Decline of forereef corals in response to recent warming linked to history of thermal exposure (original) (raw)

Rising ocean temperatures have reduced rates of coral calcification and increased rates of coral mortality, thereby negatively impacting the health of coral reef ecosystems 1,2. Nevertheless, the response of corals to thermal stress seems to vary spatially across the reef environment 3,4. Here, we show that between 1982 and 2008 in the western Caribbean Sea, skeletal extension within forereef colonies of the reef-building coral Siderastrea siderea declined with increasing seawater temperature, whereas extension rates of backreef and nearshore colonies were not impacted. These results suggest that forereef S. siderea corals are more vulnerable to ocean warming than their backreef and nearshore counterparts. This may arise from backreef and nearshore coral colonies experiencing greater baseline diurnal and seasonal thermal fluctuations than forereef colonies, which may promote acclimatization and/or adaptation to more recent anthropogenic thermal stress. These findings reveal how corals have responded to recent anthropogenic warming, offer insights into how they are likely to respond to future warming and highlight the importance of understanding cross-reef differences in coral thermal tolerance for managing coral reef ecosystems in an era of rapid regional and global climate change. The average surface temperature of the Earth has increased by about 0.2 • C per decade since the mid 1970s (ref. 5), with sea surface temperature (SST) rising by as much as 1.0 • C in the tropics and subtropics over the past century 6. SST across the region where corals reside has increased by 0.3-0.4 • C over the past four decades 7. The fourth assessment of the Intergovernmental Panel on Climate Change predicts that global SST will continue to increase owing to the rising partial pressures of greenhouse gases in the atmosphere 8. Corals and their associated ecosystems seem to be particularly vulnerable to such thermal stress 1. Tropical coral reefs exist near corals' upper thermal limits 9. Thus, even a small rise in ocean temperature may have important consequence for their health. SSTs of 1-2 • C above the mean monthly summer maximum, even for only a few weeks 10-12 , have been implicated in coral bleaching-the decrease in coral pigment concentration and/or ejection of the algal symbiont from the coral host. As reef-building zooxanthellate corals receive up to 95% of their energy from photosynthate translocated from their algal symbionts 13,14 , disruption of this symbiotic relationship impairs key functions within the coral animal, such as light-enhanced calcification, tissue growth and reproduction. Indeed, historical growth records obtained from coral cores reveal that skeletal extension of Porites spp. in the Coral Sea 15,16 ,