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School of Life Sciences' Friday Seminar

Dissertation Defense: "Coral Bleaching and Recovery Under Natural and Anthropogenic Nutrient Enrichment" Speaker: Jessica Glazner, PhD Candidate, Marine Biology Graduate Program, ÌÇÐÄÊÓƵ ²ÑÄå²Ô´Ç²¹. **** Abstract: A major driver of coral reef degradation worldwide is thermal stress that can cause coral bleaching and subsequent mortality. Coral resilience to thermal stress is influenced by local nutrient conditions, but the source of terrestrial nutrient enrichment to reefs can have contrasting effects on coral health and thermal performance. Anthropogenic inputs, such as sewage effluent, have been associated with declining reef health and bleaching resilience, whereas natural nutrient subsidies, such as from seabird guano, may enhance coral resilience. The objective of my dissertation is to investigate how natural and anthropogenic nutrient sources influence coral bleaching resistance and recovery. I combined a field study at Palmyra Atoll, examining coral physiology and feeding strategies before and during a bleaching event at sites with contrasting seabird-derived nutrient inputs, with a controlled tank experiment comparing the effects of seabird guano, sewage effluent, and inorganic nutrients on coral physiology, nutrient assimilation, and coral host–symbiont dynamics during simulated bleaching and recovery. Across both studies, natural seabird-derived nutrients were associated with enhanced coral resilience. In the field, corals near higher seabird densities increased heterotrophic feeding to support bleaching resistance through the maintenance of symbiont density and chlorophyll content during bleaching relative to the pre-bleaching year, whereas corals at the lower seabird density site did not shift their feeding strategies and experienced significantly less bleaching resistance. In the experiment, coral species differed in their nutritional flexibility and responses to nutrient enrichment. Corals that more readily utilized available nutrients showed faster recovery of photosynthetic efficiency, whereas corals with more limited nutrient uptake recovered more slowly. However, this recovery advantage was observed only in corals exposed to seabird guano, demonstrating that both coral nutritional strategy and nutrient source influence recovery from bleaching. In contrast, the anthropogenic nutrient source of sewage effluent did not enhance bleaching resistance or recovery. While we expected to see nutrient treatment effects on other coral physiology metrics, there were no significant treatment effects on symbiont density, coral growth, and lipid biomass. Together, these findings demonstrate that nutrient source influences coral responses to thermal stress. Understanding how natural and anthropogenic nutrient subsidies shape coral resilience can help explain differences in reef health across remote and populated islands and inform strategies to protect coral reefs under increasingly frequent marine heatwaves. Zoom: https://hawaii.zoom.us/j/88911577123 (passcode: MBF26JG)

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