Reefs of last resort: Locating and assessing thermal refugia in the wider Caribbean

Abstract

Ocean temperature increase is recognised as one of the major threats to the future of coral reefs. During the past 50years, global mean temperatures have risen by 0.13°C/decade, but in the Caribbean warming trends are greater, and are of the order of 0.29°C/decade. In light of this threat, some researchers have proposed that reefs may survive better in locations of naturally low thermal stress, and have hypothesised that such refugia may be located in: (1) deep areas; (2) areas of high currents; (3) upwelling; and (4) high-latitude areas. These regions have been targeted as priorities for conservation activities; however, with the exception of deep reefs, formal assessment of the efficacy of these potential refugia is lacking. Here we tested the three remaining hypotheses in the wider Caribbean region using remotely sensed data and hydrodynamic model outputs. We began by determining the location of the hypothesised refugia, and then quantified the extent to which they minimise acute and chronic thermal stress in a significant and consistent manner through time. Furthermore, recognising the increasing sea warming and the concern that high temperatures could frequently exceed lethal thresholds for many organisms in the future, we assessed the ability of these areas to slow the rates of increase of temperatures. We show that the proposed areas do not constitute meaningful refugia from acute thermal stress. However, upwelling areas in the Caribbean have conservation utility because rates of thermal warming are lower.