Degradation of Coral Reefs under Complex Impact of Natural and Anthropogenic Factors with Nha Trang Bay (Vietnam) as an Example

Until recently, the city of Nha Trang, which stretches along Nha Trang Bay of southern Vietnam, was known as the “Riviera of the South China Sea” with clean and white beaches, untouched islands, and rich coral reefs with high biodiversity. Nevertheless, complex and long-term anthropogenic impacts caused by enlargement of tourist resorts on the coast of the bay and on its islands, dredging, boom of mariculture development, and overfishing led to degradation of more than a half of the coral reefs in the bay already by the beginning of the 2010s. By that time, only a third of the remaining reefs in the seaward part of the bay were characterized by rather high coral cover and diversity. Finally, in just three years from 2017 to 2019, more than 90% of these remaining rather healthy reefs have died off as a result of an outbreak of the main coral predator: crown-of-thorns starfish Acanthaster sp. By April 2019, the abundance of this starfish reached 4.2 individuals per 100 m 2 . Such abundance is 8-fold higher than the maximum at which the coral community may exist without decline. An abrupt increase in starfish abundance in the bay was determined by an increase in phytoplankton production (food source of starfish larvae) due to eutrophication of the bay and withdrawal of all natural enemies of the starfish from the coral reef ecosystem because of overfishing. In June 2019, the subsequent strongest sea surface temperature anomaly caused bleaching and mortality of surviving coral colonies. The cascade degradation of coral reefs in Nha Trang Bay and significant degradation of coral reefs in the neighboring provinces of Vietnam do not make it possible to give an optimistic prediction on recovery of coral reefs in this area in the near future.