Nitrogen enrichment determines coral mortality during a marine heatwave

Climate change is killing foundation species worldwide and understanding how local environmental conditions interact with changing climatic conditions is a major goal in ecology. On coral reefs, marine heatwaves cause coral to die from bleaching, with the effects of marine heatwaves potentially modified by inorganic nutrients. Here we show that following an intense marine heatwave in Moorea, French Polynesia, patterns of mortality in two genera of branching coral are complex functions of heat stress and nitrogen enrichment. Two metrics of coral mortality (Prevalence: the proportion of colonies with at least some partial mortality; Severity: the proportion of a colony that died for those that exhibited mortality) were greater among Acropora than Pocillopora corals. For Acropora, nitrogen enrichment interacted with the intensity of heat stress to influence the prevalence, but not severity, of mortality. Greater nitrogen enrichment reduced prevalence at low heat stress, but increased it at high heat stress. Conversely, for Pocillopora, nitrogen interacted with heat stress to alter the severity, but not prevalence, of mortality. At low heat stress, nitrogen enrichment increased severity, but this effect declined as heat stress increased. Our work shows that nitrogen enrichment can either mitigate or exacerbate coral death from marine heatwaves depending on heat stress intensity and the taxa involved.