Sea surface warming and biodiversity loss in Caribbean coral reefs

Background/Question/Methods

Coral reefs were originally conceived under the classical equilibrium community organization model due to their apparent stability over the past 200,000 years. However, this view has been challenged by long-term ecological studies showing that these communities are highly dynamic in terms of their species composition due to their continuous exposure to disturbances. Because their recovery time typically exceeds the return time of disturbances, coral reef communities are always recovering from perturbations that prevent them from reaching equilibrium. However, how coral communities will respond to the unprecedented increase in the frequency and severity of climate-change related phenomena (e.g., sea surface warming, mass coral bleaching and mortality) is not well understood. To investigate the role of such disturbances in the dynamics of benthic communities in Caribbean coral reefs, we surveyed six sites along the eastern shelf of Puerto Rico during the 2005 mass bleaching event. Transects at <5m, 5-10 m, and >10m in each site were used to provide estimates of relative abundance of major benthic functional groups including coral species. In 2010, the same sites were revisited for assessing post-bleaching community response. A permutation analysis of variance was carried out to determine differences in pre- and post-bleaching community structure, as well as differences in species diversity index and evenness.

Results/Conclusions

The benthic community structure changed significantly after five years of experiencing the mass bleaching event at all sites (F = 20.22; p-value<0.05). Coral cover loss ranged from 77.2% at sites closer to the coast to 92.3% in remote sites, while showing an increment in macroalgae coverage of 45.4% and 250%, respectively. Species richness decreased significantly over time (F = 6.04, p<0.05). Species evenness also decreased following the disturbance with the exclusion of rare species (F=2.90, p<0.05). Significant differences in diversity index among depths were found only in 2010 suggesting that depth is a primary factor driving community response to disturbances of this magnitude. This study highlights the unprecedented coral reef community phase-shift that resulted from a single mass bleaching event in Puerto Rico. Therefore, climate change represents a principal threat to coral reef viability.