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

Abstract 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

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

Sensitivity of ecosystem condition scores depicted by correlation coefficients between individual metrics and their respective latent variables (circles-fish, squares-benthic, and crosses-corals), or between latent variables and overall ecosystem condition (bottom triangles).

<p>Error bars (thin lines) represent standard deviations associated with the differing jurisdiction and reef-habitat strata.</p

Regression models depicting the relationships between localized stressors and ecosystem condition.

<p>Fishing pressure predicted ecosystem condition on reefs > 5 km from land across Micronesia, with notable variation among islands and reef habitats (a). Independent slopes, in turn, predicted the variance among the top four non-normalized ecosystem condition scores (b, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130823#pone.0130823.s005" target="_blank">S2 Table</a>), and also the relative influence of wave exposure—coral assemblages—ecosystem condition (blue), or the fishing pressure proxy-fish assemblages—ecosystem condition (red), or a direct relationship between the two integrated variables, fishing pressure proxy—ecosystem condition (black). Conversely, interactions between fishing and pollution proxies, termed localized stressors, best predicted condition for lagoon reefs <5 km from land (c), with stressors acting equally (blue) or unequally (red) across benthic, coral, and fish latent variables. Excluding marine protected areas (MPA, dashed circles around site symbols) did not improve the regional model fit, but removing two successful MPAs on Pohnpei (circles on furthest right) did improve the localized association between stressors and ecosystem condition.</p

A map of the tropical Western Pacific Ocean and Micronesia with study islands indicated by stars.

<p>A map of the tropical Western Pacific Ocean and Micronesia with study islands indicated by stars.</p

Normalized ecosystem-condition scores across Micronesia.

<p>Monitoring sites were stratified by reef habitat, management, geography, and/or wave exposure, as appropriate (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0130823#pone.0130823.s001" target="_blank">S1 Fig</a>). Dashed lines depict the ‘effective-conservation’ threshold used to assess progress towards the Micronesia-Challenge conservation goals. Percentages indicate the proportion of sites currently above the threshold. Site-circle sizes on the maps adjacent to the bar graphs were scaled by their normalized ecosystem-condition scores. Marine protected areas are designated on the bar graphs with an asterisk (*).</p