RAPID: How does nutrient availability alter coral bleaching, mortality, and recovery on Moorea coral reefs?

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Coral reefs have enormous social and ecological value but are killed by such stresses as water pollution and unusually warm water, which can cause corals to bleach and die. The National Oceanic and Atmospheric Administration has predicted that the current warm-water El Niño event is especially strong and will likely bleach corals across the central and eastern Pacific during 2015-16. Bleaching is most likely to occur in the central South Pacific, the location of the NSF-funded Moorea Coral Reef Long Term Ecological Research (MCR LTER) site, starting in February 2016. The proposed research will support field experiments and collection of key data at the MCR LTER site to address how the coming warm-water event interacts with nutrient pollution to influence the prevalence and intensity of coral bleaching. The investigation will focus on two aspects: (1) island-wide patterns in bleaching and coral loss with respect to nutrient availability across the landscape, and (2) how different forms and sources of nitrogen-based nutrients can either facilitate or mitigate bleaching. The project will assess the interaction of two major global change drivers on coral reefs, elevated sea surface temperature and nutrient pollution. Results from this work can inform resource managers and policy makers regarding the effects of nitrogen enrichment in two different forms (nitrate vs. ammonium) from two different sources (human- vs. fish-derived) on altering the probability different types of corals will bleach during major warm-water events. Human activities can enhance nitrate pollution directly, and lower ammonium enrichment indirectly (via fishing or destruction of nursery habitat), both of which might adversely affect corals by increasing the probability they will bleach. As such, the project can provide much-needed information about how human activities can impact coral reefs. The project will contribute to the training of two graduate students who will participate in the field research.Nutrient enrichment is a major anthropogenic force altering coastal ecosystems worldwide, particularly in oligotrophic systems such as coral reefs. Based on differences in how ammonium versus nitrate enrichment affect the physiology of corals, some coral reef biologists have speculated that under stress, nitrate enrichment from anthropogenic sources should weaken the coral-dinoflagellate symbiosis, whereas recycled nitrogen excreted by reef fishes could be beneficial. Thus, different forms of nitrogen (nitrate vs. ammonium) and different sources of nutrients (fish-derived vs. anthropogenic) can have fundamentally different effects on coral biology, which may lead to contrasting effects on how they alter the susceptibility of coral to bleaching from warm-water anomalies. Further, spatial heterogeneity in nutrient availability could shape landscape scale patterns in the intensity of bleaching and subsequent community consequences. This research will assess the interaction of two major global change drivers on coral reefs, elevated sea surface temperature and nutrient pollution. Two hypotheses will be tested: (1) that nitrate enrichment from anthropogenic sources will increase the susceptibility of corals to bleaching, and (2) that nitrogen in the form of ammonium will reduce it. Given that much of the N enrichment on the reefs in the lagoons of Moorea is of anthropogenic origin, the investigators further hypothesize that on a landscape scale, corals in more nutrient enriched sites will incur greater bleaching and bleaching-associated mortality compared to corals at nutrient poor sites. These relationships will be explored for the three major genera in the Moorea system (Pocillopora, Acropora, Porites), and then scaled up to explore island-wide patterns of bleaching as a function of landscape scale heterogeneity in nutrient availability. Crucial pre-bleaching data from field surveys and WorldView-3 satellite imagery will be paired with subsequent post-bleaching data to explore bleaching patterns in relation to gradients in nutrient availability on the reef complex. In addition, a field experiment will be initiated prior to the warm-water anomaly to test the hypothesis that different forms of nitrogen will have contrasting effects on the likelihood of bleaching.