Coral Reef Islands Research Articles

Coastal flooding and mean sea-level rise allowances in atoll island

Atoll islands are among the places most vulnerable to climate change due to their low elevation above mean sea level. Even today, some of these islands suffer from severe flooding generated by wind-waves, that will be exacerbated with mean sea-level rise. Wave-induced flooding is a complex physical process that requires computationally-expensive numerical models to be reliably estimated, thus limiting its application to single island case studies. Here we present a new model-based parameterisation for wave setup and a set of numerical simulations for the wave-induced flooding in coral reef islands as a function of their morphology, the Manning friction coefficient, wave characteristics and projected mean sea level that can be used for rapid, broad scale (e.g. entire atoll island nations) flood risk assessments. We apply this new approach to the Maldives to compute the increase in wave hazard due to mean sea-level rise, as well as the change in island elevation or coastal protection required to keep wave-induced flooding constant. While future flooding in the Maldives is projected to increase drastically due to sea-level rise, we show that similar impacts in nearby islands can occur decades apart depending on the exposure to waves and the topobathymetry of each island. Such assessment can be useful to determine on which islands adaptation is most urgently needed.

Brilliantia kiribatiensis, a new genus and species of Cladophorales (Chlorophyta) from the remote coral reefs of the Southern Line Islands, Pacific Ocean.

The marine green alga Brilliantia kiribatiensis gen. et sp. nov. is described from samples collected from the coral reefs of the Southern Line Islands, Republic of Kiribati, Pacific Ocean. Phylogenetic analysis of sequences of the large- and small-subunit rDNA and the rDNA internal transcribed spacer region revealed that Brilliantia is a member of the Boodleaceae (Cladophorales), containing the genera Apjohnia, Boodlea, Cladophoropsis, Chamaedoris, Phyllodictyon, and Struvea. Within this clade it formed a distinct lineage, sister to Struvea elegans, but more distantly related to the bona fide Struvea species (including the type S. plumosa). Brilliantia differs from the other genera by having a very simple architecture forming upright, unbranched, single-celled filaments attached to the substratum by a rhizoidal mat. Cell division occurs by segregative cell division only at the onset of reproduction. Based on current sample collection, B. kiribatiensis seems to be largely restricted to the Southern Line Islands, although it was also observed on neighboring islands, including Orona Atoll in the Phoenix Islands of Kiribati, and the Rangiroa and Takapoto Atolls in the Tuamotus of French Polynesia. This discovery highlights the likeliness that there is still much biodiversity yet to be discovered from these remote and pristine reefs of the central Pacific.

Conservation implications of forage base requirements of a marine predator population at carrying capacity

SummaryPrey depletion may contribute to marine predator declines, yet the forage base required to sustain an unfished population of predatory fish at carrying capacity is unknown. We integrated demographic and physiological data within a Bayesian bioenergetic model to estimate annual consumption of a gray reef shark (Carcharhinus amblyrhynchos) population at a remote Pacific atoll (Palmyra Atoll) that are at carrying capacity. Furthermore, we estimated the proportion of the atoll's reef fish biomass production consumed by the gray reef sharks, assuming sharks either partially foraged pelagically (mean 7%), or solely within the reef environment (mean 52%). We then predicted the gray reef shark population potential of other, less remote Pacific Ocean coral reef islands, illustrating that current populations are substantially smaller than could be supported by their forage base. Our research highlights the utility of modeling how far predator population sizes are from their expected carrying capacity in informing marine conservation.

Characterizing the spatial distribution of coral reefs in the South-Central Coast region of Viet Nam using Planetscope imagery.

This study aims to understand the spatial distribution of coral reefs in the central region of Viet Nam. We classified live coral cover in Son Tra Peninsula (ST) and Cu Lao Cham Island (CLC) in the South-Central Coast Region of Viet Nam using the Maximum Likelihood Classifier on 3 m Planetscope imagery. Confusion matrices and the accuracy of the classifier were assessed using field data (1,543 and 1,560 photographs in ST and CLC, respectively). The results showed that the reef’s width ranged from 30 to 300 m across the study site, and we were able to detect live coral cover across a depth gradient of 2 to 6 m below the sea surface. The overall accuracies of the classifier (the Kappa coefficient) were 76.78% (0.76) and 78.08% (0.78) for ST and CLC, respectively. We found that 60.25% of coral reefs in ST were unhealthy and the live coral cover was less than 50%, while 25.75% and 11.46% of those in CLC were in good and excellent conditions, respectively. This study demonstrates the feasibility of utilizing Planetscope imagery to monitor shallow coral reefs of small islands at a high spatial resolution of 3 m. The results of this study provide valuable information for coral reef protection and conservation.

