Coral Reef Islands Research Articles

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.

Environmental Health Impacts on The Sustainability of The Coral Reef Ecosystem in Gede Island, Indonesia

Gede Island, located north of Rembang Regency in Central Java, Indonesia, hosts coral reefs in poor environmental health. This study aimed to assess the impact of environmental health on various dimensions of sustainability to guide stakeholder decision-making. A descriptive quantitative method was employed. Observations of coral reef variables, including type and percentage of cover, were conducted using the Point Intercept Transect (PIT) method. The sustainability status of management practices was analysed using the Multidimensional Scaling (MDS) method with Rapfish (Rapid Assessment Techniques for Fisheries) 2013 version R software. The results identified 16 genera from three sampling points: Station 1 had 30.35% hard coral cover, Station 2 had 1%, and Station 3 had 12.94%. The sustainability status of Gede Island's coral reefs was evaluated across five attributes: ecological (41.61%), social (48.05%), economic (40.08%), institutional (26.36%), and technological (57.03%). Among these, only the technological attribute was classified as moderately sustainable, while the others were deemed less sustainable. A high sustainability status offers significant potential and benefits for the local community, such as opportunities for ecotourism and fisheries sustainability.

Environmental DNA recovers fish composition turnover of the coral reefs of West Indian Ocean islands.

Islands have been used as model systems to study ecological and evolutionary processes, and they provide an ideal set-up for validating new biodiversity monitoring methods. The application of environmental DNA metabarcoding for monitoring marine biodiversity requires an understanding of the spatial scale of the eDNA signal, which is best tested in island systems. Here, we investigated the variation in Actinopterygii and Elasmobranchii species composition recovered from eDNA metabarcoding along a gradient of distance-to-reef in four of the five French Scattered Islands in the Western Indian Ocean. We collected surface water samples at an increasing distance from reefs (0 m, 250 m, 500 m, 750 m). We used a metabarcoding protocol based on the 'teleo' primers to target marine reef fishes and classified taxa according to their habitat types (benthic or pelagic). We investigated the effect of distance-to-reef on β diversity variation using generalised linear mixed models and estimated species-specific distance-to-reef effects using a model-based approach for community data. Environmental DNA metabarcoding analyses recovered distinct fish species compositions across the four inventoried islands and variations along the distance-to-reef gradient. The analysis of β-diversity variation showed significant taxa turnover between the eDNA samples on and away from the reefs. In agreement with a spatially localised signal from eDNA, benthic species were distributed closer to the reef than pelagic ones. Our findings demonstrate that the combination of eDNA inventories and spatial modelling can provide insights into species habitat preferences related to distance-to-reef gradients at a small scale. As such, eDNA can not only recover large compositional differences among islands but also help understand habitat selection and distribution of marine species at a finer spatial scale.

Biological soil crust elicits microbial community and extracellular polymeric substances restructuring to reduce the soil erosion on tropical island, South China Sea

Soil erosion stands as the preeminent environmental concern globally, attaining heightened significance, particularly within islands where land resources prove notably scarce. Biological soil crusts, referred to as biocrusts, assume a pivotal ecological role in soil conservation. Notably, they augment the horizontal stability of the substrate through the exudation of microbial extracellular polymeric substances (EPS), thereby shielding the soil against shear stress, exemplified in the form of water erosion. While extant research has delved into the anti-erosion mechanisms of biocrusts in arid landscapes, a conspicuous lacuna persists in the exploration of coral island environments. In this study, we collected and assessed 30 samples encompassing dark biocrusts, light biocrusts, and bare soil to scrutinize the potential anti-erosion efficacy of tropical coral island biocrusts within the South China Sea. Employing a cohesive strength meter, we quantified soil shear stress across various stages of biocrust development, revealing a discernible enhancement in soil erosion resistance during the formation of biocrusts. Relative to the exposed bare soil, the soil shear stress exhibited an escalation from 0.33 N m−2 to 0.61 N m−2 and 1.31 N m−2 in the light biocrusts and dark biocrusts, respectively. Mechanistically, we assayed microbial EPS contents, exposing a positive correlation between EPS and soil anti-erodibility, encompassing extracellular protein and polysaccharide. Concurrently, bacterial abundance displayed a significant augmentation commensurate with biocrust formation and development. In pursuit of elucidating the origin of EPS, high-throughput amplicon sequencing was executed to identify microorganisms contributing to biocrust development. Correlation analysis discerned Cyanobacteria, Chloroflexi, Deinococcota, and Patescibacteria as potential microbials fostering EPS production and fortifying erosion resistance. Collectively, our study presents the first evidence that biocrust from tropical coral reef island in the South China Sea promotes resistance to soil erosion, pinpointing key EPS-producing microbials against soil erosion. The findings would provide insights for island environment restoration.

