A positive trajectory for corals atLittle Cayman Island

Widespread coral bleaching and mass mortality during the 2023-2024 marine heatwave in Little Cayman

The increased frequency and intensity of marine heatwaves (MHWs) induced by continued global warming are the greatest threat to tropical coral reefs, causing mass bleaching events and widespread mortality of reef building corals. In 2023, the isolated and well-protected reefs around Little Cayman experienced a MHW of > 17 Degree Heating Weeks (DHW), far exceeding any DHW measure previously captured. During the peak of the heatwave, ~ 80% of all corals were either bleached or showing signs of mortality. On the final survey date ~54% of all corals surveyed were recorded as dead. However, we identified significant differences in bleaching susceptibility and mortality across taxonomic groups, related to different life history strategies. Notably, weedy coral taxa such as Agaricia spp., Porites astreoides, and Porites porites, experienced high bleaching and suffered extensive mortality. Meanwhile, stress-tolerant reef building taxa such as Orbicella spp., experienced bleaching, but suffered low mortality. Given Little Cayman reefs have not been exposed to previous thermal stress events, the highly sensitive weedy taxa disproportionately contributed to coral abundance. Thus, the occurrence of a high magnitude - long duration heatwave resulted in catastrophic mortality of corals in Little Cayman, despite ~57% of the coastal environment being classified as no-take Marine Protected Areas. These findings underscore that the global stressor of global climate change, which drives MHWs, cannot be mitigated by local protection and isolation, thus highlighting the need to directly tackle the cause of coral decline (i.e., global climate change).

Creation of complex reef structures through coral restoration does not affect associated fish populations on a remote, well-protected, Caribbean reef

Coral reef ecosystems are facing severe degradation due to anthropogenic activities at both local and global scales. In response, extensive restoration efforts are underway, aiming to bolster coral cover and enhance reef fish communities to foster facilitation between fish and corals. This reciprocal relationship is anticipated to improve overall restoration efficacy and enhance coral reef resilience in the face of global warming. Here, we investigate the impact of coral restoration using out-planted Acropora cervicornis colonies attached to raised domes on the associated fish community on the isolated, well-protected reef of Little Cayman Island in the Central Caribbean. Surveys were conducted immediately preceding out-planting, five days later, and 85 days later to capture temporal changes in the fish community. After 85 days of out-planting, there were no changes in fish biomass, abundance, or species richness for the entire fish community. This pattern was consistent for selected fish functional groups. Additionally, no significant differences were observed in the fish community before outplanting, five days after out-planting, or 85 days after out-planting of restoration domes. Our results underscore the limited impact of coral restoration for influencing fish communities in the isolated and highly protected reef of Little Cayman over an 85-day period. Consequently, our findings have implications for using coral restoration as a mechanism to enhance fish populations, particularly in marginally disturbed regions where structural complexity has not been lost. Future restoration programs should therefore incorporate local knowledge of environmental history and restoration needs along with an increased data-driven understanding of the intricate interaction between fish and coral populations to be successful.

A decade of study on the condition of western Cuban coral reefs, with low human impact

Background

The long-time study of coral reefs with low human impacts can provide information on the effects of regional pressures like climate change, and is an opportunity to document how these pressures are reflected in coral communities. An example of minimal local anthropogenic impacts are the Guanahacabibes coral reefs, located in the westernmost region of Cuba. The objectives of this study were: to evaluate the temporal variability of six benthic biological indicators of coral reefs, and to explore the possible relationship between predictive abiotic variables and biological response variables.

Methods

Four coral reef sites were sampled between 2008 and 2017, to analyze biological indicators (living coral cover, fleshy algae index, coral species richness, coral species abundance, coral trait groups species abundance, Functional Reef Index). Seven abiotic variables (wave exposure, sea surface temperature, degree heating week, chlorophyll-a concentration, particulate organic carbon, photosynthetically available radiation, and the diffuse attenuation coefficient) were compiled between 2007 and 2016, from remote sensing datasets, to analyze their relationship with the biological indicators. Permanova statistical analysis was used to evaluate trends in biological variables between sites and years, and Routine Analysis Based on Linear Distances (DISTLM) was used to explore some dependencies between biotic and abiotic variables.

Results

We found significant variability in the temporal analysis, with a decrease in living coral cover, a decline in the predominance of the branching and massive framework reef-building species, a decline in Orbicella species abundance, and an increase in the fleshy algae index. Some abiotic variables (average of degree heating weeks, standard deviation of the diffuse attenuation coefficient, average of the sea surface temperature, among others) significantly explained the variability of biological indicators; however, determination coefficients were low.

Conclusions

Certain decrease in the functionality of the coral reef was appreciated, taking into account the predominance of secondary and nom-massive framework reef-building species in the last years. A weak association between abiotic and biological variables was found in the temporal analysis. The current scenario of the condition of the coral reefs seems to be regulated by the global effects of climate change, weakly associated effects, and in longer terms.

Climate Change, Coral Loss, and the Curious Case of the Parrotfish Paradigm: Why Don't Marine Protected Areas Improve Reef Resilience?

