Biogeochemistrv Of Nutrients In The Changjiang (Yangtze River) Estuary And The Adjacent East China Sea

Abstract:Rivers and open sea are linked by estuary,which has complicated topography and hydrological conditions.Estuarine ecosystem has an active process of material energy exchange,and is highly sensitive to environmental change and human activities.Due to the global climate disturbance,elevated sea level,increased erosion in the estuarine delta,and intense human activities,the estuarine ecosystem is facing severe challenges.Elucidating the biogeochemical cycles of nutrients under the influence of dominant physical processes,especially those connected with their non-conservation behaviors in the estuarine area of large rivers,has long been a research hotpot.This paper takes the Changjiang Estuary and the East China Sea as the research object.The spatial distribution and seasonal variation of nutrient in the Changjiang Estuary was analyzed,and the response of the northeastern East China Sea to the Changjiang diluted water was discussed under different conditions of the Changjiang River runoff,and the structure of the phosphate composition in study area was explored,the main results are as follows:(1)Spatiotemporal distribution and seasonal variation of nutrientsSeawater samples in the Changjiang Estuary and the adjacent East China Sea(ECS)were collected in February and July 2014 during which concentrations of nutrients(including NO3-,SiO32-,PO43-,NH4+ and NO2-were measured.The distribution and variation of nutrients showed distinct seasonal variation pattern.In summer,the coverage area of seawater with high concentrations of NO3-,SiO32-and PO43-was obviously larger than that in winter,owing to the increased Changjiang discharge and the seawater stratification.On the contrary,however,the concentrations of the three nutrients in the offshore seawater were generally lower at surface in summer than those in winter.NH4+ and NO2-showed positive correlations with salinity in summer while negative relationships in winter,and this seasonal reverse pattern was largely ascribed to their sharply increased concentrations in the Changjiang freshwater end-member from summer to winter.The Changjiang runoff was transported to the pelagic oceans in separated patches,rather than in continuous plume.Comparison of nutrient concentrations among different patches validated that the Changjiang Estuary and the adjacent areas were a tremendous sink for most nutrients in summer.Losses of NO3-,SiO32-,PO43-at seawater surface,in both summer or winter,could still be observed even after removing the influence of salinity,suggesting the phytoplankton assimilation of nutrients at surface and regeneration of nutrients via mineralization at bottom.The non-conservative processes of nutrients occurred not only in the estuarine and shelf areas where freshwater and seawater mixed,but also in the Changjiang freshwater areas within the river mouth.(2)Response of nutrients in the northeastern East China Sea to Changjiang Diluted WaterIn order to further explore the influence of the expansion of the Changjiang Diluted Water on the distribution of nutrients in the East China Sea,this paper selects the date of cruises in July 2010 and July 2011 for further analysis.The west-to-east expansion range of Changjiang Diluted Water(CDW,with salinity<31)at surface in July 2010(with much higher monthly Changjiang discharge)was obviously larger than that in July 2011.However,the expansion range of surface seawater with high NO3-concentrations(>15 ?mol/L)was much smaller in July 2010.Concentrations of NO3-,SiO32-,PO43-at most stations were generally quite low due to surface phytoplankton uptake and water column stratification,and these low values maintained until to the subsurface chlorophyll maximum layer(at depths of 10-40 m)where their concentration increased sharply and then showed constant values afterwards.The concentrations of NH4+ at stations in July 2010 were commonly higher than those in July 2011,throughout the water columns and over the whole study area.Molecular ratios of SiO32-/NO3-and PO43-/NO3-decreased along the Changjiang transport pathway northeastwards from the Changjiang freshwater,to its estuarine area,and then to the northeastern ECS.The nutrient concentrations of NO3-,SiO32',and PO43-in the Kuroshio Subsurface Water and Kuroshio Intermediate Water were quite high,indicating that those water masses,together with CDW,were also important contributors to the nutrients inventories of the northeastern ECS.(3)Exploration of determination of dissolved organic phosphorusDissolved organic phosphorus is digested by persulphate oxidation under the condition of 95? for 16 h,the chromogenic time is 15 min.The linear range of PO43-standard solution is 0-400 ppb,the detection limit is 1.7 ppb,the accuracy of 100 ppb PO43-standard solution is 0.4,and the precision is within 0.6%.This method has good results for freshwater and low salinity seawater,but the results show that the method is not suitable for the determination of seawater samples.Mainly because of the matrix effect of high salinity samples,which makes the time of PO43-and chromogenic reaction greatly extended.The higher the concentration of PO43-,the more obvious the extension effect is.Samples in Datong,south of Chongming Island,North Channel and South Channel was collected,the main forms of phosphate in the three regions were PO43-,and the percentage of dissolved organic phosphorus was not more than 36.9%.