Spatial Distribution Of Coral Community And Benthic Algae And Their Ecological Impacts Across The South China Sea

Benthic algae are an important indicator for evaluating the health of coral reef ecosystems.The overgrowth of benthic algae may hinder the growth,reproduction and recovery of corals,thus,they are highly concerned.Coral reefs are widely distributed across the South China Sea(SCS),but there are rare researches on the spatial distributions of benthic algae and their ecological impacts across the coral reefs from the SCS.In this research,we focused on the benthic algae of the SCS reefs,and conducted researches on the spatial distribution of benthic algae and their ecological impact on coral communities.The latitude range(from Nansha Islands in the southern SCS to Daya Bay in the northern SCS)of this research was about 13latitudes(9–22°N,about 1500 km apart),including the Nansha Islands(Sanjiao Reef,Xian'e Reef and Xinyi Reef),Zhongsha Islands(Huangyan Island),Xisha Islands(North Reef,Yongxing Island,East Island,Huaguang Reef and Langhua Reef),Luhuitou reef in Sanya city of Hainan province,Leizhou Peninsula in Guangdong province,Weizhou Island in Guangxi province,and Daya Bay in Guangdong province.We conducted ecological surveys on 13 coral reefs of the above 7 coral reef regions during 2015–2019.In this paper,We addressed four fundamental themes:1)the spatial distributions of coral communities in the coral reefs from the SCS and their relationships with ecological factors;2)spatial variations in the resilience of coral communities across the SCS reefs and their response to human activities;3)the spatial distributions of benthic algae in the coral reefs from the SCS and their main ecological control factors;4)the ecological impacts of benthic algae on coral communities of the SCS.The surveyed ecological indicators including coral cover,coral species diversity,density of coral recruitment,coral species characteristics(i.e.,coral growth morphology and coral diameter),heat tolerance compositions of coral,benthic algal cover and their functional group compositions,the competitive interactions between benthic algae and corals,and the compositions of the symbiotic bacterial communities in algae and corals.The ecological factors we analyzed including sea surface temperature(SST),photosynthetically active radiation(PAR),chlorophyll-a(Chl-a),dissolved inorganic nitrogen(DIN),soluble reactive phosphorus(SRP),particulate organic carbon(POC),sea water transparency,significant height of combined wind waves and swell(SWH),species number and density of reef fish.The intensity for the impacts of human activities was measured by human population density(person/km²)and the reef distance to mainland(km).The main conclusions are showed as followings:(1)Live coral cover,coral species number,and diversity index of coral species all have significant differences among coral reef region and were effectively regulated by regional ecological factors.The average live coral cover across the SCS was 18.7%(based on the values of each surveyed coral reef regions).The live coral covers in coral reef regions of Daya Bay,Weizhou Island,Leizhou Peninsula,Sanya Luhuitou,Xisha Islands,Zhongsha Islands,and Nansha Islands(from the northern SCS to southern SCS)were 25.9%,8.5%,10.2%,17.4%,20.3%,9.5%,and 22.2%,and the numbers of coral species were 33,30,18,38,158,64,and 160,respectively.The changing trends of Margalef richness index and Shannon-Weaner diversity index were consistent with the number of coral species,however,the distributing trend of Simpson diversity index and Pielou evenness index were quite chaotic.The results of Random Forest models and Pearson correlation analysis showed that seawater transparency maybe the most important factor for regulating the spatial distribution of coral cover,and have a significant positive correlation with coral cover;nutrients(SRP,DIN,and Chl-a)and seawater transparency were the most important factors for predicting the spatial distribution of the coral species number,and was significant negatively and positively correlated with the numbers of coral species,respectively.Random Forest models and Pearson correlation analysis also showed that seawater transparency and SST were the most important factors for regulating the spatial distribution of the four species diversity indexes of corals,and were significantly corrected with all species diversity indexes.Furthermore,the intensity of human activities(measuring by human population density and reef distance to mainland)was negatively correlated with seawater transparency and nutrients.Thus,it was speculated that a decreasing in seawater transparency and an increasing in nutrients that caused by human activities may be the major factors for the decreasing in coral cover and species diversity in the northern SCS;while a high species diversity of corals can be maintained in the central and southern SCS was due to the high seawater transparency and high SST.