Percent recent mortality (PRM) of stony corals as an ecological indicator of coral reef condition

Abstract

The reef communities of the Florida coral reef tract play a major role in supporting the regional economy but are threatened by increased exploitation and environmental factors. Coral reef ecosystem services are vital to the economy of SE Florida where revenue and jobs depend on the status of reef resources. Here, we used an extensive, reef monitoring database collected by the Florida Reef Resilience (FRRP, 2003–2011) and the Atlantic and Gulf Rapid Reef Assessment (AGRRA, 1997–2011) programs to evaluate percent recent mortality (PRM) as a robust ecological indicator of coral colony and coral reef status. PRM, the proportion of a coral colony that has experienced recent tissue mortality so that corallite structures in the non-living parts of the coral are still intact and identifiable to species, can be attributed to disturbances taking place within days to a few months preceding the surveys.Based on data from >50,000 colonies from 11 coral species and nearly 1400 sites, we propose a benchmark level of <1% PRM and <5% prevalence of partial mortality for Florida reefs during periods of background, low-stress environmental conditions. PRM levels >1.0% and prevalence levels >5% can be used as early warning indicators of degrading conditions. Average PRM values >2% are indicative of increasingly stressful conditions as those experienced during temperature anomalies and major hurricanes. Finally, PRM values considerably >2% are reflective of significantly stressful conditions and warning signals of potential major coral mortality as evidenced by mean PRM levels of >10% recorded in Florida as a consequence of the 2010 extreme cold-water event. PRM and prevalence values from Florida reefs compared favorably with those recorded in the Caribbean and the Mesoamerican region where a benchmark of 2% for background levels of PRM under low-level, chronic stress was proposed. The status of this indicator can be easily communicated to stakeholders and will benefit managers by providing: (1) a baseline to assess the status of coral populations; and (2) early-warning indicators of unfavorable conditions that may trigger management actions such as temporary closures or the establishment of more permanent protection such as MPAs.