Influence of water-temperature variability on stony coral diversity in Florida Keys patch reefs

Annual surveys conducted by the Coral Reef Evaluation and Monitoring Project (CREMP) reported that average benthic cover of stony corals in the Florida Keys National Marine Sanctuary, USA declined from similar to 13% in 1996 to 8% in 2009. Keys-wide, mean species richness (SR) declined by similar to 2.3 species per station. Stress due to temperature extremes is suspected to be a major driver of this trend. We tested the potential for sea surface temperature (SST) variability and acute warm-temperature events (assessed with Degree Heating Weeks) to affect stony coral diversity in the Florida Keys. Benthic cover of 43 stony coral species was examined with respect to SST variability and habitat type (patch, offshore shallow, and offshore deep reefs). For each CREMP site, SST annual variance was classified as low (<7.0 degrees C-2), intermediate (7.0 to 10.9 degrees C-2), or high (>= 11.0 degrees C-2). Nonparametric MANOVA analyses showed that in the Upper, Middle, and Lower Keys regions, massive-type stony coral species (e.g. Siderastrea siderea, Pseudodiploria strigosa, Orbicella annularis complex, Montastraea cavernosa, and Colpophyllia natans) were prevalent in the patch reef habitats exposed to intermediate to high SST variability. Intermediate SST variability was also correlated with higher Shannon diversity means in patch reefs in the Upper Keys and higher SR means in the Middle Keys, indicating either that the stony coral species in these habitats are adapted to an intermediate temperature range or that individual colonies have acclimatized to that range. No significant relationships were found between stony coral diversity and SST variability in the Dry Tortugas region.