Impacts of consecutive bleaching events and local algal abundance on transplanted coral colonies in the Florida Keys

Climate change threatens many marine ecosystems, including coral reefs and the thousands of organisms that inhabit these reefs. Fluctuations in sea surface temperatures trigger bleaching events in coral colonies that lead to increased coral mortality and reef degradation. To combat the decline in coral cover, many researchers have focused on transplanting coral colonies to better understand which species are best adapted to these environmental changes. This current study examined the impacts of two consecutive thermal stress events on transplanted coral colonies in the middle Florida Keys within the Florida Keys National Marine Sanctuary. Coral fragments of two species (Porites astreoides and Siderastrea siderea) were transplanted on seven reef sites. Transplanted colonies were monitored for evidence of coral bleaching during quarterly sampling trips from June 2013 through June 2017. Bleaching was only observed during the late summer of 2014 and 2015 when Degree Heating Weeks exceeded 4 degrees C-weeks. Results indicate that P. astreoides colonies had significantly higher bleaching than S. siderea colonies. Reefs differed in their proportion of bleached and unbleached corals for P. asteroides, but these reefs did not differ in their average daily maximum temperature or Degree Heating Weeks. However, individual corals that did bleach had a higher average fleshy algal abundance (Dictyota spp.) surrounding them than those corals that remained unbleached. Corals that bleached the first year and survived were no more or less likely to bleach the second year. These findings suggest that resilience to bleaching differs not only by coral species but may also be influenced by the local abundance of fleshy algal species.