Southern Atolls Research Articles

An underwater imagery identification guide for shallow, mesophotic and deep-sea benthos in Maldives.

During the 2022 Nekton Maldives Mission, we deployed a variety of platforms (snorkelling, remotely-operated vehicles and manned submersibles) to conduct video surveys of the biodiversity and composition of shallow (< 30 m), mesophotic (30-150 m) and deep-sea (> 150 m) benthos found in the Maldives' central and southern atolls. In total, ~ 80 hrs of stereo-video footage were collected during the benthic transect surveys, which were subsequently processed using annotation software in order to evaluate benthic biodiversity and community composition. Here, we present a photographic guide for the visual, in situ identification of reef benthos encountered, including corals, sponges and other invertebrates that inhabit Maldives' nearshore habitats. We hope that this identification guide will aid future imagery-based surveys or observations of organisms during fieldwork. A total of 283 morphotypes were identified, including those belonging to Octocorallia (61), Scleractinia (57), Porifera (38), Asteroidea (22), Antipatharia (15), Decapoda (13), Hydrozoa (12), Holothuroidea (10), Actiniaria (9), Echinoidea (8), Annelida (6), Chlorophyta (5), Gastropoda (4), Bivalvia (4), Ascidiacea (3), Crinoidea (3), Bryozoa (2), Cyanobacteria (2), Zoantharia (2), Cephalopoda (1), Ceriantharia (1), Corallimorpharia (1), Ctenophora (1), Ophiuroidea (1), Rhodophyta (1) and to an unknown category (1). Out of these, we identified 40 to species level, 120 to genus, 47 to family, 14 to order and suborder, 58 to class and subclass, two to phylum and one was of unknown phylum. This represents the first attempt to catalogue the mesophotic and deep-sea benthic megafaunal diversity in the Maldives using underwater imagery.

First record of the bull shark Carcharhinus leucas (Valenciennes, 1839) from the Maldivian archipelago, central Indian Ocean.

Verified records of the bull shark Carcharhinus leucas are lacking in the Maldives. This study provides the first confirmed evidence of 23 sightings observed from 2013 to 2023 in the central and southern atolls of this archipelago. Most of the sightings occurred in close proximity to inhabited areas, where food waste is often discarded into the water, or in several dive sites, suggesting the presence of this species in different locations around central and southern atolls. Although further research is required to fully investigate the C. leucas population in the Maldives, this report documents and confirms its presence in this region.

Cytogenetic and epigenetic aberrations in peripheral lymphocytes of northwest Arkansas Marshallese

Purpose Nuclear weapons testing in the northern Marshall Islands between 1946 and 1958 resulted in ionizing radiation (IR) exposure of the thousands of Marshallese. Furthermore, numerous islands were contaminated by radioactive fallout. Significant increases in cancer and metabolic syndrome incidences have been reported among Marshallese, and potential for further increases looms due to the latency of radiation-induced health effects. The purpose of this study was to investigate the genetic and epigenetic effects of exposure to IR that could be associated with radiation-induced disease among the Northwest Arkansas (NWA) Marshallese. Materials and Methods We performed analysis of chromosomal aberrations and DNA methylation based on residential and exposure history of NWA Marshallese. Results Analysis of chromosomal aberrations demonstrated higher incidence of genetic rearrangements in women with self-reported history of radiation exposure (95% CI: 0.10, 1.22; p=.022). Further clustering of study participants based on their residential history demonstrated that participants who spent substantial amounts of time (≥6 months) in the northern atolls (thus, in the proximity of nuclear tests) before 1980 had more chromosomal aberrations than their peers who lived only in the southern atolls (95% CI: 0.08, −0.95; p=.021), and that this difference was driven by women. A relationship between the time spent in the northern atolls and increase in chromosomal aberrations was observed: 0.31 increase in chromosomal aberrations for every 10 years spent at northern atolls (95% CI: 0.06, 0.57; p=.020). Finally, significant inverse correlations between the chromosomal aberrations and the extent of DNA methylation of four LINE-1 elements L1PA2, L1PA16, L1PREC1, and L1P4B were identified. Conclusions The results of this study provide first evidence of the presence of stable genetic and epigenetic rearrangements in peripheral lymphocytes of NWA Marshallese and warrant further studies to analyze the role of radiation exposure in health disparities experienced by this Pacific Island nation.

