Atoll Research Articles

Observations of waves and currents on the fore-reef and reef flat of a coral reef atoll in the South China Sea

Understanding the wave and current conditions of coral reef ecosystems is essential for maintaining their health, as many reef processes are controlled by these hydrodynamic conditions. In this study, high-frequency measurements of tides, waves, and currents were made using acoustic, electromagnetic, and pressure instruments over a 28-d period on the fore-reef and reef flat of a coral reef atoll in the South China Sea. The research revealed wave transformation, tidal and wave modulation of flow, and wave setup conditions for the first time at this typical atoll. Three large wave processes dominated by gravity waves (GWs) are observed in the fore-reef. The GWs are significantly attenuated on the reef flat, whereas infragravity (IG) waves strengthened. The tidal modulation of GWs and IG waves on the reef flat is significant when the incident wave height exceeded 1 m. In the fore-reef, the modulation of progressive tidal waves and large waves leads to relatively stronger upper-layer currents, and weak near-bottom currents are primarily attributed to the dissipation of tidal wave energy by the rough coral terrain. In calm conditions, flow variations on the reef flat are modulated by tides, thereby allowing seawater flow to pass through the reef flat during spring tides. Conversely, during periods with large waves, tidal modulation of flow on the reef flat is less significant, and the cross-reef flow velocity increases with increasing incident wave height. The occurrence of wave setup on the reef flat enhances cross-reef flow towards the lagoon. The wave setup is positively linearly correlated with the incident wave height. The magnitude of wave setup is associated with the distance between the measurement sites on the fore-reef and reef flat.

Dismantling Ships to Revitalize Coral Reefs with Artificial Reefs

Australia's Great Barrier Reef is an immense swath of coral made up of about 2,900 reefs, 600 mainland islands, and 300 coral atolls. It stretches for 2,200 kilometers, ranging in width from 30 km to 740 km. This natural wonder is visible from space and is the largest structure in the world made solely by living organisms. The corals that form this barrier are essential for marine biodiversity and were elected as a World Heritage Site in 1981. However, the Great Barrier Reef faces serious challenges, including bleaching caused by rising temperatures and the impacts of climate change. One strategy for revitalizing reefs is the use of sunken old ships to create artificial reefs. This approach not only conserves the marine environment, but also promotes scientific research and the production of bioproducts. Artificial ship reefs provide substrates for the marine community and simulate environments similar to natural reefs, contributing to the protection and revitalization of this important barrier reef. It is essential to develop strategies to preserve the oceans and mitigate the impacts of human activities, ensuring the survival of this ecosystem and protecting it as a treasure.

The South China Sea Dispute and Regional Response

Background: Beginning in December 2013, the South China Sea dispute has emerged as a complex and highly contested issue with significant implications for regional stability in East and Southeast Asia. The Chinese military began constructing artificial islands anchored by seven coral atolls in the Spratly Islands until it had created land that it then claimed in October 2015. The construction is over 3000 acres and houses military equipment (anti-aircraft and anti-missile systems) and personnel for the People's Republic of China. Objective: From a realist perspective, the dispute reflects a struggle for power and influence among claimant states, driven by their desire to secure regional strategic interests. Neighbouring countries, recognising the implications of China's rise as a significant power, have responded mainly by using soft realist methods to accommodate or counter China's growing influence in a collective setting. This paper explores the geopolitical implications of the dispute and how other powers have chosen to engage with China and other regional actors. Methods: This article is a case study. The article analyses the case of the South China Sea dispute since 2000 through a realist lens. It first explains the lens and its focal points. Then, it examines the different facets of the dispute through this lens: economic interests, national security concerns, domestic and regional politics, and historical issues. The focus is on analysing China and ASEAN member-states as the primary parties to the conflict. Results: This lens provides valuable insights into the power struggles, self-interest, and security concerns that underlie the conflict. This paper provides a comprehensive understanding of its multifaceted nature by critically examining the dispute's territorial claims, legal frameworks, and historical context. It also explores the geopolitical implications and the involvement of major regional powers. Liberal solutions have largely failed, while more minor actors have cooperated on limited terms when their interests intersect using soft realist diplomacy and collective action within international arenas. Conclusion: The dispute has unveiled the limits of international diplomatic efforts brokered by organisations such as ASEAN to address aggressive behaviour consistently. Small and medium states are largely constrained from directly confronting China, have accommodated China, used international forums to advance power positions and prestige, and cooperated on other issues to indirectly address the dispute.