Meta‐Study of Carbonate Sediment Delivery Rates to Indo‐Pacific Coral Reef Islands

AbstractCoral reef islands are amongst the most vulnerable environments to sea‐level rise (SLR). Recent physical and numerical modeling studies have demonstrated that overwash processes may enable reef islands to keep up with SLR through island accretion. Sediment supply to these islands from the surrounding reef system is critical in understanding their morphodynamic adjustments, but is poorly constrained due to insufficient knowledge about sediment delivery rates. This paper provides the first estimation of sediment delivery rates to coral reef islands. Analysis of topographic and geochronological data from 28 coral reef islands indicates an average rate of sediment delivery of c. 0.1 m3 m−1 yr−1, but with substantial inter‐island variability. Comparison with carbonate sediment production rates from census‐based studies suggests that this represents one quarter of the amount of sediment produced on the reef platform. Results of this study are useful in future modeling studies for predicting morphodynamic adjustments of coral reef islands to SLR.

Linking Transpiration to Reef Nitrogen Supply on a Tropical Coral Island

AbstractCoral reef islands are biodiversity hotspots with high conservation value, but we have a poor understanding of how island vegetation, through transpiration, influences groundwater nutrient supply to adjacent reef systems. Here we combine stable isotope tracing, geophysical surveys, and satellite analysis to unravel the links between transpiration and the discharge of groundwater nitrate to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). Over a 2‐year study period (2020–2021) there was a net loss of freshwater from the island, that is, evapotranspiration (ET) exceeded rainfall, largely caused by high rates of transpiration (73% of ET). Transpiration was higher in forested areas, and groundwater salinity appeared to be higher there. By tracing the nitrogen stable isotope signature of groundwater nitrate into reef organism tissue we were able to map groundwater nitrogen discharge spatially and temporally. Groundwater nitrogen discharge was focussed on one side of the island and did not vary seasonally as expected, despite reduced rainfall and seabird guano inputs over the austral winter/spring. Based on our results, we propose that transpiration by island vegetation slows groundwater flow and concentrates nitrate in the groundwater being released to the surrounding reef system. High concentrations of nitrate in groundwater (up to 27 mmol L−) were observed on Lady Elliot Island and these seem to have increased since 2014, but further work is required to understand if this is a normal scenario for tropical coral islands or the result of the islands' revegetation program.

Putting the State in its Place. An Ethnographic Look at Tuvalu’s Sovereignty in the Context of the Ecological Crisis

Composed entirely of low-lying coral atolls and reef islands, the Polynesian microstate of Tuvalu is at risk of being submerged by sea levels rising due to global warming. Such circumstance raises unprecedented scenarios: were it to become uninhabitable, in fact, Tuvalu would be the first state to lose its sovereignty due to a physical damage to its territory. However, traditional understandings of sovereignty are of little use in assessing Tuvalu’s political consistency, and speculations on the future of the country, such as those that predict its disappearance, are at risk of overlooking the specificity of its history. Combining a historical and an ethnographic approach with an analysis of recent land conflicts that arose in the capital of the country, this article explores the question of Tuvalu’s future within a) the long history of institutional vicissitudes that led a disparate group of islands to become a sovereign state; and b) a study of the dynamics underlying the political texture of Tuvalu and the complex architecture of its sovereignty.

Manfaat Ekonomi Kelangsungan Ekosistem Terumbu Karang di Perairan Pulau Enggano Provisi Bengkulu: Sebuah Telaah Pustaka

Enggano Island has several potential biological resources that can improve the economy in the area, one of which is coral reefs. The biological resource potential of coral reef ecosystems in the Enggano Island area is one of the mainstays of community livelihoods, namely as a fishing ground for fishermen. This activity has been carried out for a long time and the intensity is getting higher from time to time, so it is necessary to conduct a study to calculate the value of the economic benefits of coral reef ecosystems in the Enggano Island area in the form of direct and indirect economic value for the benefit of sustainable management and utilisation. This literature review aims to provide information to various parties related to the Economic Benefits of Coral Reef Ecosystem Sustainability in Enggano Island Waters, Bengkulu Province. The results of the review of previous studies showed that the value of direct economic benefits amounted to Rp. 561,327,640, indirect benefits amounted to Rp. 46,342,500,000, optional benefits amounted to Rp. 2,516,077,983, and the benefits of the existence of the Enggano Island coral reef ecosystem amounted to Rp. 127,425,000,000. All of these economic benefits have been felt by fishing communities and other user communities.