Spatial variation of plastic debris on important turtle nesting beaches of the remote Chagos Archipelago, Indian Ocean

We report Anthropogenic Marine Debris (AMD) in Chagos Archipelago in the Indian Ocean, globally amongst the most isolated island groups. AMD on 14 island beaches in five atolls were surveyed in 2019 using two techniques: Marine Debris Tracker (MDT) along littoral vegetation and photoquadrats in open beach. Over 60 % of AMD in both beach zones was composed of plastics, especially bottles and fragments (mean = 44.9 %, 27.2 %, range = 16.5–73.2 %, 4.8–55.9 % respectively in vegetation; mean = 28.7 %, 31.5 %, range = 17.7–40.7 %, 11.6–60.0 % respectively in open beach). The density of plastic debris in littoral vegetation (MDT data: 1995 bottles, 3328 fragments per 100 m2) was 10-fold greater than in open beach (photoquadrat data: 184 bottles, 106 fragments per 100 m2). Significant latitudinal variation in vegetation AMD occurred (8-fold greater in southern atolls, p = 0.006). AMD varied within island zones: most debris observed on oceanside beaches (oceanside vs lagoon, W = 365, p < 0.001; ocean vs island tip, W = 107, p = 0.034). Standardisation of surveys using the open-source MDT App is recommended. Debris accumulation hotspots overlapped with sea turtle nesting habitat, guiding future beach clean-up prioritisation.

Last interglacial sea levels within the Gulf of Mexico and northwestern Caribbean Sea

Abstract. During the last interglacial (LIG) the volume of additional water in the world's oceans was large enough to raise global sea levels about 6–9 m higher than present levels. However, LIG sea levels vary regionally and those regional differences hold clues about the past distribution of ice sheets and local rates of subsidence and tectonic uplift. In this study, I used a standardized database template to review and summarize the existing constraints on LIG sea levels across the northern Gulf of Mexico and Caribbean shoreline of the Yucatán Peninsula. In total, I extracted 32 sea-level indicators including the insertion of 16 U-series ages on corals, 1 electron spin resonance age, 2 amino acid racemization ages, and 26 luminescence ages. Most dated sea-level indicators for the northern Gulf of Mexico are based on optically stimulated luminescence (OSL) ages of beach deposits of a mappable LIG shoreline. This shoreline extends from the Florida Panhandle through south Texas but is buried or removed by the Mississippi River across most of Louisiana. A similar feature is observed in satellite images south of the Rio Grande within the Mexican portions of the Gulf of Mexico but has yet to be dated. Elevations measured on portions of this feature close to the modern coast point to sea levels less than 1 m to ∼5 m higher than present for much of the northern Gulf of Mexico. However, a few, albeit undated, portions of the same shoreline located at more inland locations point to sea levels up to +7.2 m, attesting to up to 7 m of differential subsidence between the inland and coastal sites. Across the Yucatán Peninsula, U-series dating of corals has provided the main index points for LIG sea levels. Other carbonate coastal features such as beach ridges and eolianites have also been described but rely on corals for their dating. The maximum elevation of the LIG coral-based relative sea-level (RSL) estimates decrease from around +6 m across the Caribbean shoreline of the Yucatán Peninsula near Cancún, Mexico, to as low as −6 m to the south beneath the southern atolls of Belize, although discussion continues as to the validity of the ages for these southern corals. If these lower-elevation corals are LIG in age, their below-present elevations may be a result of vertical motion along faults dipping into the Cayman Trough. South of Belize only one purported LIG coral has been dated on the Isla de Roatán off the coast of Honduras at a likely tectonically uplifted elevation of 37.2 m. Thus the elevation of LIG sea levels within the inland siliciclastic shorelines of Guatemala and Honduras as well as the southwestern Gulf of Mexico remains poorly constrained and a potential venue for future research. The database described in this paper is available open access in spreadsheet format as Simms (2020), at this link: https://doi.org/10.5281/zenodo.4556163.