We Are Castaways: Archipelagic Imagination in Our Trembling Échos-monde

This article, here translated from the author's original lecture in Portuguese, distills his experiences of fieldwork in Latin America and reading of Caribbean authors who took his “imagination to the creole seas,” from which emerged eventually a “vision of archipelago” as a space-time splintering conducted from underwater, anti-continental perspectives. Bringing the challenge of archipelagicity to the reconfiguring of time as well as space, the lecture traces his journey from creole toward archipelago. Traversing the Mississippi delta, island clusters, and coral atolls in the Atlantic, Pacific, and Indian Ocean worlds, gathering up the poetics of dialects, pidgins, creoles, and other errant tongues, it is an “unpacific” voyage that dredges up the author's East Asian, Pacific Rim identity as a divining rod for sensing archipelagic connections between differently creolized subjectivities. This meaningful web, generated through a shared awareness of not history but its lack, constitutes a new home: “a dwelling called archipelago.”

Geochemical evidence of temporal ecosystem photosynthetic plasticity within a pristine coral atoll

Geochemical evidence of temporal ecosystem photosynthetic plasticity within a pristine coral atoll

From earthquakes to island area: multi‐scale effects upon local diversity

Tropical forests occupy small coral atolls to the vast Amazon basin. They occur across bioregions with different geological and climatic history. Differences in area and bioregional history shape species immigration, extinction and diversification. How this effects local diversity is unclear. The Indonesian archipelago hosts thousands of tree species whose coexistence should depend upon these factors. Using a novel dataset of 215 Indonesian forest plots, across fifteen islands ranging in area from 120 to 785 000 km2, we apply Gaussian mixed effects models to examine the simultaneous effects of environment, earthquake proximity, island area and bioregion upon tree diversity for trees ≥ 10 cm diameter at breast height. We find that tree diversity declines with precipitation seasonality and increases with island area. Accounting for the effects of environment and island area we show that the westernmost bioregion Sunda has greater local diversity than Wallacea, which in turn has greater local diversity than easternmost Sahul. However, when the model includes geological activity (here proximity to major earthquakes), bioregion differences are reduced. Overall, results indicate that multi‐scale, current and historic effects dictate tree diversity. These multi‐scale drivers should not be ignored when studying biodiversity gradients and their impacts upon ecosystem function.

Types and Evolution of the Miocene Reefs Based on Seismic Data in the Beikang Basin, South China Sea

During the Miocene, several reefs formed in the Beikang Basin, South China Sea, which may be potential targets for hydrocarbon exploration. This is due to the environment that developed as a result of the collision, splitting, and splicing of the Nansha Block, which was influenced by the Neogene expansion of the area. However, studies on the types, distribution, controlling factors, and evolution stages of these reefs are scarce. In this study, we used high-resolution seismic data and extensive well-drilling records to gain insights into the evolution of reefs in this particular area. Six distinct types of reefs, namely, the point reef, the platform-edge reef, the block reef, the bedded reef, the pinnacle reef, and the atoll reef, were identified based on our data. These reefs underwent four stages of development. During the initial stage, a few small-sized point reefs emerged in the basin and experienced significant growth during the early Middle Miocene. In the flourishing stage, the reefs predominantly thrived around the Central Uplift and Eastern Uplift areas. In the recession stage, the reefs began to deteriorate during the late Middle Miocene period as a result of the rapid increase in relative sea level caused by tectonic subsidence. In the submerged stage, since the Late Miocene, as the relative sea level continued to rise steadily over time, many reefs that had previously flourished surrounding the Central Uplift and Eastern Uplift areas became submerged underwater, with only a handful of atoll reefs surviving near islands located on the Eastern Uplift. This study indicated the presence of a significant number of well-preserved reefs in the Beikang Basin that have experienced minimal subsequent diagenesis and therefore exhibit high potential as reservoirs for oil and gas exploration.

