Investigating the Effects of Nutrient and Sediment Loading on Coral Thermal Tolerance in a Branching Coral Species

Coral reefs, some of the most diverse ecosystems in the world, face increased pressures from global and local-scale anthropogenic stressors. Therefore, a better understanding of the ecological ramifications of thermal anomalies and land-based pollution on coral reef ecosystems is necessary. In this study, we used thermal performance curves (TPCs) to quantify the shape of the relationship between metabolic rates (photosynthesis, respiration, and calcification) of Pocillopora acutaand temperature at high and low pollution sites in Mo'orea, French Polynesia. We also measured key physiological parameters including chlorophyll acontent, endosymbiont densities, tissue biomass, and tissue nitrogen content for all colonies. We found that low pollution sites (i.e. lower sedimentation rates and nutrient loading) exhibited higher endosymbiont densities, and chlorophyll acontent of coral tissue per unit of surface area. Furthermore, we found that corals from low pollution sites exhibited higher maximum rates of performance (Pmax) and rates at a constant temperature (b(T_c)) for gross photosynthetic and respiration rates. Respiration rates exhibited higher thermal optimums than gross photosynthetic and calcification rates, regardless of site. Gross photosynthetic rates had higher activation energy (E), Pmax, and b(T_c) than respiration and calcification rates, regardless of site. These results indicate that higher levels of land-based pollution can negatively influence corals’ metabolic response to thermal stress. Understanding how local-scale anthropogenic stressors are influencing corals’ responses to temperature is integral information for coral reef management to help mediate the influx of sediment and nutrients into coastal coral reef ecosystems.