Spatial and Temporal Changes in Coral Community Responses to Ocean Warming

Tropical Pacific sea surface temperature is projected to rise 2-3°C by the end of this century, fueling efforts to identify thermally-tolerant reef communities that have the best chance of surviving future climate change. We used skeletal indicators of thermal stress in massive Porites corals collected across the Palau archipelago to document spatial and temporal changes in community-scale tolerance to anomalous warm events associated with the 1998 and 2010 Pacific ENSOs. Within communities where bleaching was documented by visual surveys, we find a strong correlation between percent bleaching and the proportion of surviving Porites colonies exhibiting skeletal density anomalies or “stress bands”. Using this relationship, we reconstructed the intensity and spatial patterns of bleaching during the 1998 ENSO event when survey data are limited. On exposed barrier reefs and inshore fringing reefs, the proportion of corals with 1998 stress bands (60% and 40% respectively) was consistent with that expected from DHW predictions and post-bleaching surveys. Conversely, in the Rock Island bays, where ambient temperatures were highest, no 1998 stress bands were recorded. However, these corals did respond to the 2010 thermal anomaly with the appearance of stress bands and an abrupt decline in calcification. The reasons for this apparent shift in thermal tolerance in response to the relatively weak 2010 warming are not yet clear. While the interplay of temperature with other environmental variables including light and flow cannot yet be ruled out, stressors associated with an increase in human activities, including tourism, on Palau are also considered.