DIVERGENT OUTCOMES FOR DEMOCRACY FROM RECENT ELECTIONS IN FIJI AND THE MALDIVES: ANALYSING THE 2018 ELECTIONS AND ITS AFTERMATH

This paper examines the historical background and conduct of recent elections in two small island nations in Asia-Pacific, Fiji and the Maldives. It assesses the situation of democracy prior to elections, and afterwards in 2020–2021. This paper seeks to contribute towards filling the gap in literature, with regards to comparative case studies on electoral authoritarianism in small island states. The two nations with dissimilar history, religion, ethnicity and geology, held national elections in 2018. Fiji comprising volcanic and limestone islands with a multiethnic and multi-religious population is located in the Pacific Ocean. The Maldives with lowest lying coral atolls, and a homogeneous population of one ethnicity and one religion Islam, is located in the Indian Ocean. The elections were conducted under differing circumstances of electoral authoritarian rule in the aftermath of multiparty elections, with repression in the Maldives increasing, and repression in Fiji easing. But the 2018 election results saw a return to transition to democracy in the Maldives. Conversely in Fiji, the electoral authoritarian government was re-elected. Challenges to democracy remained unchanged in both nations during the COVID-19 pandemic.

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.

Beta‐diversity within coral atolls: Terrestrial species turnover increases with cyclone frequencies

AbstractAimAtolls are a widely distributed, common type of tropical ecosystem, each consisting of an annular coral reef and up to several hundred individual islets sitting on the reef platform. The small land areas and low elevation render the terrestrial communities susceptible to local extinctions from overwash and inundation due to tropical cyclones. Such recurring catastrophic disturbances should be expected to drive strong priority effects and historical contingency in species assembly, thus also promoting species turnover within atolls. This stands contradictory to the received wisdom of atolls consisting of numerous uniform, replicated island systems. We tested the hypothesis that the individual islets within an atoll exhibit compositional turnover and that this species turnover within atolls is related to cyclone frequency.LocationAtolls worldwide.Time PeriodPresent.Taxa StudiedVascular plants, reptiles, birds.MethodsWe compiled a global dataset at the level of islets for species presence/absence within atolls. We obtained long‐term (80 year) tropical cyclone frequency data for each atoll from the NOAA hurricane database and used Bayesian regression to estimate the effects of tropical cyclone frequency on species turnover within atolls.ResultsWe consistently measured high within‐atoll species turnover (i.e., between the individual islets within an atoll) on atolls worldwide. The degree of within‐atoll species turnover increases with increasing tropical cyclone frequency. Atolls that are more frequently hit by tropical cyclones show higher species turnover between their islets than those that occur outside the tropical cyclone belt.Main ConclusionsTropical cyclones are a significant driver of the community assemblages on atolls. These catastrophic disturbances promote a heterogenous atoll landscape, which challenges perceptions that the islets within an atoll are identical replicates of the entire atoll terrestrial community. Biodiversity surveys undertaken at the islet‐level (alpha‐diversity) might therefore not be representative of the entire atoll metacommunity (gamma) diversity.

Planktonic carbon metabolism of an underwater coral atoll in the oligotrophic sea: a case study of Zhongsha Atoll, Central South China Sea