Morphometrics of 39 fishes from the Seychelles artisanal fisheries

AbstractA total of 5478 fishes were sampled between 2009 and 2020 to assess length–weight, length–length and weight–weight relationships in 39 marine species from 10 families caught in the Seychelles waters by the artisanal fishery. Two types of length (total length TL, fork length FL) and three types of weight (whole weight WT, gutted weight GW and gilled-gutted weight GGW) were measured. The parameters of the relationships were estimated using the log-transformed allometric model with bias correction. Our results include length–weight, length–length and weight–weight relationships for 39, 20 and 18 species, respectively. Our length–weight data and resulting relationships were compared against FishBase database for 36 species and were in the Bayesian 95% confidence interval of the relationships available for 33 species and above for Gnathanodon speciosus, Lutjanus gibbus and Variola louti. Finally, for five abundant and widely dispersed species we tested for spatial differences in morphometric relationships between the Mahé Plateau and three southern atoll groups. Significant differences were found for two species only, but their magnitude was small. We thus argue for the regression relationships based on pooled data to be used for most types of population and community analyses. The availability of these morphometric relationships will support the application of accurate size-based analyses for Seychelles fisheries survey data, and so enhance understanding of the ecology of the reef-associated fish component of marine ecosystems and food webs, and improve fisheries research management.

Camera trapping and transect counts yield complementary insights into an endangered island endemic rail

Island endemic species are disproportionately represented in the tally of global extinctions. The island endemic Cocos buff-banded rail (Hypotaenidia philippensis andrewsi) is classified under the Australian Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) as Endangered. It is restricted to the remote Cocos (Keeling) Islands in the Indian Ocean where, until 2013, only the island Pulu Keeling supported a population of this species, following earlier extirpations of the bird from islands in the southern atoll. To establish a second viable population to mitigate against potential extinction of this subspecies, 39 rails were reintroduced from Pulu Keeling to Horsburgh Island in April 2013. As a component of post-translocation monitoring, we conducted intensive camera trapping from 29 May 2015 to 30 June 2016 (397 calendar days) to investigate recruitment success, behaviour and potential threats to the population. Biannual transect sampling to monitor and investigate long-term population density was also conducted. We found Cocos buff-banded rail persisting on Horsburgh Island with an increase in the founder population to 97 rails in February 2016. Ongoing recruitment with breeding activity on Horsburgh Island and the emigration of rails to nearby (2.5km) Direction Island is indicative of short-term success. Images from the camera traps have documented behaviours that have not previously been described for the species. House mice were also identified as a potential threat to the Horsburgh Island rail population. Island rail species have suffered a high rate of extinctions, and their conservation remains a formidable challenge, given the risks of continuing introductions of invasive mammalian predator species.

Conservation status and reintroduction of the Cocos Buff-banded Rail, Gallirallus philippensis andrewsi

An endemic subspecies of Buff-banded Rail (Gallirallus philippensis) is restricted to the Cocos (Keeling) Islands, a group of 27 islands, with total area of ~15 km2, in the north-eastern Indian Ocean. Human settlement led to marked environmental degradation of the 26 islands in the southern atoll of the group. The Cocos Buff-banded Rail declined severely, with the last confirmed record from islands in the southern atoll in 1991. The subspecies has persisted, however, with a population of ~800 individuals, on a single island, Pulu Keeling, 24 km north of the southern atoll. A recovery plan for this endangered subspecies recommended reintroduction to a suitable island in the southern atoll. This paper provides a brief overview of the history and status of the subspecies, and describes an April 2013 reintroduction of 39 rails from Pulu Keeling to the 1-km2 Horsburgh Island in the southern atoll. This program has had at least short-term success, with monitoring showing successful recruitment in the reintroduced population, and its increase to ~54 individuals by October 2014 and ~121 individuals by June 2015. Much of the world's loss of biodiversity has been from, and continues to occur on, islands: this project demonstrates that well-considered mitigation of threats and translocation programs can provide solutions to this challenge.

