Regional coral growth responses to seawater warming in the South China Sea

Abstract

Seawater temperature is one of the main environmental factors controlling coral skeleton growth. Sustained seawater warming is regarded as a major threat to coral growth and reef development. Coral reefs are widespread in the South China Sea (SCS), where the history and future of coral growth are of great concern. We integrated 99 linear extension rate series of the coral Porites from 12 locations at three regions in SCS, which include the Hainan Island (HN), the Xisha Islands (XS), and the Huangyan Island-Nansha Islands (HY-NS), and explored the regional responses of coral growth to sustained seawater warming. The sea surface temperature (SST) rose linearly by 0.47 °C, 0.71 °C, and 0.76 °C at HN, XS, and HY-HN, respectively, between 1900 and 2014. During this period, coral growth increased linearly by ~21.0% and ~0.7% at HN and XS, while HY-NS saw a decline of ~2.8% in coral growth. Moreover, interdecadal variations were found for both SST and coral growth. A nonlinear response relationship was revealed between coral growth and SST, with a thermal optimum of ~27.5 °C for Porites, which is responsible for the regional difference in the long-term trend in coral growth in SCS. In recent decades, reductions in coral growth have occurred in SCS, especially at HN, with the largest fall of ~15.1% over the past century, which is attributed mainly to intensifying human impacts instead of seawater warming. A preliminary estimate presents regional-different coral growths in SCS by the end of 21st century, with declines of ~8.9-16.3% under the atmospheric CO₂ emission scenario (RCP 8.5), implying that the overall downturn of coral growth will be inevitable under the future sustained seawater warming in SCS. The mitigation of global warming is essential to maintain coral growth and coral reef ecosystems in SCS.

Keywords: Coral Porites; Linear extension rate; Seawater warming; South China Sea; Thermal adaptation.