A Stable Isotopic Evidence Of Nutrient Sources And Dynamics For Nearshore Coral Reefs In China

Coral reefs are experiencing a recent period of decline.Nutrient over-enrichment is considered a major cause of this decline.This dissertation serves as a nearshore coral reef nitrogen research based on carbon and nitrogen stable isotopic methods.We conducted 8 cruises during 2014 to 2015 and focused on the migration and transformation of nitrogen in Xuwen?XW?and Sanya?SY?coral reef areas.Another 5 cruises were conducted on the research of Hongshi?HS?non-coral reef area and 1 cruise were conducted on the research of Daya?DY?coral reef area.We analyzed the basic environment factors of seawater and nutrient concentrations.We used the isotopic values of particular organic matter?POM?and the C/N ratios to identify the presence of anthropogenic nutrients in aquatic environments.We analyzed the fluxes of nitrogen fixation,and N2O emission,and then analyzed the possible stressors influencing the healthy of coral reef ecosystem and the contribution of coral reef ecosystem to climate change.The major conclusions are as follows:?1?The seawater qualities at three different coral reef areas were generally within the First Class Seawater Quality Standard,but heaviely affected by human activities.The nutrient concentrations were higher than those in ocean reefs.DIN/SRP values were higher in common,and the higher SRP concentration in Daya coral reef area relieved P limitation.?2?The four study areas had relatively positive values in 8 3C of POM,reflecting a marine phytoplankton derived carbon source.Despite the existance of the terrestrial derived POC in the four areas,non-coral reef area had relatively less terrestrial carbon.The ?¹⁵N values indicated a human derived terrestrial nitrogen source and seasonal variations.Hydrodynamic conditions and precipitation had a large influence on reefs.C/N values indicated the N excess conditions in coral reef areas.Therefore,continuous efforts should be made to effectively control the input and composition of human-derived nutrients in these systems.?3?The highest nitrogen fixation rate was in XW,which was 132.64±158.14?mol N m-2 d-1.Medium grade nitrogen fixation ratse were in SY and HS,which were 88.87± 99.43 ?mol N m-2 d-1 and 81.08± 83.94 ?mol N m-2 d-1,respectively.The rate in DY coral reef area was lowest,which was 4.66 ± 0.39?mol N m-2 d-1.Nitrogen fixation rate was in correlation with diazotroph species and abundance,and had significant seasonal variations.?4?The annual nitrogen fixation flux in SY,XW,DY were 7.14 × 10⁴ mol N a-1,9.68 x 10⁴ mol N a-1,and 6.04 x 10⁴ mol N a-1,respectively.The flux in Hongshi non-caoral reef area was 6.21 × 10⁴ mol N a-1.Diazotroph fixed nitrogen was non the major nitrogen source,the contribution to primary production was less than 5%.?5?Air N2O concentrations in SY,XW and HS were similar,ranging from 13 to 14 nmol N2O L-1,however N2O concentration was higher in DY,which was 15.62±1.51 nmol N2O L-1.The four study areas were N2O sources,with a highest flux in DY and lower in other three areas.The flux in SY,XW,DY and HS were 70.29±75.21,48.14 ± 49.13,241.29± 91.39 and 29.02 ± 17.56 nmol N2O m2 h-1,respectively.?6?The 815N values of POM reflecting biological process could be a tracer of marine nitrogen fixation.However,we didn't find the similar condition.Our research areas were heavily influenced by human activities,the heavier human derived 815N siginals overlapped the lighter microorganism siginals.?7?Based on the observation in SY and XW from 2014 to 2015,we found a positive correlation between N2O flux and water temperature in four single seasons.N2O emission from reefs could accelerate global warming.