Morphological Response of Coral Reef Islands to the 2019 Extreme Positive Indian Ocean Dipole Event

Morphological Response of Coral Reef Islands to the 2019 Extreme Positive Indian Ocean Dipole Event

Holocene Depositional History of Low-Lying Reef-Rim Carbonate Islets of Fakarava Atoll, Northwest Tuamotu, Central South Pacific

Assumptions about the fate of low-lying coral reef islands (atolls) facing global warming are poorly constrained, due to insufficient information on their depositional history. Based on the U/Th dating of 48 coral clasts, the chronostratigraphic analysis of excavated sections through rim islets (motu) at the windward and leeward sides of Fakarava Atoll (Tuamotu, French Polynesia) reveal that the deposition of coral detritus started approximately 2000 years ago. Most of these deposits lie on conglomerate pavements or reef flat surfaces, and are about 4500 to 3000 years old. The islet expansion at the windward sites seems to have operated coevally across the reef rim, from the ocean-facing shore lagoonwards. Meanwhile, well-developed, continuous, elongated, vegetated islets mostly occur along the windward, northeast to southeast coast, and isolated islets, vegetated or not, associated with the dense networks of conglomerates, are common on the leeward, partly submerged, western rim. Islet accretion on the windward rim sides is believed to have been mainly triggered by winter storms and occasional cyclonic events, whilst the leeward atoll parts were most likely shaped by distant-source swells from mid to high latitudes. The projections of the accelerated sea level rise in the future suggest that the long-term islet stability at Fakarava could be altered because the islets have accreted under the conditions of the falling sea level.

Potential PCR amplification bias in identifying complex ecological patterns: Higher species compositional homogeneity revealed in smaller-size coral reef zooplankton by metatranscriptomics.

PCR-based high-throughput sequencing has permitted comprehensive resolution analyses of zooplankton diversity dynamics. However, significant methodological issues still surround analyses of complex bulk community samples, not least as in prevailing PCR-based approaches. Marine drifting animals-zooplankton-play essential ecological roles in the pelagic ecosystem, transferring energy and elements to higher trophic levels, such as fishes, cetaceans and others. In the present study, we collected 48 size-fractionated zooplankton samples in the vicinity of a coral reef island with environmental gradients. To investigate the spatiotemporal dynamics of zooplankton diversity patterns and the effect of PCR amplification biases across these complex communities, we first took metatranscriptomics approach. Comprehensive computational analyses revealed a clear pattern of higher/lower homogeneity in smaller/larger zooplankton compositions across samples respectively. Our study thus suggests changes in the role of dispersal across the sizes. Next, we applied in silico PCR to the metatranscriptomics datasets, in order to estimate the extent of PCR amplification bias. Irrespective of stringency criteria, we observed clear separations of size fraction sample clusters in both metatranscriptomics and in silico datasets. In contrast, the pattern-smaller-fractioned communities had higher compositional homogeneity than larger ones-was observed in the metatranscriptomics data but not in the in silico datasets. To investigate this discrepancy further, we analysed the mismatches of widely used mitochondrial CO1 primers and identified priming site mismatches likely driving PCR-based biases. Our results suggest the use of metatranscriptomics or, although less ideal, redesigning the CO1 primers is necessary to circumvent these issues.

Risk classification of low-lying coral reef islands and their exposure to climate threats

