Anthropogenic influence on lake environmental change is one of the focus of global change research. It is necessary to identify anthropogenic sources of nitrogen in rivers and lakes. Stable nitrogen isotope ratios (δ¹⁵N) have been widely used in tracer studies of anthropogenic nitrogen sources in aquatic environments. Lake Taihu is a large, eutrophic, shallow freshwater lake in China, and is located in the Yangtze delta with high population density and economic development. It serves as an ideal site for examining human impacts on aquatic environment in China. Results of δ¹⁵N values measured in inorganic nitrogen fractions (INF) showed that a small river basin and its estuary in Yixing city were characterized by agricultural origin, while INF in Xukou Bay and East Taihu Bay by aquiculture, and INF in Meiliang Bay by discharge of human sewage. δ¹⁵N values in the macrophyte-dominated zones were much lower than those in the phytoplankton-dominated zones of the studied water body of southern estuary-East Taihu Bay-Meiliang Bay, indicating the progressive intensification of human pressure on the lake from south to north. However, elevated δ¹⁵N values in lake water which were consistent with high primary production in summer (June) suggested the consequence of biological processes. Using these values for tracing anthropogenic sources of nitrogen during algae blooms might be misleaded.