The role of coral mortality in nitrogen dynamics on coral reefs

Understanding the dynamics of coral reef nutrients, particularly nitrogen, following a coral mortality event is imperative for the prediction of further possible outcomes on impacted reefs, and as such, for the identification of potential management interventions. This study used both acetylene reduction and 15N techniques to investigate the dynamics of nitrogen associated with microbial communities that develop on coral skeletons following bleaching-induced mortality. The results show that nitrogen fixation activity increases dramatically in the initial 3 months following coral mortality with fixation rates as much as an order of magnitude higher than those observed on older hard substrate material throughout the study sites. Rates of acetylene reduction up to 70.5 nmol cm−2 h−1 were observed on post-bleached Montastraea faveolata skeletons while nitrogen fixation rates using 15N up to 112 µmol N2 m−2 h−1 were observed on post-bleached Acropora aspera skeletons. These rates are among the highest reported for coral reefs. Nitrogen fixation activity was found to be influenced by environmental parameters such as nutrient loading and light levels. The results of this study suggest that rates of nitrogen fixation following bleaching-induced mortality may (1) depend on the pre-existing characteristics of a reef, and (2) be sufficient to direct an affected ecosystem toward a phase change from coral to algal dominance.