Concurrent effect of crown-of-thorns starfish outbreak and thermal anomaly of 2020 on coral reef communities of the Spratly Islands (South China Sea)

The Spratly Islands are a remote group of more than 150 coral reefs and islands scattered in the south-central South China Sea and representing a biodiversity hotspot in the region. An integrative anthropogenic impact and increased frequency of thermal anomalies have increasingly been threatening these unique reef ecosystems over the last three decades. The repetitive surveys conducted on four coral reef sites within the Spratly Islands in 2018–2019 and in 2021 revealed a significant decrease of coral cover, particularly of acroporids, a decrease of species diversity and an outbreak of crown-of-thorns starfish (COTS) Acanthaster planci (the variation of starfish abundance was 1–18 starfish/100 m²). Moreover, one of two additional sites, located within the same study reefs, revealed a coral-killing photoautotrophic cyanosponge Terpios hoshinota, covering 56.9% of all dead scleractinian colonies on the reef slope of this site. Analysis of sea surface temperature dynamics during the last 4 years (2018–2021) in the Spratly area allowed suggesting the concurrent negative effect of the COTS outbreak and the thermal anomaly of 2020 on local coral communities. This effect may have contributed to the general trends in coral reef decline in the area of the Spratly archipelago under the condition of repetitive thermal stress and increasing anthropogenic impact.