| Since the 1980s,coral reefs（communities）around the world have been severely degraded,causing enormous ecological losses and affecting the livelihoods of millions of people.Previous studies have shown that high temperatures have been the main cause of coral reef degradation in recent years,which drove the accelerated migration of tropical marine life toward the poles.Therefore,Daya Bay,which is located in a relatively high latitude sea area,with the blessing of upwelling in summer,is considered an ideal"refuge"for corals in the process of migrating north.However,the fact is those coral communities in the Daya Bay region have degraded simultaneously as coral reefs in other sea areas of the world,furthermore,the ambiguous mechanisms of the degradation hamper the protection and restoration of coral communities.Therefore,this study conducted preliminary research on the mechanism of coral community degradation in Daya Bay over the past 40 years through on-site investigation,literature research,and simulation experiments.The results and conclusions are as follows:（1）The results of the summer 2020 on-site survey（from the outside to the inside of the bay are Sanmen Island,Dalajia Island,Xiaolajia Island,andM Baishaizhou Island）showed that the corals in the bay basically grew at a depth of 4 m or shallower,and Sanmen Island reached 6 m or deeper;a total of 39species of reef-building corals belongs to 19 genera in 9 families were identified,and the dominant species was Porites lutea;the average coverage of living corals in Daya Bay was about 14%,from the outside to the inside of the bay was36%,11%,7%,and 2%,respectively.The seawater temperature in the coral growth area was contrary to the trend of living coral cover,which gradually increased from the outside to the inside of the bay,and the overall temperature was between 26～31°C;the vertical direction of dissolved oxygen concentration in seawater was obvious,with a maximum value in the deeps of 2to 4 m,and in 4 to 10 m was maintained at about 5 mg/L or rapidly decreased to1～2 mg/L among different stations;in addition,the N/P ratio of seawater nutrient salt in some stations was still higher than 50:1.（2）Since the 1980s,the coral communities in Daya Bay have entered a period of rapid degradation.Taking coral reefs in Dalajia for example,the coverage of live corals has dropped from 72.7%in 1983-84 to 11%now;The most severe decline was from 1983-84 to 1991,with a 56%decrease in coverage,typically characterized by the near-disappearance of the dominant species,Acropora pruinosa.The analysis showed that coral reef communities degradation in Daya Bay might be triggered by independent or combined events such as intermittent high temperatures,low dissolved oxygen,and sewage discharge.（3）Simulation experiments showed that high temperature（30°C）,low salt（19‰）and low dissolved oxygen（<4 mg/L）could independently lead to coral bleaching or even death;when these factors synergized,it would even cause the increase in bacterial abundance of Halodesulfovibrio sp.and the production of H2S gas in the water,which resulted in the death of corals.In summary,through on-site investigation,historical data analysis and simulation experiments,this study found that the coral communities in Daya Bay have been seriously degraded in the past 40 years,and global warming and hypoxia might have become the main factors limiting the distribution of corals in the Daya Bay. |
