Threshold Decline in Mesoamerican Coral Growth and Resiliency

Abstract

Caribbean coral reefs have been massively altered in recent decades due to human impacts, resulting in a dramatic reduction of live coral cover but quantitative data before the 1970s have not been available to assess how these changes came about1,2,3. We produced master chronologies of growth rates in massive Montastrea faveolata corals from the Mesoamerican reef tract that provide a method for extending records of coral health further back into the past. Our records reveal a unique reduction in growth rate associated with the 1998 coral bleaching event that has no parallel in the past 75 to 150 years. Of 92 cores collected from live coral heads in 2006-2007, 94% have severely reduced growth rates and 14% exhibit partial mortality scars in 1998-1999 whereas only 3 stress bands are found in single cores prior to 1998, and none of these cores exhibited earlier partial mortality. Some corals returned to pre-1998 extension rates by 2001, although corals in areas affected by sediment-laden runoff or high human population density still had not fully recovered by the time of sample collection eight years later. Previous episodic stresses like hurricane strikes and a warming event in 1983 more severe than 1998 had little to no effect on M. faveolata growth rates. The 1998 event apparently surpassed a threshold in coral tolerance precipitating a catastrophic shutdown in growth that had lasting effects throughout the Mesoamerican reef system but was particularly prolonged in areas exposed to other stressors. These findings suggest that projected increases in global temperatures over the next century are likely to result in drastic reductions in growth rather than a gradual decline in coral health, but that corals with fewer local stresses will be better able to survive bleaching events, underscoring the need for local conservation measures.