Reef islands have continually adjusted to environmental change over the past two millennia

Global environmental change is identified as a driver of physical transformation of coral reef islands over the past half-century, and next 100 years, posing major adaptation challenges to island nations. Here we resolve whether these recent documented changes in islands are unprecedented compared with the pre-industrial era. We utilise radiometric dating, geological, and remote sensing techniques to document the dynamics of a Maldivian reef island at millennial to decadal timescales. Results show the magnitude of island change over the past half-century (±40 m movement) is not unprecedented compared with paleo-dynamic evidence that reveals large-scale changes in island dimension, shape, beach levels, as well as positional changes of ±200 m since island formation ~1,500 years ago. Results highlight the value of a multi-temporal methodological approach to gain a deeper understanding of the dynamic trajectories of reef islands, to support development of adaptation strategies at timeframes relevant to human security.

Massive corals maintain a positive carbonate budget of a Maldivian upper reef platform despite major bleaching event

Coral reefs experienced the third global bleaching event in 2015–2016 due to high sea-surface temperature (SST) anomalies. Declines in net carbonate production associated with coral bleaching are implicated in reef structural collapse and cascading impacts for adjacent coral reef islands. We present the first carbonate budget study of a reef platform surface (reef crest and reef flat) in the southern Maldives and the first record of upper reef flat condition in the central Indian Ocean post the 2015–2016 coral bleaching event. Scleractinian corals were the primary carbonate producers, with live coral cover averaging between 11.1 ± 6.5 and 31.2 ± 21.8% and dominated by massive corals. Gross carbonate production rates averaged 5.9 ± 2.5 G (kg CaCO₃ m² yr⁻¹). Bioerosion was estimated at 3.4 ± 0.4 G, resulting in an average net carbonate production rate of 2.5 ± 2.4 G. Comparison of results with a study of the fore-reef slope highlights major differences in post-bleaching carbonate budget state between the fore-reef slope and the reef platform surface. The positive reef flat carbonate budget is attributed to the persistence of massive corals (Porites spp. and Heliopora spp.) through the bleaching event.

Predicting wave overtopping thresholds on coral reef-island shorelines with future sea-level rise

Wave-driven flooding is a serious hazard on coral reef-fringed coastlines that will be exacerbated by global sea-level rise. Despite the global awareness of atoll island vulnerability, little is known about the physical processes that control wave induced flooding on reef environments. To resolve the primary controls on wave-driven flooding at present and future sea levels, we present a globally applicable method for calculating wave overtopping thresholds on reef coastlines. A unique dataset of 60,000 fully nonlinear wave transformation simulations representing a wide range of wave energy, morphology and sea levels conditions was analysed to develop a tool for exploring the future trajectory of atoll island vulnerability to sea-level rise. The proposed reef-island overtopping threshold (RIOT) provides a widely applicable first-order assessment of reef-coast vulnerability to wave hazards with sea-level. Future overtopping thresholds identified for different atoll islands reveal marked spatial variability and highlight distinct morphological characteristics that enhance coastal resilience.

Sea-level rise will exacerbate wave overtopping on low-lying coral reef islands. Here the authors present a novel method that quantifies wave overtopping thresholds and associated reef-island vulnerability trajectories based on differences in local wave climate, reef morphology and island height.

Skeletal extension and calcification of reef-building corals in the central Indian Ocean

Skeletal extension, density and calcification rate of 12 scleractinian coral species comprising 7 morphological groups were examined on the reef crest of Vabbinfaru platform, Maldives. Growth rates of coral specimens were measured over the period of February 2010-March 2011 using alizarin staining and direct measurements. Skeletal extension rate was highly variable between coral species. Colony morphology was a major control on the skeletal extension and calcification of coral specimens. Growth rates of Acropora and Porites corals were comparable to existing data recorded for Caribbean and Indo-Pacific reef provinces. Skeletal density was less variable between species and was typically consistent among morphological groups. Findings represent the first estimates of coral growth in the central Indian Ocean region and add to the limited growth studies available that have examined a broad range of coral growth morphologies in other reef-building regions.