| The distribution characteristic of nutrients, organic pollutants, dissolved oxygenand phytoplankton biomass and status of eutrophication in the Yangtze River Estuarywere analyzed. According to conceptual model of coastal eutrophication, thesensitivity of different zones in the Yangtze River Estuary to nutrient enrichment wasdiscussed. The characteristics of eutrophication were analyzed with exploratory dataanalysis. Additionally, the effect of water storage of the Three Gorges Project oneutrophication in the Yangtze River Estuary was discussed.The main results are as follows:DIN in the Yangtze River Estuary was abundant, 30 percent of whosemeasurements reached up to or exceeded the fourth grade of sea water qualitystandard in November 2003 and August 2004 and the pollution of DIN was enhancingfrom November 2004 to September 2005. Pollution of phosphate was less serious,most of whose measurements were less than the second grade of sea water qualitystandard except August from November 2003 to August 2004 and the pollution ofphosphate was enhancing. The organic pollution was not so serious that manymeasurements were under second grade of sea water quality standard. According toNational Estuarine Eutrophication Assessment, the Yangtze River Estuary inNovember 2003 and May 2004 was under medium eutrophicaiton condition as far asChl a was concerned. In May and August 2004, the oxygen content was low;especially in May, 50 percent of measurements in the whole water column reached upto or exceeded the fourth grade of sea water quality standard and large area ofhypoxia(< 2mg·L-1) in bottom water offshore existed.The mean concentrations of nutrients in surface water were highest in inner estuaryand Turbidity Maximum zone. The maximum Chl a concentration existed in theadjacent sea in spring and summer. An analysis of historical and recent data revealedthat in the last 40 years, nitrate and phosphate concentrations increased about 9-foldand 3 or 4-fold, respectively. Meanwhile, N:P ratios also increased. In parallel withthe N and P enrichment, an increase of Chl a was detected and the mean DOconcentration in bottom waters decreased.In comparison with other estuaries, the Yangtze River Estuary was characterized byhigh DIN and silicate concentrations, with low phosphate concentrations. Despite thehigher nutrient concentrations, Chl a concentrations were lower in the inner estuaryand Turbidity Maximum zone than in the adjacent sea. The phytoplankton growth wasnot limited by nutrients;however, in inner estuary and Turbidity Maximum zone,phytoplankton growth was suppressed by high turbidity, large tidal amplitude andshort residence time. Therefore, the inner estuary and Turbidity Maximum zone wasnot sensitive to nutrient enrichment. By comparison, in adjacent sea, thephytoplankton growth was not limited by nutrients;furthermore, phytoplanktongrowth was not suppressed by turbidity, tidal amplitude and residence time. What wasmore important, water stratification was also an important factor that resulted in agreater phytoplankton biomass and lower DO concentrations. Hence, the open seawas unexpectedly more sensitive to nutrient enrichment and related eutrophicationprocesses.Three principal components (PCs) were extracted with principal componentanalysis: N nutrients and COD for PC1, Chl a and cell abundance for PC2 and Pnutrients for PC3. A west-east decreasing gradient in the PC1 indicated N nutrientsand organic pollution originated from Yangtze River. Influenced by anthropogenicsewage, PC1 near Wusongkou and Balonggang outlets was higher than other stations.N nutrients and organic pollution in stations of inner estuary in winter are mostserious. PC1, PC2, PC3 in spring and summer are higher so eutrophication level inspring and summer is more serious than other seasons. A good linear relationshipbetween PC1 and salinity exists especially in autumn and winter so N nutrients andorganic pollution are predicted to a good extend on the basis of a linear model inwhich salinity is the independent variable. NO3ˉ, TN, TP and phytoplankton cellabundance are dominating factors in assessing the variation of eutrophication in theYangtze Estuary and its adjacent area.The concentration of nutrients changed significantly before and after water storageof Three Gorges Project. Concentration of DIN and silicate increased while that ofphosphate decreased after water storage. N:P and Si:P ratios increased significantlybut Si:N ratio declined after water storage. Potential P limitation was more seriousand the area of potential P limitation was larger after water storage. Concentration ofChl a was higher than before water storage and area of high Chl a increased so area ofhigh phytoplankton biomass was expanded after water storage.
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