Coral atolls are widely recognized as oases of remarkable biodiversity and productivity within the oligotrophic open ocean. However, considerable debate surrounds the net carbon metabolism of planktonic communities in oceanic coral atolls and their contribution to the overall carbon budget of the ocean. Zhongsha Atoll, situated in the central South China Sea, represents one of the largest submerged reef atolls globally. Despite its significance, the ecological environment and characteristics of community production and metabolism at Zhongsha Atoll have been scarcely studied, and it remains uncertain whether these features differ from those observed in barrier-type reefs. In this study, we examined the gross primary production (GPP), community respiration (CR), and net community production (NCP) of the planktonic community in Zhongsha Atoll and its surrounding waters from 22 June to 6 July 2020. We also analyzed the potential influences of their distribution patterns. Our findings revealed that CR did not vary significantly with depth and it was considerably higher than GPP. As a result, the waters at the euphotic depth of Zhongsha Atoll were found to be heterotrophic, with negative NCP. Additionally, the correlation analysis demonstrated a negative relationship between NCP and CR. The NCP values were -35.28 ± 26.73 and -53.18 ± 31.77 mg C m-3 d-1 for the surface and chlorophyll maximum (DCM) layers, respectively. The NCP for the upper water column was -3023 mg C m-2 d-1. The waters above the reef flat (FL) exhibited higher primary productivity, with GPP in the surface and DCM layer being 1.61 and 2.71 times greater, respectively, than in the surrounding oceanic regions. However, the FL displayed a greater level of heterotrophy due to its stronger CR. In conclusion, the carbon metabolism of the planktonic community in submerged Zhongsha Atoll acts as a source of atmospheric CO2, and the distribution of coral reefs considerably contributes to the efficiency of carbon cycling within the atoll.

Reef‐wide and long‐term skeletal growth records of the mountainous star coral (Orbicella faveolata) from Belize barrier and atoll reefs (Central America)

AbstractCoral reefs are vulnerable marine ecosystems and reef‐building corals are especially sensitive to the impacts of environmental change. Skeletal growth records of corals (Scleractinia) can be used as archives of ecological and climatological change. This study focusses on massive Orbicella faveolata coral skeletons from the Belize barrier and atoll reefs. In total, 11 drill cores from 10 coral colonies were studied. Their skeletal growth records range from 17 to 186 years and span time windows from the early 19th century to the beginning of the 21st century. Based on these records, master chronologies have been compiled for the entire 20th century. The data indicate uniform skeletal growth across the various reef environments of offshore Belize. Skeletal density is increasing reef‐wide, whereas linear extension is declining, accompanied by a slightly declining calcification rate. Apparently, a more densely packed coral skeleton does not compensate for the reduced linear growth and calcification sufficiently in O. faveolata populations across the Belize reefs. The longest analysed coral sample contains >186 years of skeletal growth, which exhibits periodicities of 40–80 years. Such cycle lengths likely reflect the Atlantic Multidecadal Oscillation, lending further support to coral skeletal records being valuable archives for long‐term oceanographic change.

Sympatric species of coral trout (Plectropomus) show contrasting patterns of genomic structure across isolated atoll reefs

Sympatric species of coral trout (Plectropomus) show contrasting patterns of genomic structure across isolated atoll reefs

Impact of Climate Change on Coral reef and Marine Life of the Lakshadweep – A Short Review

Lakshadweep, a group of islands with one of the least studied coral atolls enclosing lagoons, submerged reefs and banks situated in the Arabian Sea, is now vulnerable due to rapid erosion, turbulent seas and rising ocean temperatures by climate change. Moreover, rising global sea levels have gradually become a critical threat that is going to impact small islands in the upcoming years. This study depicts the present status of the coral reef and the marine life of the Lakshadweep and how climate change might pose a major threat to the islands in the near future.

Temporal variability in seawater Sr/Ca ratios within a coral atoll as an indicator of marine calcifier community diversity

