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The Influence of Nutrient and Sediment Loading on Multiple Facets of Coral Functionality in Branching Coral Species
Coral reefs, some of the most diverse ecosystems in the world, face increasing pressures from global and local-scale anthropogenic stressors. Nutrient enrichment and sediment loading could make corals more vulnerable to global climate change by suppressing calcification and reducing their photosynthetic performance. Yet, some nutrient-enriched reef environments have exhibited higher coral growth rates, coral productivity, and coral thermal performance. To properly inform management in coastal coral reef ecosystems, a better understanding of warming and land-based inputs (i.e. sedimentation rates and nutrient loading) on coral reef ecosystems is necessary. This thesis is comprised of two studies that address the ecological and physiological influence of land-based inputs on two branching coral species to thermal stress. The first study tested how a natural nutrient and sedimentation gradient affected multiple facets of coral functionality, including algal endosymbiont and coral host response variables, holobiont metabolic responses, and percent cover of Pocillopora acuta colonies on the north shore fringing reefs in Mo'orea, French Polynesia. Algal endosymbiont % nitrogen content, algal endosymbiont densities, and total chlorophyll a content increased with nutrient input, while algal endosymbiont nitrogen content cell-1 decreased, likely representing competition among algal endosymbionts. Nutrient and sediment loading decreased coral metabolic responses to thermal stress in terms of their thermal performance and metabolic rate processes, and the percent cover of P. acuta colonies decreased by nearly two orders of magnitude along the nutrient gradient. The second study focused on the direct effects of nutrient enrichment on the thermal performance of corals on a fore reef in Mo'orea, French Polynesia. In this study, I measured how chronic nutrient enrichment affected coral physiology, including endosymbiont and coral host response variables, and holobiont metabolic responses of Pocillopora spp. colonies in Mo'orea, French Polynesia. I experimentally enriched corals with nitrogen and phosphorus combined for 15 months on the oligotrophic fore reef of Mo'orea. I first characterized symbiont and coral physiological traits due to enrichment and second used thermal performance curves to quantify the relationship between metabolic rates and temperature for experimentally enriched and control coral colonies. I found that the nutrient-enriched coral's endosymbiont densities, total chlorophyll a content, and tissue biomass were 54 %, 38 %, and 22 % higher, respectively, while algal endosymbiont nitrogen content cell-1 was 74 % lower relative to the control colonies. In addition, thermal performance metrics indicated that the maximal performance rate for gross photosynthesis was 29 % higher in enriched colonies, while the rate of metabolism at a reference temperature (26.8 °C) for gross photosynthesis was 33% higher compared to the control colonies. These findings illustrate that the relationship between land-based inputs, such as nutrient and sediment loading, and coral physiology is dependent on the local environmental context. Together these results help us better understand how local-scale stressors influence coral functionality and thermal performance in varying reef habitats. By understanding the complexities of coral-nutrient dynamics, research-based reef management approaches can be taken, particularly on where to mediate the influx of land-based inputs into coastal coral reef ecosystems.