Assessment of coral bleaching using symbiotic zooxanthellae density and satellite remote sensing data in the Nansha Islands, South China Sea

Abstract

Coral bleaching, characterized by a significant loss of symbiotic zooxanthellae, is the primary cause of mass coral mortality and reef degradation throughout the world. The characteristics, processes, and resistance of corals to bleaching varies significantly and is dependent on environmental conditions. We documented a mass coral bleaching event in June 2007 at the Meiji and Zhubi Reefs, Nansha Islands (NS), South China Sea using ecological surveys and measurement of coral zooxanthellae density and sea surface temperatures (SST). More than 35 species of corals (between 0–20 m in depth) were bleached. These bleached corals accounted for 15.6% of total corals in the investigated quadrats. The branching corals Pocillopora and Acropora were the most vulnerable species whereas the massive corals Porites and Favia were more tolerant of the high SSTs. Surprisingly, we found no evidence of bleaching in Agariciidae corals suggesting that this family is resistant to thermal stresses. The bleached corals had lost 72%–90% of their symbiotic zooxanthellae. Furthermore, corals that had no visual signs of bleaching had also lost 31%–53% of their zooxanthellae suggesting that most corals were experiencing the early stage of bleaching. The monthly mean SST during June 2007 was 30.8°C, the highest since 1998. Based on measurements of SST and the Hotspots and DHW data (NOAA), we conclude that it the extremely high SSTs triggered this coral bleaching event. Our results suggest that the previously accepted temperature thresholds used to predict coral bleaching based on satellite data are likely to underestimate the extent and intensity of coral bleaching, at least in the NS.