Coral reefs are marine environments where calcium carbonate production and dissolution along with photosynthesis and respiration can have a large impact on the daily biogeochemical cycles of seawater dissolved inorganic carbon (DIC), total alkalinity (TA), oxygen (O2), and major nutrients. Using seawater samples from Tetiaroa Atoll, French Polynesia, we apply isotope dilution thermal ionization mass spectrometry (ID-TIMS) to show that these daily cycles of variability extend to the dissolved seawater Sr/Ca ratio (Sr/Casw). Hourly measurements over a 27+ hour period indicate that Sr/Casw covaries with DIC and TA and exhibits up to a 2% relative difference compared to the regional Pacific Ocean source water Sr/Casw value of 8.53 mmol/mol. We hypothesize that the primary mechanism of hourly variability in Sr/Casw is carbonate precipitation/dissolution by corals and other marine calcifiers on the Tetiaroa reef, while sedimentary redox processes may significantly contribute to an apparent decoupling between net calcification rates estimated from TA compared to dissolved calcium [Ca]sw concentrations. Using a simple Rayleigh distillation model and a two-member mixing equation, we estimate the net partition coefficient of Sr (KD-Sr) for each hourly sample as a mixture of coral (aragonite; KD-Sr ∼ 1.06) and crustose coralline algae (high Mg-calcite; KD-Sr ∼ 0.39) contributions to the measured variability of [Ca]sw and Sr/Casw. As the high Mg-calcites of crustose coralline algae are (1) integral components to the structure and stability of the reef framework and (2) more soluble than aragonite under more acidic conditions, we propose that continued analyses of Sr/Casw within coral reef environments could be a useful tool for monitoring the ecological distribution and contributions of marine calcifiers on reefs as ocean warming and acidification intensify in response to anthropogenic climate change. Additionally, seasonal, interannual, and/or centennial-scale offsets between the Sr/Casw of open ocean source seawater and ambient reef seawater may have significant implications for interpreting past changes in sea surface temperatures derived from the coral Sr/Ca paleothermometer.

9000 years of change in coral community structure and accretion in Belize reefs, western Atlantic

Tropical coral reefs, as prominent marine diversity hotspots, are in decline, and long-term studies help to improve understanding of the effects of global warming, sea-level rise, ocean acidification, deterioration of water quality, and disease. Here, we evaluated relative coral abundance and reef accretion rates over the past 9000 years in Belize barrier and atoll reefs, the largest reef system in the Atlantic Ocean. Acropora palmata and Orbicella spp. have been the most common corals. The abundance of competitive, fast-growing acroporids was constant over multi-millennial timescales. A decline in A. cervicornis abundance, however, and three centennial-scale gaps in A. palmata occurrence, suggest that the modern decline in acroporids was not unprecedented. Stress-tolerant corals predominate at the beginning of Holocene successions. Following the improvement of environmental conditions after inundation of the reef pedestal, their abundance has decreased. The abundance of weedy corals has increased during the Holocene underlining the importance of fecundity for the coral community. Reef-accretion rate, as calculated based on 76 new U-series age dates, has decreased over the Holocene and the mean value of 3.36 m kyr−1 is at the lower end of global reef growth compilations and predicted future rates of rise in sea level.

Carbonate budgets in Lakshadweep Archipelago bear the signature of local impacts and global climate disturbances

Predicted sea-level rise and increased storm frequency caused by climate change drastically threaten low-lying inhabited coral atolls. Coral reef frameworks are the atolls’ primary defence from these changes. The growth and integrity of these frameworks is reflected in their carbonate budgets, a dynamic balance between biogenic accretion through coral growth and erosional forces, both of which are affected by factors acting at different spatial scales. We explored how carbonate budgets, estimated using the ReefBudget methodology, vary between three inhabited atolls that face differing anthropogenic stressors in the Lakshadweep Archipelago in the Northern Indian Ocean. We surveyed ten reefs, at two depths each, across the three islands. Overall, net carbonate budgets of reefs across all atolls were below optimal production rates needed to continue protecting shorelines (5 G measured on healthy reefs). This was a result of repeated mass bleaching events as well as local impacts. Carbonate production was influenced by a recent mass bleaching event in 2016 and a cyclone in 2018, and varied between depths and exposures, potentially due to differential recovery and mortality dynamics. Erosional processes were locally mediated with both urchin and parrotfish density showing large differences between islands, possibly linked to nutrient outflow and fishing intensity. We also find that by the year 2100, a large proportion of shallow sites will experience an increase in water depth above half a metre under moderate and high emissions scenarios, but none will breach this threshold under a low emissions scenario. Our results show that patterns of carbonate production were largely mediated by the history of global/regional disturbances, while erosional rates were much more dependent on local factors.