The bio-physical responses of low-lying coral islands to climate change are of concern. These islands exist across a broad range of bio-physical conditions, and vulnerabilities to rising and warming seas, ocean acidification and increased storminess. We propose a risk-based classification that scores 6 island eco-morphometric attributes and 6 bio-physical ocean/climate conditions from recent open-access data, to assign islands with respect to 5 risk classes (Very Low, Low, Moderate, High and Very High). The potential responses of 56 coral islands in Australia's jurisdiction (Coral Sea, NW Shelf and NE Indian Ocean) to climate change is considered with respect to their bio-physical attributes and eco-morphometrics. None of the islands were classed as Very Low risk, while 8 were classed as Low (14.3 %), 34 were Moderate (60.7 %), 11 were High (19.6 %), and 3 were Very High (5.4 %). Islands in the Very High risk class (located on the NW Shelf) are most vulnerable due to their small size (mean 10 Ha), low elevation (mean 2.6 m MSL), angular/elongated shape, unvegetated state, below average pH (mean 8.05), above average rates of sea-level rise (SLR; mean 4.6 mm/yr), isolation from other islands, and frequent tropical storms and marine heatwaves. In contrast, islands in the Low (and Very Low) risk class are less vulnerable due to their large size (mean 127 Ha), high elevation (mean 8.5 m MSL), sub-angular/round shape, vegetated state, near average pH (mean 8.06), near average SLR rates (mean 3.9 mm/yr), proximity to adjacent islands, and infrequent cyclones and marine heatwaves. Our method provides a risk matrix to assess coral island vulnerability to current climate change related risks and supports future research on the impacts of projected climate change scenarios. Findings have implications for communities living on coral islands, associated ecosystem services and coastal States that base their legal maritime zones on these islands.

Human visitation disrupts natural determinants of breeding seabird communities on coral reef islands

Coral Reef Islands (CRIs) support key colonies of tropical seabirds, whose long-term sustainability is under growing threat from climate change and small-scale, island-specific pressures such as invasive species or human disturbance. Unlike most compartments of reef ecosystems, CRI seabirds have been poorly studied from a biogeographical perspective, which questions the relevance of existing conservation policies. Here, we investigate the effects of ecological, geographical and anthropogenic drivers on the diversity and abundance of CRI seabird communities, using survey data acquired over the 2000–2023 period in 100 CRIs of New Caledonia, South-western Pacific. Our results show that the diversity and biomass of breeding communities are mostly constrained by the availability of breeding habitat and by the characteristics of neighboring marine environments. We observe a negative effect of human visitation on both diversity and biomass, indicating that the mitigation of human disturbance combined with habitat maintenance are key for sustaining seabird colonies on CRIs. Our findings highlight the complementary roles of remote and coastal islands, which host distinct species assemblages, and demonstrate that although isolated islands are often seen as optimal conservation targets, they only host part of the diversity of CRI seabirds. This study emphasizes the importance of combining species and site monitoring for assessing the ability of managers to effectively restore the state of seabird communities and contribute to the health of adjacent reef ecosystems. In addition to local policies, we also underline that future management of CRI-dependent seabird communities must integrate multi-scale, long-term assessments of reef decline and coastal erosion.

Honduran Reef Island Shoreline Change and Planform Evolution over the Last 15 Years: Implications for Reef Island Monitoring and Futures

Assessing the vulnerability of low-lying coral reef islands is a global concern due to predictions that climate and environmental change will increase reef island instability and cause reef island populations to be among the first environmental refugees. Reef islands in the Pacific and Indian Oceans are highly dynamic environments that morphologically adjust to changing environmental conditions over annual-decadal timescales. However, there is a paucity of reef island shoreline change data from the Caribbean where sea-level rise, ecological and environmental disturbance and hydrodynamic regimes are considerably different than in other oceans globally. Here we present shoreline change analysis of 16 reef islands in northern Honduras, at the southern end of the Mesoamerican Barrier Reef. Satellite imagery from a maximum period of 12.4 years from Utila (2006–2019), and 2.4 years from Cayos Cochinos (2018–2021) was analysed to quantify island shoreline change and planform morphological adjustments. We identified accretion as the dominant island behaviour in Utila, where 5 of 7 islands increased in area and 61.7% of shorelines accreted, contributing to an overall net area increase of 9.4%. Island behaviour was more variable in Cayos Cochinos, where 55.7% of shorelines eroded, 5 of 9 islands remained stable, and net island area change was insignificant (2%). Conversely, the 4 smallest Cayos Cochinos islands (all <1500 m2) experienced significant shoreline change, potentially highlighting a new size threshold for considering reef island evolution. Across both sites, reef islands demonstrated a range of modes of planform change, including lateral accretion and erosion, and migration. Consequently, we provide the first empirical evidence of the dynamic nature of Caribbean reef islands during a period coincident with sea-level rise and highlight the heterogeneous nature of reef island evolution between and within two neighbouring sites at timescales relevant for island adaptation efforts.

