Predicting coral-reef futures from El Niño and Pacific Decadal Oscillation events

Past disturbances and local conditions influence the recovery rates of coral reefs

Corals are being increasingly subjected to marine heatwaves. Theory suggests that increasing the intensity of disturbances reduces recovery rates, which inspired us to examine the recovery rates of coral cover following marine heatwaves, cyclones, and other disturbances at 1921 study sites, in 58 countries and three oceans, from 1977 to 2020. In the Atlantic Ocean, coral cover has decreased fourfold since the 1970s, and recovery rates following disturbances have been relatively slow, except in the Antilles. By contrast, reefs in the Pacific and Indian Oceans have maintained coral cover and recovery rates over time. There were positive relationships between rates of coral recovery and prior cyclone and heatwave frequency, and negative relationships between rates of coral recovery and macroalgae cover and distance to shore. A recent increase in the variance in recovery rates in some ecoregions of the Pacific and Indian Oceans suggests that some reefs in those ecoregions may be approaching a phase shift. While marine heatwaves are increasing in intensity and frequency, our results suggest that regional and local conditions influence coral recovery rates, and therefore, effective local management efforts can help reefs recover from disturbances.

Assessing spatiotemporal change in coral reef social-ecological systems

Coral reef resilience is eroding at multiple spatial scales globally, with broad implications for coastal communities, and is thus a critical challenge for managing marine social-ecological systems (SESs). Many researchers believe that external stressors will cause key coral reefs to die by the end of the 21st century, virtually eliminating essential ecological and societal benefits. Here, we propose the use of resilience-based approaches to understand the dynamics of coral reef SESs and subsequent drivers of coral reef decline. Previous research has demonstrated the effectiveness of these methods, not only for tracking environmental change, but also for providing warning in advance of transitions, possibly allowing time for management interventions. The flexibility and utility of these methods make them ideal for assessing complex systems; however, they have not been used to study aquatic ecosystem dynamics at the global scale. Here, we evaluate these methods for examining spatiotemporal change in coral reef SESs across the global seascape and assess the subsequent impacts on coral reef resilience. We found that while univariate indicators failed to provide clear signals, multivariate resilience-based approaches effectively captured coral reef SES dynamics, unveiling distinctive patterns of variation throughout the global coral reef seascape. Additionally, our findings highlight global spatiotemporal variation, indicating patterns of degraded resilience. This degradation was reflected regionally, particularly in the Pacific Ocean and Indian Ocean SESs. These results underscore the utility of resilience-based approaches in assessing environmental change in SESs, detecting spatiotemporal variation at the global and regional scales, and facilitating more effective monitoring and management of coral reef SESs.

Moderate chlorophyll-a environments reduce coral bleaching during thermal stress in Yap, Micronesia

Thermal-stress events on coral reefs lead to coral bleaching, mortality, and changes in species composition. The coral reefs of Yap, in the Federated States of Micronesia, however, remained largely unaffected by major thermal-stress events until 2020, when temperatures were elevated for three months. Twenty-nine study sites were examined around Yap to determine geographical and taxonomic patterns of coral abundance, bleaching susceptibility, and environmental predictors of bleaching susceptibility. Island-wide, 21% (± 14%) of the coral cover was bleached in 2020. Although inner reefs had a greater proportion of thermally-tolerant Porites corals, the prevalence of bleaching was consistently lower on inner reefs (10%) than on outer reefs (31%) for all coral taxa. Corals on both inner and outer reefs along the southwestern coast exhibited the lowest prevalence of coral bleaching and had consistently elevated chlorophyll-a concentrations. More broadly, we revealed a negative relationship between bleaching prevalence and (moderate) chlorophyll-a concentrations that may have facilitated resistance to thermal stress by reducing irradiance and providing a heterotrophic energy source to benefit some corals exposed to autotrophic stress. Southwestern reefs also supported a high but declining fish biomass, making these bleaching-resistant and productive reefs a potential climate-change refuge and a prime target for conservation.

Potential local adaptation of corals at acidified and warmed Nikko Bay, Palau

Ocean warming and acidification caused by increases of atmospheric carbon dioxide are now thought to be major threats to coral reefs on a global scale. Here we evaluated the environmental conditions and benthic community structures in semi-closed Nikko Bay at the inner reef area in Palau, which has high pCO₂ and seawater temperature conditions with high zooxanthellate coral coverage. Nikko Bay is a highly sheltered system with organisms showing low connectivity with surrounding environments, making this bay a unique site for evaluating adaptation and acclimatization responses of organisms to warmed and acidified environments. Seawater pCO₂/Ω_arag showed strong gradation ranging from 380 to 982 µatm (Ω_arag: 1.79-3.66), and benthic coverage, including soft corals and turf algae, changed along with Ω_arag while hard coral coverage did not change. In contrast to previous studies, net calcification was maintained in Nikko Bay even under very low mean Ω_arag (2.44). Reciprocal transplantation of the dominant coral Porites cylindrica showed that the calcification rate of corals from Nikko Bay did not change when transplanted to a reference site, while calcification of reference site corals decreased when transplanted to Nikko Bay. Corals transplanted out of their origin sites also showed the highest interactive respiration (R) and lower gross photosynthesis (Pg) to respiration (Pg:R), indicating higher energy acquirement of corals at their origin site. The results of this study give important insights about the potential local acclimatization and adaptation capacity of corals to different environmental conditions including pCO₂ and temperature.