Temporal and Spatial Variabilities of Chemical and Physical Parameters on the Heron Island Coral Reef Platform

Temporal and Spatial Variabilities of Chemical and Physical Parameters on the Heron Island Coral Reef Platform

Global decline in capacity of coral reefs to provide ecosystem services

Global decline in capacity of coral reefs to provide ecosystem services

Indonesian coral reef habitats reveal exceptionally high species richness and biodiversity of diatom assemblages

We present the results of the first ever research on biodiversity and biogeography of marine benthic diatoms from coral reefs of the tropical islands of Indonesia, including West Borneo, East Java, South Celebes, Komodo, Rinca and Flores. The sampled islands offer a broad range of coral reef microhabitats and host diatom assemblages from seaweeds, sea grass, dead corals, scrapes from other biological surfaces, sand, rocks and other solid surfaces. The results of this study are based on the light (LM) and scanning electron microscopy (SEM) identification of diatoms only. Our taxonomic analysis revealed the presence of 911 diatom taxa representing 188 genera. Biodiversity indices such as Shannon, Simpson and Evenness as well as species richness and diatom relative abundance were calculated for each sample examined. We find that these indices were higher for Indonesia than those published for tropical oceanic islands or regions from both hemispheres, e.g. Galapagos and Martinique, but comparable to Madagascar. The non-metric multidimensional analysis showed that the type of habitat from which a particular sample was taken was of greater significance in the taxonomic composition than the geographical aspect. In terms of biogeography, cosmopolitan taxa were dominant in terms of relative abundance, but species richness was higher within tropical zonal and local forms. Many taxa were identified only at the genus level, and may turn out to be new to science.

Numerical modelling of the wave interaction with revetment breakwater built on reclaimed coral reef islands in the South China Sea—Experimental verification

The South China Sea (SCS) is an important channel, which plays a significant role in global economic trade and in the maintenance of world energy security. A series of artificial lands have been successfully built on the top of natural coral reefs in the SCS by the way of reclamation in recent years. In order to prevent those artificial lands from wave scouring and impacting, a great number of revetments and breakwaters have been constructed along the margin of these artificial lands. The revetment breakwaters have great significance and practical value to ensure the stability of these reclaimed lands, and to guarantee their normal long-term service performance. In this study, taking the reclamation project in the SCS as the engineering background, a computation model for the interaction between ocean waves, revetment breakwater and its calcareous coral sand foundation is established by taking the CFD solver OlaFlow as the computation platform which was developed based on the open source library OpenFOAM. Then this established computation model is verified by some laboratory testing data of wave profile and wave impact which have been measured in several wave flume physical model tests. The comparison between the testing data and the computational results indicates that the computation model established adopting OlaFlow can reliably simulate the wave generation, propagation, the dissipation of wave energy as well as the complicated interaction between ocean wave, the revetment breakwater and its calcareous coral sand foundation. This verification work will be a solid basis for the subsequent investigation of the interaction between severe ocean waves and the revetment breakwaters in large-scale, as well as the quantitative evaluation of the stability of the revetment breakwater build on reclaimed coral sand foundation in the SCS.

Multi-decadal planform changes on coral reef islands from atolls and mid-ocean reef platforms of the equatorial Pacific Ocean: Gilbert Islands, Republic of Kiribati

Constructed of poorly lithified carbonate sediments with elevations of <3–5 m above MSL, coral reef islands are considered extremely vulnerable to the effects of anthropogenic climate change, particularly sea-level rise. While some studies have shown islands are dynamic landforms with the ability to physically adjust their shape and position on the reef platform, large geographical gaps still exist in reef island change records. This study presents a multi-decadal analysis of shoreline changes and planform geomorphic adjustments across coral reef islands in the equatorial Pacific from the Gilbert archipelago, Republic of Kiribati. Shoreline positions of 71 islands from 3 atolls and 4 mid-ocean reef platforms were analysed by comparing historical aerial photographs (from 1940s and 1960s) and recent satellite imagery covering a period of local sea-level rise rate of ~2.2 mm/year. Results show ~47% of the shorelines were characterised by statistically significant accretion leading to a net increase of 274.07 ha (2.45%) of planform land area. A comparative examination of changes on platform-islands and atoll-islands revealed platform-islands were relatively more stable (mean rate of 0.01 m/year) with significantly less variability in shoreline change rates, while atoll-islands were characterised by a higher mean rate of accretion (0.09 m/year). Further analysis of change rates across various shoreline exposure categories, demonstrate high dynamism across atoll-island shorelines is a function of distinctive local-scale processes occurring within atoll settings; lagoon-facing shorelines were characterised by the highest rate of accretion (0.14 m/year), spits were found to be highly dynamic and embayment infilling accounted for ~9% of the net increase in land area across the study islands. Results are instructive in defining styles of morphological change across various geomorphic settings to inform projections of future island adjustments, assessments of attribution of island change and formulation of adaptation responses across atoll nations.