Numerical modelling of the physical response of coral reef Sandy island to sea level rise by considering seasonal patterns

Sea level rise (SLR) can increase the bathymetry of coral reef platforms, thereby increasing the potential for island erosion and flooding to damage coastal buildings. However, SLR is a threat to the coastal buildings of islands not only in this way, but also in the destruction of coastal buildings due to island morphology changes caused by SLR. Based on sediment, offshore wave, and tidal data from Yongxing Island (sandy island), Xuande Atoll, South China Sea, a process-based morphodynamic model, XBeach-G, was used to predict the morphological evolution of coral reef sandy island under future SLR based on the RCP8.5 scenario. As waves and tides in the South China Sea are distinctly seasonal, the influence of seasonal patterns was considered in the model. Sensitivity analysis of the effects of water level on the reef, offshore significant wave height, sediment porosity and sediment size on the morphological evolution of the islands was conducted. Numerical simulations indicate that under SLR and extreme wave action, the sandy islands eventually retreat landward and the foreshore height increases and the slope steepens and as the SLR rate increases, the island retreats more towards land and the foreshore height rise decreases. While this natural adjustment of the coral reef island increases the island's potential to offset future flood events and support future physical persistence, there is a negative impact on the safety of coastal buildings. This paper highlights the need to develop reasonable conservation policies based on the island's specific environment and conditions. Excessive use of artificial shorelines to protect islands can damage their natural adjustment to resist disasters such as flooding, while artificial defenses are necessary in the case of threats to coastal buildings or the disappearance of certain islands.

DECADAL EVOLUTION OF CORAL ISLANDS IN A CHANGING OCEAN AND CLIMATE

Coral reef islands are under threat from warming and rising seas, ocean acidification and increased storminess. Coral islands are low-lying accumulations of sediment derived from the shells and skeletons of calcifying reef organisms. IPCC predications and COP26 highlight that the future of coral islands is not certain as islands are morphologically active. It is unclear what tipping points are causing negative impacts on some coral reef islands and not others. A better understanding of island stability and vulnerability is urgently needed. This has direct implications for over 200 million people that rely on reefs and their islands for their livelihoods (Ferrario et al., 2014). Further, coastal States such as Australia, USA, the Philippines and Small Island Developing States (SIDS) use coral reef islands as legal baselines to support their maritime jurisdictions (UNCLOS, 1982). We focus on 31 of Australia’s offshore coral reef islands on 10 reefs across the Coral Sea and NW Shelf. These islands are environmentally and geo-politically important as they support and extend Australia’s maritime jurisdiction. Island morphology (e.g., shoreline positions, areas, shapes) in historical aerial and satellite imagery (1977-now) were compared to ocean and climate data (e.g., storm tracks, NOAA Coral Watch). We used this data to identity tipping points that drive decadal changes in coral reef island stability.

Widespread retreat of coastal habitat is likely at warming levels above 1.5 °C.

Several coastal ecosystems-most notably mangroves and tidal marshes-exhibit biogenic feedbacks that are facilitating adjustment to relative sea-level rise (RSLR), including the sequestration of carbon and the trapping of mineral sediment1. The stability of reef-top habitats under RSLR is similarly linked to reef-derived sediment accumulation and the vertical accretion of protective coral reefs2. The persistence of these ecosystems under high rates of RSLR is contested3. Here we show that the probability of vertical adjustment to RSLR inferred from palaeo-stratigraphic observations aligns with contemporary in situ survey measurements. A deficit between tidal marsh and mangrove adjustment and RSLR is likely at 4 mm yr-1 and highly likely at 7 mm yr-1 of RSLR. As rates of RSLR exceed 7 mm yr-1, the probability that reef islands destabilize through increased shoreline erosion and wave over-topping increases. Increased global warming from 1.5 °C to 2.0 °C would double the area of mapped tidal marsh exposed to 4 mm yr-1 of RSLR by between 2080 and 2100. With 3 °C of warming, nearly all the world's mangrove forests and coral reef islands and almost 40% of mapped tidal marshes are estimated to be exposed to RSLR of at least 7 mm yr-1. Meeting the Paris agreement targets would minimize disruption to coastal ecosystems.

