Nutrient dynamics linked to hydrological condition and anthropogenic nutrient loading in Chaohu Lake (southeast China)

Chaohu Lake, a large (770 km(2)) shallow lake in the Yangtze River basin, has experienced serious eutrophication over the past three decades. To track its nutrient history and the causes, multi-proxies (diatom, geochemical indicators, magnetic susceptibility, and grain size) were analyzed on a 120-cm long core from the lake. Nutrient dynamics of the past 500 years were reconstructed using sedimentary diatom assemblages and an established diatom-inferred total phosphorus (DI-TP) transfer function. Between about 1500 and 1740 AD, the lake remained in an upper-mesotrophic state with the DI-TP oscillating around 60 mu g l(-1). Together with enhanced agricultural activities, a drier and warmer climate led to a decrease in water level and water exchange volume with the Yangtze River, thus triggering the first phase of eutrophication (ca. 1740-1820 AD). After this eutrophic episode, the lake had exhibited a recovery in nutrient status until the 1960s. However, the lake became susceptible to nutrient input due to a sharp decrease in water exchange volume after its impoundment in 1962. A large amount of anthropogenic nutrient input has exacerbated eutrophication in the lake since the late 1970s. Redundancy analysis using a range of sedimentary proxies indicated that the two eutrophication phases were mainly attributed to anthropogenic nutrient loading and altered hydrological conditions. The hydraulic connection with the Yangtze River should be the primary self-adjustment mechanism of the lake against anthropogenic nutrient input.