Coral reefs degradation under complex impact of natural and anthropogenic factors with Nha Trang Bay (Vietnam) as an example

Up to recently, Nha Trang City stretched along Nha Trang Bay of the 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 impact stipulated by enlargement of touristic resorts on the coast of the bay and on its islands, dredging, boom of mariculture development, and overfishing have led to degradation of more than a half of coral reefs in the bay already at the beginning of 2010s. To that time, only a third of remaining reefs in the seaward part of the bay were characterized by rather high coral cover and diversity. Finally, only for three years from 2017 till 2019 more than 90% of these remained 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 an abundance of this starfish reached 4.2 individuals per 100 m2. Such abundance is 8-fold higher than a maximum at which coral community may exist without decline. An abrupt increase of starfish abundance in the bay was determined by an increase of phytoplankton production (food source of starfish larvae) due to eutrophication of the bay and withdrawal of all natural enemies of the starfish from coral reef ecosystem because of overfishing. In June 2019, the subsequent strong sea surface temperature anomaly caused bleaching and mortality of survived coral colonies. The cascade coral reef reduction in Nha Trang Bay and significant degradation of coral reefs in the neighboring provinces of Vietnam do not allow optimistic prediction on coral reef recovery to occur in this area in the near future.