Long-term impacts of sea surface warming trends in the northeastern Caribbean: Case studies from Puerto Rico

Background/Question/Methods

There is increasing evidence that sea surface warming trends are causing unequivocal impacts on coral reef ecosystems at a planetary scale. Sea surface temperature (SST) has shown a significant increase over the last few decades across circumtropical oceans. Impacts have included significant % coral cover decline, declining biodiversity, community phase shifts favoring dominance by algae and other non reef-building taxa, declining coral growth, and calcification rates. These might have negative indirect consequences in reef fish communities, particularly in small sized reef dwellers. The 1998 and 2005 massive bleaching events were followed by coral disease and mortality outbreaks across the Caribbean, affecting the principal reef builder taxa, the star coral, Montastraea annularis species complex, with 70 to 100% losses. This study is aimed at documenting several case studies from Puerto Rico regarding the long-term changes associated to increasing SST trends over the last decades. We present evidence of: 1) long-term changes in benthic community structure; 2) declining trends in M. annularis; 3) modeling efforts to predict the fate of M. annularis populations given the current trends of increasing SST; and 4) sclerochronological evidence of long-term impacts of increasing SST on coral skeletal extension, density, and calcification rates in Solenastrea bouirnoni.

Results/Conclusions

Summer SST significantly increased in Puerto Rico over the last three decades. There was also a net loss of up to 70-80% of living coral cover between 1997 and 2010. The 2005 massive bleaching event accompanied by a record-setting accumulation of 14.3 degree heating weeks caused a 50% loss of living coral cover relative to 2005 pre-bleaching levels. There was a significant community phase shift favoring macroalgae, cyanobacteria and algal turfs. It also caused a significant decline in species richness and colony abundance, including a mean 80% loss in the M. annularis species complex living cover. Stochastic modeling efforts showed: 1) population growth in the absence of massive bleaching; 2) long-term population stability at bleaching frequency of 5%;  3) significant population decline at bleaching frequency of 10%; and 4) rapid population collapse at bleaching frequency of 20%. Preliminary modeling efforts of surviving physiological fragments of M. annularis on partially killed colonies suggest that population collapse of small-sized fragments is largely possible in the near future under current predicted scenarios. Further, preliminary sclerochronological evidence obtained from S. bouirnoni cores suggest that increasing SST caused a decline in coral skeletal extension, calcification rates, and an increase in sekeletal density.