High frequency dependence of sound speed and attenuation in coral sand sediments

The acoustic properties of seafloor coral sand sediments are of significant meaning for accurately modeling underwater sound propagation in sea areas around coral reef islands. To ultrasonically measure coral sand sediments with different grain sizes, sediment samples from coral islands were screened and remolded into five types of samples with different grain sizes. This study measured the sound speed and attenuation in the frequency range of 27–247 kHz and analyzed the frequency dependence of the acoustic properties. The results showed that the sound speed increased significantly with frequency, suggesting an approximately linear relation. The sound speed dispersion in the coarse-grained sediments was larger than that in the fine-grained sediments. The measured attenuation data showed a variable frequency dependence among the different sediment types. The measured data were compared with Biot-Stoll model (BS model) predictions, and the fitting results showed that intergranular pores can lead to sound speed data-model mismatches. The attenuation data-model comparisons revealed that sediments with different grain sizes have different sound wave energy loss mechanisms. The BS model with grain contact squirt flow and shear drag (BICSQS model) can match the attenuation data more consistently than the BS model. The results show that scattering is an important factor in sound attenuation in coral sand sediment and the attenuation scattering factors for different grain size sediments were provided.

Experimental study of filtration & fine particles migration of calcareous sand in coral reef island

Experimental study of filtration & fine particles migration of calcareous sand in coral reef island

Quiet oceans: United States Virgin Islands coral reef soundscapes in the age of coronavirus

The COVID-19 pandemic has led to unprecedented changes in human behavior around the world. These changes have impacted local soundscapes in both terrestrial and marine environments resulting in a global ‘quieting’ event that has allowed for the opportunity to evaluate the activity of soniferous organisms in unique acoustic environments. Across the world’s oceans, patterns of commercial, research, and recreational vessel traffic have all been altered, with corresponding alterations in the anthropogenic contribution to underwater noise. To assess these changes we used long-term acoustic recorders at three different coral reef sites on the Southern coast of the island of St. John in the United States Virgin Islands (USVI). We compared conditions pre-coronavirus outbreak (2019) and post-outbreak (2020) and found a decrease in the incidence of boat noise over the course of the pandemic. Because anthropogenic noise tends to overlap with the low frequencies used by marine fishes and invertebrates for sound production and detection, we predicted that changes in boat noise patterns would lead to concurrent changes in biological sound production of coral reef organisms. Understanding the impacts of this “quiet time” on marine organismal activity in the USVI could play a role in worldwide coral reef acoustic studies and conservation efforts.

Sediment supply dampens the erosive effects of sea-level rise on reef islands

Large uncertainty surrounds the future physical stability of low-lying coral reef islands due to a limited understanding of the geomorphic response of islands to changing environmental conditions. Physical and numerical modelling efforts have improved understanding of the modes and styles of island change in response to increasing wave and water level conditions. However, the impact of sediment supply on island morphodynamics has not been addressed and remains poorly understood. Here we present evidence from the first physical modelling experiments to explore the effect of storm-derived sediment supply on the geomorphic response of islands to changes in sea level and energetic wave conditions. Results demonstrate that a sediment supply has a substantial influence on island adjustments in response to sea-level rise, promoting the increase of the elevation of the island while dampening island migration and subaerial volume reduction. The implications of sediment supply are significant as it improves the potential of islands to offset the impacts of future flood events, increasing the future physical persistence of reef islands. Results emphasize the urgent need to incorporate the physical response of islands to both physical and ecological processes in future flood risk models.

Grimontia sedimenti sp. nov., isolated from benthic sediments near coral reefs south of Kuwait.

