Award: OCE-1619697

Coral reefs provide critical ecosystem services to coastal human populations who depend on them for their livelihoods, and they are also among the most threatened coastal systems. In 2015-16, for example, there was a worldwide coral bleaching event that resulted in widespread death of corals on tropical reefs. Until recently, coral reefs demonstrated the capacity to recover from disturbances such as cyclones and bleaching that cause extensive loss of live coral. Reefs were ecologically resilient in that coral would readily become re-established in the years following a disturbance. However, increasingly coral reefs have been observed to transition to a state where seaweeds, not live coral, dominate the landscape. The causes of these state shifts have been attributed to a combination of human-induced drivers that lower the ecological resilience, with nutrient loading hypothesized to be one key driver, as it can foster growth of seaweeds. Nutrient enrichment is a major anthropogenic force altering coastal ecosystems worldwide. On coral reefs, nutrient enrichment can result in rapid growth of seaweeds that can then dominate the reef and exclude corals. One major source of nutrients entering coral reef systems is freshwater runoff, which can contain fertilizers (such as nitrates) from agriculture as well as wastewater, and these have been argued to be detrimental to the health of corals. By contrast, nutrients such as urea and ammonium have been thought to be beneficial to corals as they are naturally produced in the ecosystem by fishes and other organisms and are taken up by corals. These nutrients could enhance coral health and potentially help corals survive a bleaching event. This research explored the effects of nutrient enrichment on the ability of coral to withstand bleaching. In particular, it assessed whether nutrients typical of agricultural fertilizers (nitrate) impacted corals differently than nutrients that are normally recycled in the coral reef system (urea and ammonium) during the bleaching event of 2015-16. The worldwide coral bleaching event of 2015-16 provided us an unparalleled opportunity to test explicit hypotheses regarding the influence of human- versus fish-derived nutrients in altering the susceptibility of three major types of corals to bleaching, as well as to map spatial patterns of bleaching onto landscape scale patterns of nutrient availability. We obtained crucial ?pre-bleaching? data from field surveys and satellite imagery, paired with subsequent ?post-bleaching? data to explore bleaching patterns in relation to gradients in nutrient availability on the reef complex that surrounds Moorea, French Polynesia. In addition, a large field experiment initiated prior to the warm-water anomaly enabled a test of the hypothesis that different forms of nitrogen would have contrasting effects on the bleaching probability of a coral colony. Overall our results indicated that nutrient enrichment (nitrate) from anthropogenic sources (but not naturally derived urea or ammonium) was harmful to corals and predisposed them to bleaching and mortality. The research project had a participatory, integrative-training component that provided hands-on experience for eight graduate students, one undergraduate and one postdoctoral student. As part of that training the students had the unique experience of working alongside and in collaboration with the Principal Investigators and were exposed to their expertise in marine science. The students also received hands-on field training as they conducted field sampling and experiments in Moorea. This research contributes to our understanding of the socio-cultural, economic, and environmental influences on health of coral reefs. Our findings will guide local community groups on Moorea and other locations with coral reefs in developing sustainable agricultural and wastewater management practices along with strategies for conservation and management of marine resources. To disseminate the project?s findings as widely as possible, the Principal Investigators and graduate students have given presentations at conferences, scientific workshops, and public meetings, as well as produced technical publications in the scientific literature. Last Modified: 05/23/2018 Submitted by: Sally J Holbrook