Darwin’s atolls revisited: lagoon infilling and closure has ecological consequences to North Keeling Atoll

Charles Darwin’s seminal theory of atoll formation was developed during his epic voyage on the Beagle. The Cocos (Keeling) Islands (12°12′ S, 96°54′ E) were the only atolls Darwin visited and comparisons with his 1836 observations reveal sediment-infilling of the south-east lagoon entrances at the southern atoll (Smithers et al. 1992). Here, we report that the same processes have caused the closure of the only lagoon entrance (south-east) at the northern atoll in 2005. In 2014 the lagoon entrance remained closed and had been colonised by plants. Prior to closing, the lagoon contained coral, seagrass, sand, and mud flats, which were inhabited by turtles, mud crabs (Scylla sp.), blacktip sharks, and at least 20 other fish species. The closure caused significant changes to the lagoon community. In 2008, we observed extensive cyanobacteria mats (10–50 cm thick) smothering the lagoon substrate. Removing this mat released a sulfur-like smell and revealed a thin layer (1–2 cm) of black sand covered in dead gastropod shells (1–2 cm in length). No seagrass was seen and the only animals observedwere milkfish (Chanos chanos) and small glass shrimp. Similar changes in habitat and community composition occurred at Clipperton Atoll following the lagoon closure, and were linked to three changes in water condition: desalination of surface water, a deeper water layer of anoxic H2S, and high nutrient input from seabirds (Charpy et al. 2010). It is likely that these changes also occurred at North Keeling Atoll, given its moderate rainfall, abundant nesting seabirds, and benthic layer of lagoon water smelling like H2S. We conducted detailed snorkel surveys around the entire outside of the atoll (at 1–15 m depth) and did not locate any seagrass, bonefish or juveniles of fishes previously reported from the lagoon. Species reliant on seagrass or lagoonal habitats may have become locally extinct. The endemic endangered buff-banded rail may also be affected because it forages on seagrass-associated crustacea (Cochrane 2004). The northern atoll was declared a national park in 1995 to protect the unique biodiversity, with the lagoon, considered an important nursery habitat, also listed as a Ramsar Wetland (Cochrane 2004). The Ramsar listing criteria are still met because it is a rare and unique natural wetland, and the surrounding Communicated by B. W. Hoeksema

The influence of repetitive thermal stresses on the dominance of reef-building Acropora spp. (Scleractinia) on coral reefs of the Maldive Islands

The distribution and size-age structure of Acropora corals were studied in two Maldivian atolls that differ in their geographic position and sea surface temperature regimes. The frequency and strength of thermal anomalies for the last 2 decades had a significant influence on the abundance, mortality rates, and age structures of acroporid communities. The long-term temperature amplitude was higher and the maxima were more pronounced in the northernmost Ihavandippolu Atoll than those in the equatorial South Huvadhoo Atoll. These differences resulted in a 10.4% mean cover of Acropora at Ihavandippolu Atoll, whereas the Acropora cover in the South Huvadhoo Atoll reached 59.5%. In the northern atoll, the coral mortality rate after the 2010 thermal anomaly was 3 times higher than that in the southern atoll. Younger acroporid colonies (up to 2 years old) dominated the northern atoll reefs, while the southern atoll showed a high proportion of older mature colonies. In both atolls, healthy table colonies of Acropora cytherea with a disk diameter greater than 2 m were observed that apparently survived three thermal anomalies since 1998. The mechanisms of acclimatization of Acropora and the prospects for its dominance in the Maldives under changing environmental conditions are discussed.