A Gram-stain-negative, rod and rod-curved shaped motile bacterium designated strain S25T was obtained from benthic sediment collected near the Kubbar Island coral reefs south of Kuwait. Phenotypic analysis revealed that strain S25T was slightly halophilic, mesophilic and facultative anaerobic, fermenting d-glucose, d-ribose, d-mannose, d-mannitol, maltose, fructose, gentiobiose, cellobiose, melibiose, trehalose and sucrose. It was positive for oxidase and indole production and negative for arginine dihydrolase and lysine and ornithine decarboxylases. It contained C16 : 1 ω7c/C16 : 1 ω6c (summed feature 3), C18 : 1 ω7c (summed feature 8) and C16 : 0 as the major fatty acids. Strain S25T grew optimally at 30 °C and pH 8 in the presence of 3 % (w/v) NaCl. Phylogenetic analysis based on 16S rRNA sequences revealed that strain S25T is related to species of the genus Grimontia, having 99.15 % similarity to ‘Grimontia indica’ AK16T, 99.08 % to Grimontia celer 96-237T and 98.66 % to Grimontia marina IMCC 5001T. The DNA G+C content was 48.8 mol% and the full genome analysis for the strain S25T showed that the bacterium has a genome size of 5 158 621 bp and contains 4730 predicted protein-encoding genes. The average nucleotide identity values between the S25T genome and the genomes of its nearest matches ranged between 81.39 and 94.16 %. The strain was distinguishable from the phylogenetically related genera through differences in several phenotypic properties. On the basis of the phenotypic, phylogenetic and genetic data, strain S25T represents a novel species in the genus Grimontia, for which the name Grimontia sedimenti sp. nov. is proposed. The type strain of Grimontia sedimenti is S25T (=DSM 28878T=LMG 28315T).

Development of a comprehensive assessment model for coral reef island carrying capacity(CORE-CC)

Coral reef islands provide precious living space and valuable ecological services for human beings, and its sustainability cannot be ignored under the pressure of human activities. Carrying capacity (CC) assessment has gradually become an important means to measure sustainability of islands. However, there is little comprehensive evaluation of the carrying capacity of coral reef islands, and traditional evaluation methods are difficult to express the social-ecological characteristics of coral reef islands. The present paper proposes a comprehensive assessment model for coral reef island carrying capacity (CORE-CC) which comprises dimensions of resources supply, environmental assimilative, ecosystem services, and socio-economic supporting. According to the characteristics of the coral reef islands, the core factors and indicators of each dimension are selected and the corresponding assessment index system of "pressure-support" is constructed. The assessment involves (1) identification of carrying dimensions and core factors, (2) pressure/support measurement and (3) assessment of carrying state. A case study is conducted in Zhaoshu Island of China, demonstrating the applicability of CORE-CC model and serving as a reference for adaptive management.

Numerical Modelling of the Creep Subsidence of an Ocean Lighthouse Constructed on a Reclaimed Coral Reef Island

This work is to investigate the creep subsidence of a large-scale ocean lighthouse constructed on a reclaimed calcareous coral sand foundation (abbreviated to RCCSF thereafter) in the South China Sea (SCS). Firstly, two modified Burgers creep models, referred to as modified HBM and KBM, respectively, are proposed and validated based on several triaxial creep test results, and then both are implanted into ABAQUS. Secondly, the model parameters of the modified Huang’s Burgers Creep Model (HBM) and Kong’s Burgers Creep Model (KBM) for the RCCSF are carefully calibrated for the subsequent numerical modelling according to the creep tests conducted by us. Thirdly, based on the modified HBM and KBM proposed in this study and the calibrated model parameters, the subsidence of an ocean lighthouse constructed on a reclaimed land in the South China Sea is numerically analyzed. The numerically predicted results illustrate that the creep subsidence of the ocean lighthouse in the future 50-year is 17.2 mm to 42.5 mm, meeting the mandatory requirement (<200 mm) stipulated by the China national design code. Finally, the sensitivity of the predicted creep subsidence to the model parameters is screened by performing parametric analysis.

Role of Future Reef Growth on Morphological Response of Coral Reef Islands to Sea‐Level Rise

AbstractCoral reefs are widely recognized for providing a natural breakwater effect that modulates erosion and flooding hazards on low‐lying sedimentary reef islands. Increased water depth across reef platforms due sea‐level rise (SLR) can compromise this breakwater effect and enhance island exposure to these hazards, but reef accretion in response to SLR may positively contribute to island resilience. Morphodynamic studies suggest that reef islands can adjust to SLR by maintaining freeboard (island crest elevation above still water level) through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remains unknown. Here we show, using a process‐based morphodynamic model, that, although reef growth significantly affects wave transformation processes and island morphology, it does not lead to decreased coastal flooding and island inundation. According to the model, reef islands evolve during SLR by attuning their elevation to the maximum wave runup and islands fronted by a growing reef platform attain lower elevations than those without reef growth, but have similar overwash regimes. The mean overwash discharge Qover across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a Qover value of O (10 l m−1 s−1) separating island construction from destruction. Islands, therefore, can grow vertically to keep up with SLR via flooding and overwash if specific forcing and sediment supply conditions are met, offering hope for uninhabited and sparely populated islands. However, this physical island response will negatively impact infrastructure and assets on developed islands.

