Role Of Atmospheric Nitrogen Deposition In Coral-Algae Phase Shift And Coral Bleaching In Remote Reefs

In recent decades,the coastal shallow coral reefs have large-scale degraded.The remote or deeper coral reefs are considered as coral refuge because of the stable environment and the distance with nearshore human activities.However,terrestrial nutrients can be long distance transported by air mass,which has became the main nutrient input in the remote region.But the impact of atmospheric nitrogen(N)deposition on coral reef is still unclear.In this paper,the mid-northern South China Sea has selected as study region,analyse the ecological status of the coral reefs;estimate the atmospheric N deposition flux;and study the effects of nitrogen deposition on coral reefs by laboratorial simulation experiments,the results as follow:According to the data of study region,include 9 islands,20 sections,and 81study sites,the live coral cover averaged at 16.3%,ranging from 0.1 to 72.9%;the macroalgae cover averaged at 10.1%,ranging from 0 to 59.5%;the algae-coral ratio of 28.4%of the sites were higher than 1.The live coral cover of deep water is lower than that in shallow or mid-depth water in a half of sections,and live coral cover of 44.1%stations is lower than 10%.The distribution rule of the live coral cover in habitats were lower slope>upper slope>reef flat>lagoon.The38.9%of the sections shows higher macroalgae cover in lower slope habitat than reef flat.These results show that,remote deeper habitat has no obvious advantage in coral cover status,but facing the risk of macroalgae over growth.The observation data show that the NH₄⁺concentration of atmospheric deposition in the SCS ranging from 0.8?mol/L to 7890?mol/L,the high value appeared at 2000,2006 and 2013.The concentration of NO₃^-ranging from 0.5?mol/L to 406.8?mol/L,the high value appeared at 2001-2004.The N deposition generally show fluctuated increase trend.The maximum NH₄⁺and NO₃^-monthly average concentration mainly in April to September,the minimum value mainly in January February and December.During the summer monsoon,the atmospheric N deposition flux of coral reefs in the mid-northern SCS is estimated at about 11 mmol N/m²month.By analyzing the response of 9 coral species of 7 genera to heat stress(26?32?)and the differences between genera and species under different N concentrations and N/P,it was found that(1)In the condition that without nitrogen deposition(control group),the Fv/Fm of zooxanthallae decreased to below 0.5 on the 5th day,indicating that the photosynthesis of zooxanthalla was inhibited.In addition to Lithophyllon sp.and Favites sp.,Acropora sp.1?Acropora sp.2 and Montipora sp.bleached on the 20th day;The zooxanthalla density of Acropora sp.1 decreased by 70%.(2)When the N/P was balance(N/P=16:1),under medium N deposition concentration(7?mol/L),the corals survived for 41 days except Montipora sp.;Within 20 days,the Fv/Fm value remained above 0.5,and the zooxanthalla density of Acropora sp.1 decreased about 10%.At high concentration of N deposition(30?mol/L),the survival time of corals was close to the control group,but the survival time of Acropora sp.1and Porites sp.was 2?3 days shorter than control group,and the zooxanthalla density of Acropora sp.1 decreased by about 40%.(3)When the N/P value was out of balance,medium concentration of nitrogen deposition significantly shortened the survival time of Acropora sp.3 and Montipora sp.,and only survived for 4?5 days(N/P=36:1)and 7?8 days(N/P=8:1).In the same period of control group,the Fv/Fm values of corals were above 0.5,and there was no bleaching coral colonies.(4)Nitrogen deposition can significantly promote the growth of macroalgae.In 22 days,medium concentration nitrogen deposition increased the macroalgae cover from 18.3%to 50%;high concentration nitrogen deposition increased the macroalgae cover from 21.7%to 70%then stop increase.In conclusion,the benthic community undergoes a coral-algae phase shift in the remote reefs of the mid-northern SCS,the deeper water habitat shows no obvious advantage compared with shallow water habitat.The flux of N deposition in the SCS reefs has obvious intra-and interannual variations,during the summer monsoon.Medium concentration of N deposition can improve corals survive through the maximal temperature month in the SCS;high concentration of N deposition has no improve,but even exacerbate coral bleaching;in the occlusion reef area with weak water exchange,N deposition can easily lead to imbalance N/P and further aggravate coral bleaching.Atmospheric N deposition can make macroalgae grow rapidly,and may be the main cause of coral-algae phase shift.