Analysis of scleractinian coral communities and influencing factors in Daya Bay, Guangdong

Using the point intercept transect sampling method, the scleractinian corals in the waters near Yangmeikeng (YMK) and Dalajia Island (DLJ) in Daya Bay, Guangdong, were surveyed in June 2022. Substrate types, coral species composition, dominant coral species, coral community characteristics, and key indicator species within coral distribution areas were analyzed. Differences of coral communities between island and nearshore areas in Daya Bay were examined. The results of horizontal and vertical transect surveys were compared. Furthermore, long-term trends in coral communities were assessed using historical data. The results indicate that gravel has the highest coverage, constituting approximately 60% of the coral distribution areas near YMK and DLJ, followed by sand. The coverage of live scleractinian corals ranges from 1.8% to 20.0%, with an average of 9.9%. Massive live corals dominate, accounting for approximately 67.5% of the live total coral coverage. The average coral coverage is 13.6% near YMK, and 6.2% near DLJ. The distribution of corals is patchy and uneven. The highest coral coverage is observed at the outer edge of the coral distribution area near YMK, while near DLJ, the highest coverage occurs between the low-tide line and 1 m water depth. A total of 44 coral species were identified near YMK and DLJ, belonging to 17 genera across 9 families. A total of 37 species from 9 families were identified near YMK, and 25 species from 9 families near DLJ. A significant difference in coral species composition between YMK and DLJ is revealed by One-Way ANOSIM (ppPorites solida is the most dominant species near YMK (Y=0.24) and DLJ (Y=0.35). However, PCO analysis indicates that the four corals Platygyra carnosa, Turbinaria peltata, Goniopora columna, and Porites annae significantly contribute to the differentiation of coral communities between YMK and DLJ. The horizontal and vertical transect methods are complementary for coral system surveys. Both methods should be utilized and combined to achieve a more comprehensive understanding of coral distribution patterns in baseline investigations. Over the past 40 years, the coral communities in Daya Bay have severely degraded. The dominant species in Daya Bay have undergone succession, with their morphology shifting from branching forms to encrusting and massive forms. Additionally, the average coverage of live scleractinian corals has declined from 76% to approximately 10%. Amid global climate change, the coral communities in Daya Bay continue to face multiple threats, including ocean acidification, marine engineering projects, overfishing, marine ecological disasters, and other human activities. The protection and restoration of coral communities in Daya Bay are urgently needed.