Impact of repetitive thermal anomalies on survival and development of mass reef‐building corals in theMaldives

AbstractThe distribution of dominant coral genera and their response to the recent sea temperature anomalies in the last three decades are analyzed across reefs of the Maldivian Archipelago, which spans 860 × 120 km in the central Indian Ocean. The Maldives suffered one of the worst coral mortality rates in the Indian Ocean in the 1998 warming event and experienced two sub‐lethal thermal anomalies in 2003 and 2010. The results showed that theAcroporaproved an important driver of post‐1998 recovery and has become a major dominant genus in most Maldivian reefs at present. The average coral cover and proportion of acroporids have decreased from south to north and represent correspondently 70.6/59% for the southern atoll, 62/53% for the central atoll and 33/10.3% for the northernmost atoll.Stylophora,Seriatoporaand the hydrocoralMilleporawere not found and are considered candidates for local extirpation from the Maldives. Higher thermal variability and frequency of thermal stress might lead to the decrease in the abundance of susceptible taxa in the northernmost atoll. No phase‐shift to algae‐dominated reefs was observed in any of the three locations. Factors contributing to coral recovery and acclimatization are discussed.

Island shadow effects and the wave climate of the Western Tuamotu Archipelago (French Polynesia) inferred from altimetry and numerical model data

To implement a numerical model of atoll lagoon circulation, we characterized first the significant wave height (Hs) regime of the Western Tuamotu Archipelago and the local attenuation due to the protection offered by large atolls in the south Tuamotu. Altimetry satellite data and a WAVEWATCH III two-way nested wave model at 5km resolution from 2000 to 2010 were used. Correlation between altimetry and model was high (0.88) over the period. According to the wave model, the archipelago inner seas experienced attenuated Hs year-long with a yearly average Hs around 1.3m vs a minimum of 1.6m elsewhere. The island shadow effect is especially significant in the austral winter. In contrast with southern atolls, Western Tuamotu experienced only few days per year of Hs larger than 2.5m generated by very high Hs southern swell, transient western local storms, strong easterly winds, and during the passage of distant hurricanes.

RADIATION DOSES AND CANCER RISKS IN THE MARSHALL ISLANDS ASSOCIATED WITH EXPOSURE TO RADIOACTIVE FALLOUT FROM BIKINI AND ENEWETAK NUCLEAR WEAPONS TESTS: SUMMARY

