Nitrogen Biogeochemical Processes And Geochemical Record Of Anthropogenic Nutrient Loading In Coastal Regions Of Beibu Gulf,guangxi Province

As an important transition zone between land and sea,the estuarine and coastal area is under great pressure from the development of coastal cities and human activities,and is more prone to various environmental problems,such as eutrophication and harmful algal blooms,which have posed serious threats on the coastal ecosystem and economic development.It is of great scientific significance to understand the migration,transformation and sedimentary history of nutrients for mitigating and preventing coastal environmental problems.In this study,the coastal area of Guangxi Beibu Gulf was selected as the research object,and the main nitrogen biogeochemical processes,the nutrients budgets,and the sedimentation records of nutrient elements were studied.The main research results are as follows:?1?Spatiotemporal distribution of nutrient in surface seawater,overlying water and pore water was studied in our study area.Nutrient concentrations in Yintan Coast,Maowei Sea and Lianzhou Bay were significantly higher than those in other regions.Nutrient concentrations decreased from the estuaries to outer bays,indicating that terrigenous inputs were the main nutrient source.Nutrient concentrations in summer were higher than those in winter,which was caused by the seasonal difference of terrigenous inputs.In addition,nutrient fluxes at the sediment-water interface?SWI?were studied with the Fick's first law of diffusion.It all showed all positive and indicated nutrient diffused from sediment to water column.This suggested that sediment could be a second nutrient source.High temperature and low oxygen condition can promote the nutrient fluxes at the SWI.?2?Through the periodic investigation and LOICZ box model,the budgets of nitrogen,phosphorus and silica were estimated.Due to the seasonal difference of water budgets and nutrient concentration,nutrient budgets in wet season were significantly higher than those in dry season.Overall,the integrated budgets of all estuaries revealed that the coastal regions of northern Beibu Gulf behaved as a sink of all nutrient species.Nutrient derived from the submarine groundwater discharge?SGD?was the dominant nutrient source,accounting for more than 50%of all sources.River-borne nutrient inputs were also a significant nutrient importing pathway.Both residual flows and water exchange flows transported nutrients off the estuaries and water exchange flows were the main output way.Additionally,the net nitrogen biogeochemistry was-0.14 mol m^-2yr^-1,indicating that the amounts of denitrification and anammox were higher than that of nitrogen fixation.?3?Nitrogen fixation rate in surface water was studied with the Acetylene Reduction Assay.The average nitrogen fixation rate was 0.33±0.17 nmol N/L/h in summer and 0.23±0.11 nmol N/L/h in winter.It mainly occurred in the outer bays with high temperature,low nitrate concentration and low ratio of nitrogen and phosphorus?N/P<16?.Comparisons of the nitrogen fixation rates between the two size fractions indicated that the contribution of unicellular diazotrophs was more important than filamentous diazotrophs,accounting for 69.76±7.93%.Besides,field investigation revealed that Richelia was the dominant species in filamentous diazotrophs.The abundance and distribution of Richelia was related to Si/N and the species and abundance of diatoms.Nitrogen fixation contributed a little to the primary production in the study area.?4?Denitrification and anammox in surface sediments were investigated with isotope pairing method.Denitrification rate was 3.19±1.11 nmol N/m L/h in summer and 2.24±1.43 nmol N/m L/h in winter.Denitrification was mainly related to the nitrate concentration in pore water and organic matter in the sediment,while anammox was mainly regulated by the concentration of nitrate and nitrite in pore water.Additionally,temperature,dissolved oxygen?DO?and salinity also had an impact on denitrification and anammox.The net areal yield of nitrogen biogeochemical processes was estimated to be-1079t/a,as an important pathway of nitrogen removal.?5?Concentrations and speciation of N,P,Si in surface sediments were measured and it showed a general decreasing pattern of all nutrients from estuaries to offshore area,probably due to terrigenous inputs.Bioavailable N and P accounted for 12.89%and 43.01%of total N and P in surface sediments,indicating bioavailable nutrients in surface sediments may perform a secondary source in certain conditions.Source identification indicated that organic matters were derived from terrigenous plants and riverine phytoplankton in rivers and estuaries,while organic matters came from terrigenous plants and marine phytoplankton in outer bays.?6?Sediment cores were ²¹⁰Pb dated and analyzed for the geochemical records.Multi-sedimentary proxies,such as elemental C,N,P and Si concentrations and?¹³C and?¹⁵N values,were used to understand the anthropogenic changes in nutrient loading and trophic status over the last century.The results showed that after 1980,the nutrient load of TOC,TN,TP and BSi began to increase gradually,then the growth rate accelerated after 2010,and peaked in 2017,which is basically consistent with the development trend of the coastal areas of Guangxi Province.The burial rate of nutrients has also increased significantly since 1980,which is directly related to the increase in sedimentation rate caused by human activities and land use.The results also showed that between 1960 and 2010,the organic matters in sediments mainly came from terrestrial inputs,while in recent years the organic matters came from the combination of terrestrial plants and marine phytoplankton.The historical change of TOC content is positively correlated with the GDP and population density of each urban area,which once again proves the impact of human activities on the input of organic matter in the near shore.