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.

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

El Niño Southern Oscillation (ENSO) events modulate oceanographic processes that control temperature and productivity in tropical waters, yet potential interactions with low frequency climate variability, such as the Pacific Decadal Oscillation (PDO), are poorly understood. We show that ENSO and PDO together predicted (i) maximum sea-surface temperatures (SST), which were associated with coral bleaching and declines in coral cover, and (ii) maximum chlorophyll-a concentrations, which were associated with high densities of coral-predatory Acanthaster starfish, across the tropical north Pacific Ocean since 1980. Asynchrony between the positive PDO and negative ENSO (i.e., La Niña) was associated with peaks in annual SST. By contrast, synchrony between the positive PDO and positive ENSO (i.e., El Niño) was associated with peaks in chlorophyll-a. Both conditions led to ecological disturbances and significant loss of coral cover, however, spatial models revealed where impacts to reefs were expected under varying climate scenarios.  The 2015/17 ENSO event was coupled with a positive PDO and resulted in high SST and Acanthaster abundances in eastern Micronesia, while positive coral growth occurred in western Micronesia.  Our novel approach for forecasting coral growth into the future may be applicable to other oceanic regions with differing oceanographic modulators.

The Micronesia Challenge: Assessing the Relative Contribution of Stressors on Coral Reefs to Facilitate Science-to-Management Feedback.

Fishing and pollution are chronic stressors that can prolong recovery of coral reefs and contribute to ecosystem decline. While this premise is generally accepted, management interventions are complicated because the contributions from individual stressors are difficult to distinguish. The present study examined the extent to which fishing pressure and pollution predicted progress towards the Micronesia Challenge, an international conservation strategy initiated by the political leaders of 6 nations to conserve at least 30% of marine resources by 2020. The analyses were rooted in a defined measure of coral-reef-ecosystem condition, comprised of biological metrics that described functional processes on coral reefs. We report that only 42% of the major reef habitats exceeded the ecosystem-condition threshold established by the Micronesia Challenge. Fishing pressure acting alone on outer reefs, or in combination with pollution in some lagoons, best predicted both the decline and variance in ecosystem condition. High variances among ecosystem-condition scores reflected the large gaps between the best and worst reefs, and suggested that the current scores were unlikely to remain stable through time because of low redundancy. Accounting for the presence of marine protected area (MPA) networks in statistical models did little to improve the models' predictive capabilities, suggesting limited efficacy of MPAs when grouped together across the region. Yet, localized benefits of MPAs existed and are expected to increase over time. Sensitivity analyses suggested that (i) grazing by large herbivores, (ii) high functional diversity of herbivores, and (iii) high predator biomass were most sensitive to fishing pressure, and were required for high ecosystem-condition scores. Linking comprehensive fisheries management policies with these sensitive metrics, and targeting the management of pollution, will strengthen the Micronesia Challenge and preserve ecosystem services that coral reefs provide to societies in the face of climate change.