The Ecological Mechanism of Coral–Algal Phase Shifts: A Case Study of Wenchang in Hainan Province

The interaction between corals and algae is of great significance in maintaining the health of coral reef ecosystems. Globally, tropical coral reefs are being degraded by human activities, resulting in the decline of reef-building corals and an increase in macroalgae. The study aims to determine the ecological mechanism of coral-algal phase shifts occurring in the coastal waters of Wenchang in Hainan Province, which are affected by human activities. The field survey results revealed that 177 species of reef-forming corals belonged to 49 genera and 18 families, which were dominated by coral taxa <i>Favites</i>, <i>Porites</i>, <i>Galaxea</i>, and <i>Montipora</i>. Furthermore, there existed obvious competition between dominant corals and algae, especially <i>Lobophora</i>, which showed stronger competitive advantages compared to other algae. The salinity, NO₂ ^-, and NH₄ ⁺ were the key environmental drivers that affected macroalgal abundance. There was a negative correlation between macroalgae and live coral cover and a positive correlation with coral mortality. Notably, the dominant algal species <i>Lobophora</i> had a significant negative correlation with the dominant coral taxa <i>Favites</i>, <i>Galaxea</i>, and <i>Montipora</i>. Further correlation analysis showed that significant interspecific competition existed among corals and algae themselves, which may function as the internal driving factor for the phase shifts of coral-algal relationships. These results will help us understand the role of different functional groups of algae in the degradation of coral reef ecosystems and lay the research foundation for the development of scientific and rational coral reef protection strategies.