Managing local impacts to protect coral reefs
Photo: Luiz Rocha/California Academy of Sciences, ABC News
Climate change is increasing the severity and frequency of coral bleaching events as a result of rising sea surface temperatures. The coral animals that build the foundation framework of coral reef ecosystems are sensitive to these thermal stress events that occur in the marine environment and can respond to this stress in a visually striking way known as coral “bleaching”. When corals bleach, the symbiotic algae held in their tissues are lost, generating the stark white color giving this response its name. Under normal conditions, these algae provide the corals with sugars and nutrients produced through photosynthesis and, in turn, receive protection and nutrients back from the coral. As a consequence of the destabilization of this symbiosis, corals lose their primary source of energy and nutrition and in cases of severe or sustained bleaching, mortality can occur, negatively impacting the function and stability of this important ecosystem.
As bleaching and mortality events continue to affect reefs worldwide, scientists and managers are working to identify strategies to better preserve and manage reefs on a local scale to protect reefs from climate change and global stressors. On a local scale, nitrogen pollution is one factor that is suspected to compromise the health of corals and make them more susceptible to bleaching. Excess nitrogen in coastal oceans, released through fertilizer or sewage run-off, can upset the sensitive relationship between corals and their algae. However, the role of nitrogen in driving bleaching across larger scales is still not well understood. To address this gap in knowledge, researchers from University of California Santa Barbara and Pennsylvania State University studied the role of nitrogen pollution in exacerbating coral bleaching on a large scale on reefs in Moorea, French Polynesia.
During a heat wave event in Moorea in 2016, the researchers surveyed bleaching of corals of two dominant genera, Pocillopora and Acropora, and monitored the nitrogen content in the water. For both Pocillopora and Acropora corals, high heat and nitrogen pollution increased the severity of coral bleaching, measured as the proportion of bleached tissue on each colony. Even in situations of more mild heat stress, nitrogen pollution led to a twofold increase in bleaching severity.
As nitrogen pollution can increase coral sensitivity to bleaching, the authors of this study suggest that mitigation of nitrogen pollution may provide a strategy to protect corals during bleaching events. Continuing to identify local strategies to preserve reef health is critical to boosting reef resilience to stress while taking action to address climate change.
Mary K. Donovan, Thomas C. Adam, Andrew A. Shantz, Kelly E. Speare, Katrina S. Munsterman, Mallory M. Rice, Russell J. Schmitt, Sally J. Holbrook, Deron E. Burkepile. Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape. Proceedings of the National Academy of Sciences, 2020; 201915395 DOI: 10.1073/pnas.1915395117