Nitrogen Identity Drives Differential Impacts of Nutrients on Coral Bleaching and Mortality
Nitrogen pollution increases the susceptibility of corals to heat-induced bleaching. However, different forms of nitrogen (nitrate vs. ammonium/urea) may have different impacts on thermal tolerance of corals. We used an 18-month field experiment on the oligotrophic fore reef of Moorea, French Polynesia, to test how different forms of nitrogen (nitrate vs. urea) impacted coral bleaching. The experiment spanned two moderate thermal stress events in 2016 and 2017. Nitrate increased bleaching prevalence in Acropora by up to 100% and in Pocillopora by up to 60% compared to control corals. Urea exposure often had intermediate effects on bleaching (not different from either control or nitrate-exposed corals) in both taxa. Importantly, nitrate prolonged bleaching in both Acropora and Pocillopora as nitrate-exposed corals remained bleached even after thermal stress ended, while control and urea-exposed corals had mostly recovered. Nitrate exposure also increased the prevalence of partial mortality in Pocillopora colonies and more than tripled the number of colonies that completely died. Our data are the first to show contrasting effects of different forms of nitrogen on coral bleaching and mortality in a natural reef environment, linking previous patterns from large-scale correlative studies with results from more mechanistic laboratory experiments. Most importantly, we showed that corals exposed to nitrate exhibited more frequent bleaching, bleached for longer duration, and were more likely to die than corals in low nitrogen conditions. Exposure to excess nitrogen, particularly anthropogenic nitrogen, may lower the temperature threshold at which corals bleach, triggering bleaching events on polluted reefs even when typical thermal stress thresholds have not been crossed.
Keywordsnutrient pollution climate change coral reef eutrophication symbiosis anthropocene
National Science Foundation Grants OCE-1619697 to SJH, DEB, and RJS, OCE-1547952 to DEB, and OCE-1236905 and OCE-1637396 for the Moorea Coral Reef LTER to RJS and SJH, and a Hong Kong Research Grants Council Grant GRF# 17100014 to DMB supported this research. We thank M. Anskog, A. Duran, C. Fuchs, K. Landfield, S. Leung, K. Neumann, E. Schmeltzer, K. Seydel, A. Simoes Correa, A.T.S. Tang, A. Thurber, R. Vega Thurber, R. Welsh, and S. Wise for field and laboratory assistance. Research was completed under permits issued by the Territorial Government of French Polynesia (Délégation à la Recherche) and the Haut-commissariat de la République en Polynésie Francaise (DTRT) (Protocole d’Accueil 2015-2017).
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