The Distribution Characteristics Of Turf Algae And Its Potential Effect On Scleractinian Coral In The Reefs Of Mid-northern South China Sea

Turf algae is an important indicator of the health of coral reefs.When coral reefs are subjected to large-scale bleaching or severe disturbances,turf algae usually quickly dominate,thus accelerating coral reef degradation.So far,coral reefs in the South China Sea are widely distributed,but little research has been done on the turf algae.Base on the ecological survey data of coral reefs(Weizhou Island in Beibu Gulf,Yongxing Island in Xisha Islands and Huangyan Island in Zhongsha Islands)at different offshore distances in the mid-northern South China Sea in summer 2015,this study analyzed the coverage and spatial distribution of turf algae and the relationship between the coverage,spatial distribution,diversity of scleractinian coral and recruitment;based on the data of human activities,natural factors,the potential effects of turf algae on the growth of scleractinian coral were analyzed.The main conclusions are as follows:(1)There were significant differences between turf algae and scleractinian coral in coral reefs with different offshore distances(p<0.05).With the increase of offshore distance,the coverage of turf algae in Weizhou Island,Yongxing Island and Huangyan Island were 22.93%,14.35%and 2.75%,respectively,and the coverage of scleractinian coral were 6.05%,6.46%and 15.59%,respectively.The spatial distribution of turf algae was lower in the offshore and higher in the inshore,while the coverage of scleractinian coral was higher in the offshore and lower in the inshore.Excluding natural factors(such as sea surface temperature and natural disasters),human activities in different degrees in inshore and offshore reefs may be the main reason for the spatial distribution differences of turf algae and scleractinian coral.(2)Pearson correlation analysis showed that the coverage of turf algae was negatively correlated with the coverage of scleractinian coral and recruitment(/p<0.01),while the coverage of crustose coralline algae was positively correlated with the coverage of scleractinian coral and coral recruitment(p<0.01).Furthermore,the coverage of turf algae was negatively correlated with that of crustose coralline algae(p<0.05).In the inshore Weizhou Island and offshore Yongxing Island with high coverage of turf algae,the recruitment was 3.62 ind/m2,4.60 ind/m2 respectively,and the coverage of crustose coralline algae was 2.45%and 3.68%respectively;In the offshore Huangyan Island with low turf algae coverage,the recruitment is up to 15.63 ind/m2,and the coverage of crustose coralline algae is up to 17.93%.These results indicate that high coverage of turf algae not only inhibits the growth of crustose coralline algae,but also potentially affects the replenishment of coral larvae and the growth of scleractinian coral.(3)The Weizhou Island and Yongxing Island in the mid-northern South China Sea have undergone a process of ecological phase shift from coral dominance to turf algae dominance due to the great influence of human activities,and are in a state of rapid degradation.The coverage of scleractinian coral in Weizhou Island decreased from 69.28%in 1991 to 6.05%in 2015,and that in Yongxing Island decreased from?90%in 1980 to 6.46%in 2015.The coverage of scleractinian coral is low and still declining.The coverage of turf algae in Weizhou Island and Yongxing Island is 3.79 and 2.22 times of that of scleractinian coral,respectively.However,Huangyan Island,which is less affected by human activities,maintains a higher coverage of scleractinian and a lower coverage of turf algae without ecological phase shift.