Shoreline changes in coral reef islands of the Federated States of Micronesia since the mid-20th century

With projections of accelerating sea-level rise, changing weather and wave regimes and increasing intensity of storm events associated with anthropogenic climate change, the physical persistence and consequently habitability of coral reef islands is a global concern. This study presents an analysis of shoreline changes on 104 coral reef islands from 16 atolls in the western equatorial Pacific nation of the Federated States of Micronesia across a period coincident with rising local sea level and a high frequency of storm events. Aerial photographs from the mid-1940s and 1970s were analysed alongside recent high-resolution satellite imagery to document shoreline changes and planform morphological adjustments in islands. Results revealed accretion has been the predominant mode of shoreline change, with 46% of the studied shorelines showing statistically significant accretion leading to a net increase of 64.37 ha (~3%) of planform land area across the archipelago. Three new islands were formed with a cumulative land area of 2.05 ha, conversely, three islands were completely eroded leading to a loss of net land area of 3.20 ha, providing a record of concurrent island formation and loss within a high storm frequency setting. Reef islands in the FSM demonstrated a range of styles of planform geomorphic adjustments including island rotation, expansion, contraction, spit extension, and migration of islands on the reef platform. Results provide empirical evidence of shoreline accretion despite local sea-level rise as well as of heterogeneity in magnitude and style of geomorphic responses of reef islands across FSM with considerable variability at inter and intra-atoll scales.

Diversity and distribution of scleractinian corals from Mandapam group of Islands in Gulf of Mannar marine national park, South East coast of India

The Mandapam group of Islands being a part of the Gulf of Mannar Marine National Park shows a variety of marine biodiversity in each of the seven Islands. A Scleractinian coral shows a rich diversity in the study islands during the study period. The present study will help us to get a summarized species distribution in the Mandapam group of Islands. The maximum percentage of hard coral cover was recorded in the Poomarichan Island followed by Single Island, Manoliputti Island, Manoli Island, Hare Island, Kurusadai Island and Pullivasal Island. A total of 69 species of scleractinian corals were listed from the coral reef Islands in the study sites of Mandapam group. Statistical analysis such as Principal Component Analysis and Correspondence Analysis supported the status of maximum coral cover in Poomarichan Island.

Spatial modelling of Acropora muricata and Porites lutea distribution using environmental descriptors across Lakshadweep–Chagos Archipelago

Globally, climatic and anthropogenic forcings are causing the catastrophic decline of coral reef ecosystems, which sustain a plethora of marine life and support the livelihoods of several millions of people. Lakshadweep–Maldives–Chagosarchipelago (LMC) forms one of the largest chains of atoll systems in the world, and due to remoteness from the mainland, its islands boast a unique set of flora and fauna. The coral reefs of these tropical islands are highly vulnerable to stressors such as climate change, overfishing, monsoon runoff, and ocean acidification. To understand and manage these sensitive ecosystems, knowledge about the existing coral cover and distribution patterns are essential. In the present study, habitat modelling of the two corals Acropora muricata (Linnaeus, 1758) and Porites lutea (Milne Edwards & Haime, 1851) were carried out using the Maximum Entropy (MaxEnt) model to predict the probability of occurrence using remotely sensed environmental variables as predictors. The average test AUC values of 0.980 and 0.974, respectively, for A. muricata and P. lutea as estimated by MaxEnt shows that the model performance for both the species is outstanding. The average uncertainty (standard deviation) was about 0.012 and 0.021 respectively. It is found that the bathymetry is the variable having the highest contribution followed by Calcite and Phosphate for the distribution of both the species. The results of this study throw light on the probable occurrence of coral reefs in many of the hitherto unknown areas, especially the submerged banks and seamounts in the region. Much of these areas are less explored and have strategic positional advantages in increasing the ecosystem connectivity of the region. Furthermore, the relationship between coral distribution and the environmental variables as predicted by this study will be valuable in future conservation activities and designing marine protected areas.