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

## TL;DR

This study explores how human activities and ecological competition between corals and algae are causing shifts in coral reef ecosystems in Wenchang, Hainan.

## Contribution

The study identifies key environmental and competitive factors driving coral-algal phase shifts in a human-impacted region of China.

## Key findings

- Dominant coral species like Favites, Porites, Galaxea, and Montipora compete with algae, particularly Lobophora.
- Salinity, NO2−, and NH4+ are key environmental drivers of macroalgal abundance.
- Macroalgae abundance correlates negatively with live coral cover and positively with coral mortality.

## Abstract

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 Favites, Porites, Galaxea, and Montipora. Furthermore, there existed obvious competition between dominant corals and algae, especially Lobophora, 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 Lobophora had a significant negative correlation with the dominant coral taxa Favites, Galaxea, and Montipora. 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.

This study selected the coral reef areas in the coastal waters of Wenchang, Hainan, China, which were significantly affected by human activities. Through four consecutive years of field investigations, the key driving factors and potential mechanisms of ecological phase shifts between corals and macroalgae were identified, thus providing a scientific basis for strengthening coral reef management under the influence of human activities.