(2)Density of coral recruitment(small corals with a diameter of 0–5 cm)and genera composition of juvenile corals(corals with a diameter of 0–10 cm)were differ significantly among coral reef regions,and were closely related to the impacts of human activities.The average density of coral recruitment in the SCS was 13.6 ind m^-2.The density of coral recruitment showed a gradient increasing trend from the northern to southern SCS.The density of coral recruitment in coral reef regions of Daya Bay,Weizhou Island,Leizhou Peninsula,Sanya Luhuitou,Xisha Islands,Zhongsha Islands,and Nansha Islands were 6.0 ind m^-2,3.4 ind m^-2,2.7 ind m^-2,8.0ind m^-2,8.3 ind m^-2,15.7 ind m^-2,and 26.1 ind m^-2,respectively.The dominant genera of juvenile corals in the northern SCS including massive Porites,Favia,Favites,Pavona,and Goniopora.However,the dominant genera of juvenile corals(with different coral growth form)in the central and southern SCS including branching Pocillopora and Acropora,and massive Porites,Favia,Favites,and Goniastrea.Due to the increasing human activities and global warming,the massive coral genera,which are more tolerant to environmental pressure,are gradually becoming the coral dominant group of the SCS reefs.Based on the compositions of coral diameter,growth morphology,and heat tolerance,we found that the northern SCS were charactered by simple three-dimensional structure and lower coral resilience,while the central and southern SCS were charactered by complex three-dimensional structure and higher resilience;furthermore,coral communities across the SCS reefs were tolerance to environmental stress on a large spatial scale.(3)Benthic algal covers including turf algae,macroalgae,and crustose coralline algae(CCA)showed significant spatial variation in the SCS reefs and were mainly controlled by gradient changing ecological factors(except for macroalgae).The average cover of benthic algae across the SCS was 29%(based on the values of each surveyed coral reef regions),of which turf algae and CCA both account for 12%,and macroalgae only account for 5%.The spatial distribution of turf algae covers gradually decreases(from 22%to 6%)from the northern to the southern SCS(along with the increasing of the reef distance to mainland),conversely,the spatial distribution of CCA covers gradually increases(from 2%to 18%),but the gradient changing trends of macroalgae cover was not obvious.Redundancy analysis and Pearson correlation analysis showed that cover of turf algae and CCA were significant correlated to 10 ecological factors,namely SST,PAR,Chl-a,DIN,SRP,POC,sea water transparency,SWH,and species number and density of reef fish;while the corrections between macroalgae cover(including total macroalgae,fleshy macroalgae,and Halimeda)and the 10 ecological factors were quite weak or even non-existent.Results of random forest models showed that nutrients(DIN and SRP),density of reef fish,and sea water transparency were the most important factors for regulating the spatial distribution of turf algae cover,moreover,SST,sea water transparency,DIN,and species number of herbivorous fish were the most important factors for regulating the spatial distribution of CCA cover.However,the spatial distributions of macroalgae could not accurately predict by mostly ecological factors,because the natural relationships between macroalgae and ecological factors may have been destroyed by the overgrowth of turf algae and the effect of nutrient subsidies.(4)Benthic algae(especially turf algae)exerted pressures on healthy corals by changing the symbiotic microbial communities of corals,and therefore influenced the competitions between benthic algae and corals,and ultimately leads to changes in coral cover,species diversity of coral,and coral resilience.The analysis of symbiotic microorganisms of corals and algae showed that turf algal contacts may lead to the reorganization and homogenization of symbiotic bacterial communities of corals by promoting the proliferation of symbiotic bacterial communities of corals,increasing potential pathogenic bacteria,and reducing coral probiotics.Thereby,turf algal contacts could endangering nearby coral health.Comparing with other benthic algal group,an increase in turf algal cover had the most extensive ecological impacts on the coral communities in the SCS,and could reduce the coral cover;furthermore,both turf algae and macroalgae have a significant negative impacts on the species diversity and resilience of corals,while the impacts of CCA were just the opposite.The competitive impacts(namely contacting and damaging impacts)of turf algae on individual corals were 2-18 times higher than that of macroalgae and CCA in the SCS.In addition,the ecological impacts(including competitive impacts)of benthic algae on corals were also affected by the intensity of human activities,coral genus,coral growth form,coral diameter,and diameter of coral polyp.(5)The dominance of coral community for most coral reefs of the SCS has been replaced by benthic algae;the ecological impacts of benthic algae on corals were directly related to the types and abundance of benthic algae functional groups.The total cover of benthic algae was higher than that of corals and any other benthic organisms in mostly coral reef regions(except for Daya Bay).Turf algal cover gradually decreased from the northern to the southern SCS,meanwhile,proportions of turf algal contact along the coral edges gradually decreased,however the proportions of turf algal damage along the coral edges did not change gradually.The proportions of macroalgal contact and damage along the coral edges were significantly higher in Weizhou Island and Xisha Islands than that of other coral reef regions.Because macroalgal cover in the coal reefs of Weizhou Island and Xisha Islands was significantly higher than other coral reef regions.CCA cover gradually increased from the northern to the southern SCS,meanwhile,the proportions of CCA contact and damage along the coral edges increased significantly and gradually.Although,turf algal cover was lower in the central and southern SCS,turf algae can also cause the most damaging impacts on corals,whilst the highest cover of CCA yielded fewer damaging impacts on corals.This suggested that turf algae has a more extensive ecological impacts on corals across the SCS.The intensification of human activities and global warming has caused the continuous degradation of coral reef ecosystems.However,the ecological impact of benthic algae will become a critical factor of the health and evolution of coral reef ecosystems in the future.