Hydrodynamic and atmospheric drivers create distinct thermal environments within a coral reef atoll

Within coral reefs, different thermal environments can be found at locations separated by less than 100 s of meters and can generate fine-scale patterns of thermal stress and subsequent bleaching. In this study, we use an 11-month record of in situ temperature measurements, coupled with oceanographic and atmospheric data to examine the role of surface and advective heat fluxes in driving spatial patterns of temperature variability across several reef zones (i.e., fore-reef, reef flat, channel and lagoon) within an individual coral reef atoll. We show that advection of heat (driven by a combination of wave and tidal flows) was dominant across all sites and surface heating was more important across shallow areas or areas of low net exchange (i.e., reef flat and lagoon zones). Tidal flows were important in driving short term variability in the transport of heat across the atoll, but their contribution to the net transport of heat (cooling vs heating) was less significant over the longer timescales (days to weeks) that are typically used to assess thermal stress experienced by coral reef communities (e.g., Degree Heating Weeks). Conversely, although the wave-driven advection of heat contributed minimally to reef temperature changes over short timescales, the net transport of heat over daily to weekly timescales had a significant influence on persistent temperature anomalies. By parameterising the mechanisms driving temperature variability across the reef flat and lagoon zones, we demonstrate how satellite measurements of sea surface temperatures can be corrected to provide robust temperature estimates at the reef scale.

A facies- and sequence stratigraphy-based reservoir model for a carbonate reef complex: South Swan Hills oil pool, Alberta

We present a reservoir model for the South Swan Hills oilfield, Alberta, Canada, developed in a Devonian atoll reef, based on facies analysis and sequence stratigraphy. The analysis illustrates the complex reservoir geometries in such a reef, caused by facies variation from reef margin to reef interior and fluctuations in relative sea level. The latter resulted in cyclic deposition of facies and formation of field-scale sequence boundaries. The model is built on lithofacies analysis from high-resolution core descriptions that recorded lithologies, bioclast type and characteristics, and sedimentary structures. Core descriptions were correlated to well logs, enabling lithofacies to be mapped across the field. Lithofacies distributions permit definition of three distinct reef environments: a reef interior, a reef margin and a terminal shoal. Vertical variability is characterized using sequence stratigraphic interpretations. Two 3rd order and four 4th order sequence boundaries were identified and correlated across the reef. The analysis illustrates the complex reef geometries linked to facies variation from reef margin to reef interior and fluctuations in relative sea level. The latter resulted in cyclic deposition of facies and formation of field-scale unconformities. These and the intervening deposits represent the response to fluctuating sea level.The geological analysis was transformed into a 3-D geocellular model using a sequential indicator simulator. The simulator models lithofacies and, in turn, porosity and permeability distributions based on strong relationships between lithofacies and petrophysical properties for the three representative regions; the interior, margin and terminal shoal. The interior deposits formed forms the bulk of non-reservoir material in the centre of the reef build-up. The margin surrounded surrounds the interior and formed forms a continuous 1–2 km band of reservoir around it. Reservoir thickness is 20–30 m and primarily composed of reef flat deposits. The terminal shoal forms a reservoir body that caps the reef complex, but is disconnected from the bulk of the underlying reef by a thick layer of impermeable clay associated with a sequence boundary. The high-resolution realization displays the geometry and continuity of the reservoir and exposes demonstrates the compartmentalized and variable producibility of the atoll.

Trait-based approaches reveal that deep reef ecosystems in the Western Indian Ocean are functionally distinct

Tropical deep reefs (>30 m) are biologically and ecologically unique ecosystems with a higher geographic reach to shallow (<30 m) reefs. Yet they are poorly understood and rarely considered in conservation practices. Here, we characterise benthic and fish communities across a depth gradient (10–350 m) in remote coral atolls in Seychelles, Western Indian Ocean. Using taxonomic and trait-based approaches we present the taxonomic and functional composition of shallow and deep reef communities, with distinct communities and traits dominating different depths. Depth-related changes in community metrics (taxa richness, abundance and biomass) and functional diversity metrics (richness, dispersion, and evenness) indicate complex relationships across different biological components (fish, benthos) that differ between shallow and deep reefs. These in turn translate into different patterns of reef resilience against disturbance or species invasions with depth. Notably, deep reefs host on average fewer and less abundant taxa but with higher functional contribution and originality scores, some of which are of conservation concern. Overall, the results highlight the unique nature of deep reefs that requires their explicit consideration in conservation and management activities.