The Nitrogen Isotope Characteristic Study Of Eutrophication In Changjiang Estuary

Based on the investigation work carried out in Changjiang Estuary inFebruary, May, September and November 2005, the δ¹⁵Np, the δ¹⁵N ofthe particulate organic nitrogen in surface water of the estuary, and otherrelated parameters were measured;then the distribution of δ¹⁵Np and thecorrelation between δ¹⁵Np and other parameters in the waters, includingthe surface waters of the entire Changjiang estuary, the bottom layerwaters of Changjiang River and the max mixture zone, and the verticalwaters of typical upwelling and non-upwelling region, were discussed;atlast, the land derived nitrogen and the biogeochemical processes on thenitrogen eutrophcation of Changjiang Estuary was analyzed and someconclusions were got listed below:The fact that both the δ¹⁵Np and its scatteration degree in the surfacewaters of Changjiang Estuary are higher and variable according to theseason change indicated that the proportion of the nitrogen derivedfrom agriculture and air deposition is comparatively high, the varietyof land derived nitrogen sources , the degree of nitrogenbiogeochemical process and its nitrogen fractionation is high andvariable to the season.Compared with dry season, the δ¹⁵Np in the surface waters ofChangjiang River in wet season are comparatively higher, thediversity among different investigation sections is smaller, thesources of SPON are more closer, and the proportion of industry,lifewastewater is higher. The correlations between δ15Np and otherparameters changes among different sections are weak both in dryand wet season, and the δ 15Np fractionation caused by thebiogeochemical behavior of SPON is not obvious;the main factorsinfluencing the distribution of δ15Np are the SPON source and itsrelated δ15Np background.As a whole, the degree of nitrogen biogeochemical processes ishigher in spring and summer and lower in autumn and winter. Inautumn, the area influenced by the POM decomposition was thelargest, and the proportion of the assimilation is the highest in winter.In alongshore area, the influence on the nitrogen distribution,transformation and nitrogen isotope fractionation posed by the POMdecomposition is the greatest in summer, followed by those in springand autumn, and it is little in winter, besides, it is greater in the maxmixture zone than those in Changjiang River. In the open sea, theinfluence of the assimilation is close to those of the POMdecomposition. The influence of the POM decomposition in the opensea is the greatest in autumn, followed by those in spring and summer,and it is little in winter;the influence of the assimilation is thegreatest in winter, followed by those in spring and summer, and it islittle in autumn. The assimilation degree and its influence is higher inthe north part and the south part of the open sea, however, thedecomposition degree and its influence is higher in middle part andnear-land part of the open sea.Compared with other seasons, the spring is the season when thedifference of the land derived nitrogen source , nitrogenbiogeochemical processes and nitrogen fractionation betweendifferent layers,different area in the Changjiang River and the maxmixture zone is larger;compared with bottom layer waters, thedifference between different season,different area in the surfacelayer waters is greater in Changjiang River and less in max mixturezone. The influence of the decomposition on the nitrogendistribution,transformation and nitrogen isotope fractionation isclose to those of the assimilation in the surface waters of ChangjiangRiver, however, in the bottom layer waters, the influence of thedecomposition is the main influence. The situation is comparativelycomplex in the max mixture zone.Compared with the non-upwelling region, the seasonal change ofδ15Np in vertival waters in the upwelling region waters is bigger, and thebackground δ15Np is lower, which is casued by the influence of theTaiwan Warm Current, especially in fall. The processes andparameters which influence the nitrogen eutrophication and nitrogenfractionation in the nearland 17 and 41 position region waters aremore obvious.