Increasing frequency and severity of disturbances is causing global degradation of coral reef ecosystems. This study examined temporal changes in live coral cover and coral composition in the central Maldives from 1997 to 2016, encompassing two bleaching events, a tsunami, and an outbreak of Acanthaster planci. We also examined the contemporary size structure for five dominant coral taxa (tabular Acropora, Acropora muricata, Acropora humilis, Pocillopora spp, and massive Porites). Total coral cover increased throughout the study period, with marked increases following the 1998 mass-bleaching. The relative abundance of key genera has changed through time, where Acropora and Pocillopora (which are highly susceptible to bleaching) were under-represented following 1998 mass-bleaching but increased until outbreaks of A. planci in 2015. The contemporary size-structure for all coral taxa was dominated by larger colonies with peaked distributions suggesting that recent disturbances had a disproportionate impact on smaller colonies, or that recruitment is currently limited. This may suggest that coral resilience has been compromised by recent disturbances, and further bleaching (expected in 2016) could lead to highly protracted recovery times. We showed that Maldivian reefs recovered following the 1998 mass-bleaching event, but it took up to a decade, and ongoing disturbances may be eroding reef resilience.

Coral growth is an important component of reef health and resilience. However, few studies have investigated temporal and/or spatial variation in growth of branching corals, which are important contributors to the structure and function of reef habitats. This study assessed growth (linear extension, density, and calcification) of three branching coral species (Acropora muricata, Pocillopora damicornis and Isopora palifera) at three distinct locations (Lizard Island, Davies/Trunk Reef, and Heron Island) along Australia’s Great Barrier Reef (GBR). Annual growth rates of all species were highest at Lizard Island and declined with increasing latitude, corresponding with differences in temperature. Within locations, however, seasonal variation in growth did not directly correlate with temperature. Between October 2012 and October 2014, the highest growth of A. muricata was in the 2013–14 summer at Lizard Island, which was unusually cool and ~0.5 °C less than the long-term summer average temperature. At locations where temperatures reached or exceeded the long-term summer maxima, coral growth during summer periods was equal to, if not lower than, winter periods. This study shows that temperature has a significant influence on spatiotemporal patterns of branching coral growth, and high summer temperatures in the northern GBR may already be constraining coral growth and reef resilience.

Climate change is the greatest threat to coral reef ecosystems. In particular, increasing ocean temperatures are causing severe and widespread coral bleaching, contributing to extensive coral loss and degradation of coral reef habitats globally. Effects of coral bleaching are not however, equally apportioned among different corals, leading to shifts in population and community structure. This study explored variation in bleaching susceptibility and mortality associated with the 2016 severe mass bleaching in the Central Maldives Archipelago. Five dominant coral taxa (tabular Acropora , Acropora humilis , Acropora muricata , Pocillopora and massive Porites ) were surveyed in February 2016 and October 2017 to test for changes in abundance and size structure. Substantial taxonomic differences in rates of mortality were observed; the most severely affected taxa, Acropora , were virtually extirpated during the course of this study, whereas some other taxa (most notably, massive Porites ) were relatively unaffected. However, even the least affected corals exhibited marked changes in population structure. In February 2016 (prior to recent mass-bleaching), size-frequency distributions of all coral taxa were dominated by larger size classes with over-centralized, peaked distributions (negatively skewed with positive kurtosis) reflecting a mature population structure. In October 2017, after the bleaching, coral populations were dominated by smaller and medium size classes, reflecting high levels of mortality and injury among larger coral colonies. Pronounced changes in coral populations and communities in the Maldives, caused by coral bleaching and other disturbances (outbreaks of crown-of-thorns starfish and sedimentation), will constrain recovery capacity, further compounding upon recent coral loss.