Drivers of shoreline change on Pacific coral reef islands: linking island change to processes

Reef islands are considered highly vulnerable to the impacts of anthropogenic climate change. While existing studies provide evidence of high dynamism and predominant stability over recent periods of sea-level rise, efforts to statistically link recorded morphological changes to processes are limited. We analyse shoreline changes across an expansive region of diverse oceanographic and climatic conditions and assess those observations relative to the processes potentially driving change. This record comprises 568 islands from 42 atolls spanning the western to the central Pacific Ocean from the Federated States of Micronesia, Marshall Islands, Kiribati and Tuvalu, and is one of the first robust attempts to statistically attribute the drivers of observed island changes within the past half-century. Our analyses demonstrate that island change and a range of its potential controls, including oceanographic, climatic and local-scale morphometric characteristics of reefs and islands, are not correlated, underscoring that no single variable could predict a uniform response across the study islands. The lack of association between sea-level rise and shoreline change on this large sample of islands addresses the common notion of a linear relationship between rising sea levels and magnitude of erosion in reef island systems. We emphasise the critical need to move from simplistic assessments of island change, highlight the importance of incorporating multiple predictors and the need for more computationally robust models.

Coral reef island shoreline change and the dynamic response of the freshwater lens, Huvadhoo Atoll, Maldives

Low-lying coral reef islands have been projected to become uninhabitable by the end of the century due to sea level rise, but such projections of vulnerability assume that reef islands are static landforms that flood incrementally with sea level rise. In fact, GIS-based reef island shoreline analyses have demonstrated that reef islands are highly dynamic landforms that may adjust their shorelines in response to changing environmental conditions. However, the vast majority of reef island shoreline analyses have been undertaken in the Pacific Ocean, leaving our understanding of changes in the Indian Ocean more limited. Further, our knowledge of how island dynamics can impact groundwater resources is restricted due to the assumption that islands will exhibit purely erosional responses to sea level rise. Here, we analyse shoreline evolution on 49 reef islands over a 50-year timeframe in Huvadhoo Atoll, Maldives. Additionally, rates of shoreline change were used to undertake numerical modelling of shifts in freshwater lens volume in 2030, 2050 and 2100 in response to changes in recharge. Despite sea level rising at 4.24 mm/year (1969-2019), accretion was prevalent on 53% of islands, with the remaining islands eroding (25%) or remaining stable (22%). Average net shoreline movement was 4.13 m, ranging from -17.51 to 65.73 m; and the average rate of shoreline change (weighted linear regression) was 0.13 m/year, ranging from -0.07 to 2.65 m/year. The magnitudes and rates of reef island evolution were found to be highly site-specific, with island type found to be the only significant predictor of either net shoreline movement or weighted linear regression. Results suggest that freshwater lens volume was substantially impacted by shoreline change compared to changes in recharge whereby accretion and erosion led to large increases (up to 65.05%) decreases (up to -50.4%) in les volume, respectively. We suggest that the capacity of reef islands to both (1) adjust their shorelines, and even accrete, under conditions of sea level rise; and (2) increase their storage of groundwater over the coming decades represents highly valuable geomorphic ecosystem services.

Quantifying Mechanisms Responsible for Extreme Coastal Water Levels and Flooding during Severe Tropical Cyclone Harold in Tonga, Southwest Pacific

The South Pacific region is characterised by steep shelves and fringing coral reef islands. The lack of wide continental shelves that can dissipate waves makes Pacific Island countries vulnerable to large waves that can enhance extreme total water levels triggered by tropical cyclones (TCs). In this study, hindcasts of the waves and storm surge induced by severe TC Harold in 2020 on Tongatapu, Tonga’s capital island, were examined using the state-of-the-art hydrodynamic and wave models ADCIRC and SWAN. The contributions of winds, atmospheric pressure, waves, and wave-radiation-stress-induced setup to extreme total water levels were analysed by running the models separately and two-way coupled. The atmospheric pressure deficit contributed uniformly to the total water levels (~25%), while the wind surge was prominent over the shallow shelf (more than 75%). Wave setup became significant at locations with narrow fringing reefs on the western side (more than 75%). Tides were dominant on the leeward coasts of the island (50–75%). Storm surge obtained from the coupled run without tide was comparable with the observation. The wave contribution to extreme total water levels and inundation was analysed using XBEACH in non-hydrostatic mode. The model (XBEACH) was able to reproduce coastal inundation when compared to the observed satellite imagery after the event on a particular coastal segment severely impacted by coastal flooding induced by TC Harold. The coupled ADCIRC+SWAN underestimated total water levels nearshore on the reef flat and consequently inundation extent as infragravity waves and swash motion are not resolved by these models. The suite of models (ADCIRC+SWAN+XBEACH) used in this study can be used to support the Tonga Meteorological Service Tropical Cyclone Early Warning System.