[Spatial distribution pattern and stock estimation of nutrients during bloom season in Lake Taihu]

Based on the data of high density spatial sampling in July 2013, we analyzed the spatial distribution pattern of nutrients and estimated their amount during bloom season in Lake Taihu to discuss the correlation of algal bloom in different types of ecological water and nutrients in large shallow lake and the representative of its sampling sites. The research showed that nutrients and chlorophyll-a concentration (CHL) in Lake Taihu tended to reduce from northwest to southeast in general during bloom season. Nitrogen was mainly present in dissolved form, accounting for 76.28 percent of the total nitrogen (TN), and phosphorus was mainly present in particulate form, accounting for 66.38 percent of the total phosphorus (TP). The sampling points in the whole lake could be divided into four sections with significant difference between each other using principal component analysis and cluster analysis: The first section was located in the district of northwestern Lake Taihu, which represented the heavy eutrophic lake areas with serious blooms; the second section mainly included Meiliang Bay and area of river inflow into lake in South of Lake Taihu, which stood for moderate eutrophication of water quality; The third section included the central area and the southwest of lake, which represented the water area with medium water pollution, but blooms were frequent; And the fourth area was the remainder areas including Gonghu Bay, Xukou Bay, and Eastern Taihu, which stood for the region of weaker blooms and better water quality. Different factors also affected the growth of planktonic algae in different sections: From the point of the whole lake, CHL was significantly correlated with TP, TN, total dissolved nitrogen (TDN) and nitrate nitrogen (NO3(-) -N); while in the first section, CHL was significantly correlated to TP and TDN; CHL was correlated to TN and TDN in the second section; in the third section, the influencing factors were TP, reactive phosphate (PO4(3-) -P), TDN; PO4(3-)-P, total dissolved phosphorus (TDP) and nitrite nitrogen (NO2(-) -N) were the influencing factors for the fourth section. The study showed that the values of TN, TDN, TP and TDP respectively were 12 800 tons, 9 800 tons, 445 tons and 150 tons during the research period. As a large shallow lake, Lake Taihu showed high spatial heterogeneity in nutrients during bloom season, which was resulted from the space migration accumulation characteristics of cyanobacteria blooms and the alienation characteristics of ecological type. Therefore, when monitoring and evaluating the large shallow lakes, sampling points should be set rationally and the results should be interpreted properly, to avoid overgeneralization due to improper monitoring points and statistical methods.