Scientists have advocated for local interventions, such as creating marine protected areas and implementing fishery restrictions, as ways to mitigate local stressors to limit the effects of climate change on reef-building corals. However, in a literature review, we find little empirical support for the notion of managed resilience. We outline some reasons for why marine protected areas and the protection of herbivorous fish (especially parrotfish) have had little effect on coral resilience. One key explanation is that the impacts of local stressors (e.g., pollution and fishing) are often swamped by the much greater effect of ocean warming on corals. Another is the sheer complexity (including numerous context dependencies) of the five cascading links assumed by the managed-resilience hypothesis. If reefs cannot be saved by local actions alone, then it is time to face reef degradation head-on, by directly addressing anthropogenic climate change-the root cause of global coral decline.

Implications of high rates of sexual recruitment in driving rapid reef recovery in Mo'orea, French Polynesia

Coral abundance continues to decline on tropical reefs around the world, and this trend suggests that coral reefs may not persist beyond the current century. In contrast, this study describes the near-complete mortality of corals on the outer reef (10 m and 17 m depth) of the north shore of Mo’orea, French Polynesia, from 2005 to 2010, followed by unprecedented recovery from 2011 to 2017. Intense corallivory and a cyclone drove coral cover from 33–48% to <3% by 2010, but over the following seven years, recovery occurred through rapid population growth (up to 12% cover y⁻¹) to 25–74% cover by 2017. The thirteen-year, U-shape trajectory of coral cover over time created by the loss and replacement of millions of corals through sexual reproduction underscores the potential for beneficial genetic responses to environmental conditions for at least one genus, Pocillopora. The high ecological resilience of this coral community appears to have been enhanced by variation among genera in the susceptibility to declining cover, and the capacity for population growth (i.e., response diversity). These results suggest that the outer coral communities of Mo’orea may be poised for genetic changes that could affect their capacity to persistence.

Mortality, recovery, and community shifts of scleractinian corals in Puerto Rico one decade after the 2005 regional bleaching event

This work analyzes the mortality, recovery, and shifts in the composition of scleractinian corals from Puerto Rico one decade after the 2005 regional coral bleaching event. Temporal and spatial patterns of coral community structure were examined using a stratified, non-random sampling approach based on five permanent transects per reef at 16 reef stations. A negative correlation between percent coral cover loss and light attenuation coefficient (Kd₄₉₀) was observed, suggesting that light attenuation, as influenced by water turbidity and depth, played a major role in coral protection during the bleaching event (“sunblock effect”). Responses of coral assemblages varied after the bleaching event, including shifts of cover from massive corals (Orbicella spp.) to opportunistic (Porites astreoides) and branching corals (Madracis auretenra, P. porites) and/or turf algae; partial recovery of reef substrate cover by O. annularis complex; and no measurable changes in coral assemblages before and after the event.

A study on the recovery of Tobago's coral reefs following the 2010 mass bleaching event

In 2010, severe coral bleaching was observed across the southeastern Caribbean, including the island of Tobago, where coral reefs are subject to sedimentation and high nutrient levels from terrestrial runoff. Here we examine changes in corals' colony size distributions over time (2010-2013), juvenile abundances and sedimentation rates for sites across Tobago following the 2010 bleaching event. The results indicated that since pre-bleaching coral cover was already low due to local factors and past disturbance, the 2010 event affected only particular susceptible species' population size structure and increased the proportion of small sized colonies. The low density of juveniles (mean of 5.4±6.3 juveniles/m(-2)) suggests that Tobago's reefs already experienced limited recruitment, especially of large broadcasting species. The juvenile distribution and the response of individual species to the bleaching event support the notion that Caribbean reefs are becoming dominated by weedy non-framework building taxa which are more resilient to disturbances.

Effects of Protection and Sediment Stress on Coral Reefs in Saint Lucia

The extent to which Marine Protected Areas (MPAs) benefit corals is contentious. On one hand, MPAs could enhance coral growth and survival through increases in herbivory within their borders; on the other, they are unlikely to prevent disturbances, such as terrestrial runoff, that originate outside their boundaries. We examined the effect of spatial protection and terrestrial sediment on the benthic composition of coral reefs in Saint Lucia. In 2011 (10 to 16 years after MPAs were created), we resurveyed 21 reefs that had been surveyed in 2001 and analyzed current benthic assemblages as well as changes in benthic cover over that decade in relation to protection status, terrestrial sediment influence (measured as the proportion of terrigenous material in reef-associated sediment) and depth. The cover of all benthic biotic components has changed significantly over the decade, including a decline in coral and increase in macroalgae. Protection status was not a significant predictor of either current benthic composition or changes in composition, but current cover and change in cover of several components were related to terrigenous content of sediment deposited recently. Sites with a higher proportion of terrigenous sediment had lower current coral cover, higher macroalgal cover and greater coral declines. Our results suggest that terrestrial sediment is an important factor in the recent degradation of coral reefs in Saint Lucia and that the current MPA network should be complemented by measures to reduce runoff from land.