Nuclear weapons testing conducted at Bikini and Enewetak Atolls during 1946-1958 resulted in exposures of the resident population of the present-day Republic of the Marshall Islands to radioactive fallout. This paper summarizes the results of a thorough and systematic reconstruction of radiation doses to that population, by year, age at exposure, and atoll of residence, and the related cancer risks. Detailed methods and results are presented in a series of companion papers in this volume. From our analysis, we concluded that 20 of the 66 nuclear tests conducted in or near the Marshall Islands resulted in measurable fallout deposition on one or more of the inhabited atolls of the Marshall Islands. In this work, we estimated deposition densities (kBq m(-2)) of all important dose-contributing radionuclides at each of the 32 atolls and separate reef islands of the Marshall Islands. Quantitative deposition estimates were made for 63 radionuclides from each test at each atoll. Those estimates along with reported measurements of exposure rates at various times after fallout were used to estimate radiation absorbed doses to the red bone marrow, thyroid gland, stomach wall, and colon wall of atoll residents from both external and internal exposure. Annual doses were estimated for six age groups ranging from newborns to adults. We found that the total deposition of 137Cs, external dose, internal organ doses, and cancer risks followed the same geographic pattern with the large population of the southern atolls receiving the lowest doses. Permanent residents of the southern atolls who were of adult age at the beginning of the testing period received external doses ranging from 5 to 12 mGy on average; the external doses to adults at the mid-latitude atolls ranged from 22 to 59 mGy on average, while the residents of the northern atolls received external doses in the hundreds to over 1,000 mGy. Internal doses varied significantly by age at exposure, location, and organ. Except for internal doses to the thyroid gland, external exposure was generally the major contributor to organ doses, particularly for red bone marrow and stomach wall. Internal doses to the stomach wall and red bone marrow were similar in magnitude, about 1 mGy to 7 mGy for permanent residents of the southern and mid-latitude atolls. However, adult residents of Utrik and Rongelap Island, which are part of the northern atolls, received much higher internal doses because of intakes of short-lived radionuclides leading to doses from 20 mGy to more than 500 mGy to red bone marrow and stomach wall. In general, internal doses to the colon wall were four to ten times greater than those to the red bone marrow and internal doses to the thyroid gland were 20 to 30 times greater than to the red bone marrow. Adult internal thyroid doses for the Utrik community and for the Rongelap Island community were about 760 mGy and 7,600 mGy, respectively. The highest doses were to the thyroid glands of young children exposed on Rongelap at the time of the Castle Bravo test of 1 March 1954 and were about three times higher than for adults. Internal doses from chronic intakes, related to residual activities of long-lived radionuclides in the environment, were, in general, low in comparison with acute exposure resulting from the intakes of radionuclides immediately or soon after the deposition of fallout. The annual doses and the population sizes at each atoll in each year were used to develop estimates of cancer risks for the permanent residents of all atolls that were inhabited during the testing period as well as for the Marshallese population groups that were relocated prior to the testing or after it had begun. About 170 excess cancers (radiation-related cases) are projected to occur among more than 25,000 Marshallese, half of whom were born before 1948. All but about 65 of those cancers are estimated to have already been expressed. The 170 excess cancers are in comparison to about 10,600 cancers that would spontaneously arise, unrelated to radioactive fallout, among the same cohort of Marshallese people.

Erosion vs. Recovery of Coral Reefs after 1998 El Niño: Chagos Reefs, Indian Ocean

Three years after most corals died on the central Indian Ocean reefs of Chagos, erosion and recovery were studied to 30 m depth. Mortality was near-total to 15 m deep in northern atolls, and to > 35 m in central and southern atolls. Some reef surfaces have 'dropped' 1.5 m due to the loss of dense coral thickets. Coral bioerosion is substantial, reducing 3-D reef 'structure' and forming unconsolidated rubble. Juvenile corals are abundant, though mostly on eroding or unstable substrates, and are of less robust species. Reef fish abundance and diversity at 15 m depth remains high; species dependent on corals have diminished, while some herbivores and detritivores have increased. A new sea surface temperature (SST) data set shows that mean SST has risen 0.65 degree C since 1950. The critical SST causing the mortality in Chagos was 29.9 degrees C.

Thyroid Neoplasia in Marshall Islanders Exposed to Nuclear Fallout

We studied the risk of thyroid neoplasia in Marshall Islanders exposed to radioiodines in nuclear fallout from the 1954 BRAVO thermonuclear test. We screened 7266 Marshall Islanders for thyroid nodules; the islanders were from 14 atolls, including several southern atolls, which were the source of the best available unexposed comparison group. Using a retrospective cohort design, we determined the prevalence of thyroid nodularity in a subgroup of 2273 persons who were alive in 1954 and who therefore were potentially exposed to fallout from the BRAVO test. For those 12 atolls previously thought to be unexposed to fallout, the prevalence of thyroid nodules ranged from 0.9% to 10.6%. Using the distance of each atoll from the test site as a proxy for the radiation dose to the thyroid gland, a weighted linear regression showed an inverse linear relationship between distance and the age-adjusted prevalence of thyroid nodules. Distance was the strongest single predictor in logistic regression analysis. A new absolute risk estimate was calculated to be 1100 excess cases/Gy/y/1 X 10(6) persons (11.0 excess cases/rad/y/1 million persons), 33% higher than previous estimates. We conclude that an excess of thyroid nodules was not limited only to the two northern atolls but extended throughout the northern atolls; this suggests a linear dose-response relationship.