Tropical marine ecosystems are richly diverse, but are experiencing growing pressure from coastal development and tourism. Assessing the status of coral reef communities along gradients of human pressure is necessary to predict recovery capacity of reefs exposed to acute events such as mass bleaching or storm destruction. Islands in the central Maldives Archipelago, which experience three different management regimes (four for each category: local community, uninhabited, and resort islands), were sampled during the International Union for Conservation of Nature (IUCN)-REGENERATE Cruise in 2015. Assessments were carried out using the FoRAM Index (FI), based on relative abundances of larger foraminiferal shells in reef sediments.Overall, FI values (> 5) indicate that water quality currently should support active accretion by reef-building corals and larger benthic foraminifers. The highest median FI values (5.9) were recorded from sites associated with the uninhabited islands. Slightly, but significantly lower medians were recorded at sites near community and resort islands (FI = 5.3 and 5.1, respectively) that host permanent human settlement, indicating possible local deterioration of water quality by disposal of domestic wastes. Note that the FI was designed to assess suitability of local water quality and not to assess responses to regional to global changes associated with temperature stress or ocean acidification.

No abstract

Coral reefs are threatened worldwide by a variety of natural and human-induced stressors; anomalous temperatures are presently among the most serious threats by causing extensive coral bleaching. Amphistegina spp. exhibit similar bleaching as corals in the presence of photo-oxidative stress induced by either light or temperature, especially during times of maximum solar irradiance. At 11 islands (34 sampling sites) in the North Ari Atoll in the Maldives, bleaching in Amphistegina was observed a few weeks before the onset of an extensive El Niño-related coral bleaching that was more severe than expected for this region. Assessment using the Amphistegina Bleaching Index (ABI) showed that the proportions of bleached specimens of Amphistegina in April–May 2015 can be explained by photo-inhibitory stress associated with temperatures exceeding 30°C during peak seasonal solar irradiance and water transparency. Importantly, the ABI indicates that environmental conditions are suitable for Amphistegina and other calcifying symbioses at most of the investigated sites, and that either chronic or relatively recent onset of photo-oxidative stress was present at the time of sampling. The observed bleaching in Amphistegina further demonstrates the potential of these unicellular protists to identify stressors in coral reefs; such applications should be considered in future reef-management plans.

Climate change is threatening the persistence of coral reef ecosystems resulting in both chronic and acute impacts which include higher frequency and severity of cyclones, warming sea surface temperatures, and ocean acidification. This study measured net ecosystem primary production (NEP) and net ecosystem calcification (NEC) on a reef flat after the most severe El Nino-driven mass bleaching event on Australia's Great Barrier Reef (GBR) in 2016 and again in 2018 after another consecutive bleaching event in 2017. Our results indicate temporal changes in reef metabolism likely as result of both the continuing press disturbance of ocean acidification and severe pulse disturbances (cyclones and bleaching events). In 2016, NEP was within the range of values reported in past studies, however, it declined in 2018. NEC over a 12-h period was lower in 2016 than 2018; but when compared with past studies there was a severe decline in daytime net calcification from 2008-2009, to 2016 followed by an increase in 2018 (but still NEC remained lower than values reported in 2008-2009). Conversely, nighttime net calcification was similar to that reported in 2009 indicating nighttime dissolution did not increase over the past decade. Overall coral cover remained stable following recent disturbances, however, algal turf was the dominant benthic component on the reef flat, while calcifiers (corals and calcified algae) were minor components (<20% of total benthic cover). This study documented temporal changes in community function following major pulse disturbances (bleaching events and cyclones) within the context of ongoing OA at the same location over the last decade. Repeated pulse disturbances could jeopardize the persistence of the reef flat as a net calcifying entity, with the potential for cascading effects on other ecosystem services.

This study aimed to assess how the size-frequency distributions of coral genera varied between reefs under different fishing pressures in two contrasting Indian Ocean locations (the Maldives and East Africa). Using generalized linear mixed models, we were able to demonstrate that complex interactions occurred between coral genera, coral size class and fishing pressure. In both locations, we found Acropora coral species to be more abundant in non-fished compared to fished sites (a pattern which was consistent for nearly all the assessed size classes). Coral genera classified as 'stress tolerant' showed a contrasting pattern i.e. were higher in abundance in fished compared to non-fished sites. Site specific variations were also observed. For example, Maldivian reefs exhibited a significantly higher abundance in all size classes of 'competitive' corals compared to East Africa. This possibly indicates that East African reefs have already been subjected to higher levels of stress and are therefore less suitable environments for 'competitive' corals. This study also highlights the potential structure and composition of reefs under future degradation scenarios, for example with a loss of Acropora corals and an increase in dominance of 'stress tolerant' and 'generalist' coral genera.