simulink software to simulate and analyse its operational characteristics in various seasonal scenarios. The simulation results indicate that the surplus from wind power and photovoltaic systems, after supplying the load for over 70 % of the day, constitutes more than 40 % of the load demand. The energy storage system can maintain a maximum charging rate for more than 50 % of the day and a maximum discharge rate for less than 16.7 % of the day. The electrolysis tank produces hydrogen for more than 71 % of the day and simultaneously consumes up to 750 % of the load. The fuel cell consumes hydrogen for less than 12.5 % of the day and simultaneously provides up to 27 % of the load. Analysis revealed that the hydrogen system improves the energy utilisation, the control strategy realises stable supply and demand within the system.

Modeling of extreme waves in the South China Sea under the influence of tropical cyclones: long-term statistical perspective in Xisha reef islands

Modeling of extreme waves in the South China Sea under the influence of tropical cyclones: long-term statistical perspective in Xisha reef islands

Spatial prioritization of dugong habitats in India can contribute towards achieving the 30 × 30 global biodiversity target

Indian coastal waters are critical for dugong populations in the western Indian Ocean. Systematic spatial planning of dugong habitats can help to achieve biodiversity conservation and area-based protection targets in the region. In this study, we employed environmental niche modelling to predict suitable dugong habitats and identify influencing factors along its entire distribution range in Indian waters. We examined data on fishing pressures collected through systematic interview surveys, citizen-science data, and field surveys to demarcate dugong habitats with varying risks. Seagrass presence was the primary factor in determining dugong habitat suitability across the study sites. Other variables such as depth, bathymetric slope, and Euclidean distance from the shore were significant factors, particularly in predicting seasonal suitability. Predicted suitable habitats showed a remarkable shift from pre-monsoon in Palk Bay to post-monsoon in the Gulf of Mannar, indicating the potential of seasonal dugong movement. The entire coastline along the Palk Bay-Gulf of Mannar region was observed to be at high to moderate risk, including the Gulf of Mannar Marine National Park, a high-risk area. The Andaman Islands exhibited high suitability during pre- and post-monsoon season, whereas the Nicobar Islands were highly suitable for monsoon season. Risk assessment of modelled suitable areas revealed that < 15% of high-risk areas across Andaman and Nicobar Islands and Palk Bay and Gulf of Mannar, Tamil Nadu, fall within the existing protected areas. A few offshore reef islands are identified under high-risk zones in the Gulf of Kutch, Gujarat. We highlight the utility of citizen science and secondary data in performing large-scale spatial ecological analysis. Overall, identifying synoptic scale ‘Critical Dugong Habitats’ has positive implications for the country's progress towards achieving the global 30 × 30 target through systematic conservation planning.

Thermal tolerance as a driver of reef fish community structure at the isolated tropical Mid-Atlantic Ridge Islands

Reef fish communities are shaped by historical and ecological factors, including abiotic and biotic mechanisms at different spatial scales, determining species composition, abundance and biomass. The oceanic islands in the Mid-Atlantic Ridge (St. Peter and St. Paul's Archipelago - SPSPA, Ascension, and St. Helena), exhibiting differences in community structure along a 14-degree latitudinal and a 10 °C thermal gradient. We investigate the influence of sea surface temperature, area, age, isolation and phosphate on reef fish community structures. Reef fish trophic structure varies significantly across the islands, with planktivores and herbivore-detritivores showing the highest abundances in SPSPA and Ascension, while less abundant in St. Helena, aligning with the thermal gradient. Variations in reef fish community structures were predominantly influenced by thermal regimes, corroborating the expansion of species' thermal niche breadth at higher latitudes and lower temperatures. This study highlights that in addition to biogeographic factors, temperature is pivotal on shaping oceanic island reef fish community structure.