Environmental changes in Chaohu Lake (southeast, China) since the mid 20th century: The interactive impacts of nutrients, hydrology and climate

摘要

Chaohu Lake, the fifth largest freshwater lake in the Yangtze floodplain, is faced with multiple stresses from anthropogenic disturbances and climate change. To explore the ecological changes in Chaohu Lake since the mid 20th century, we examined diatoms, geochemical indicators and particle size in ~(210)Pb-dated sediment core from the lake. Diatom succession revealed that the lake had switched to a eutrophic state since the late 1970s. Redundancy analysis using limnological data, hydrological and meteorological variables showed that sedimentary total phosphorus (TP) and total organic carbon (TOC), annual mean temperature, annual mean wind velocity, and water-level amplitude (WLA) were five significant factors influencing diatom succession. Diatom assemblages from 1950 till 1978 were driven by WLA and wind. The establishment of Chaohu Dam baffled hydrological connectivity between the lake and the Yangtze River in 1962, and reducing water exchange-induced flow. Meanwhile, weak wind velocity reduced the wind-induced flow in the 1960s. Due to the weak hydrodynamic intensity, the dominant species (Aulacoseira granulata, a species with high sinking rate) became less important during this period. From 1979 till 2006, diatom assemblages were mainly driven by TP, TOC and temperature, which were highly correlative. Increasing nutrient loading promoted the blooms of eutrophic species (e.g., Cyclostephanos dubius). In addition, rising temperature would indirectly influence diatom assemblages by mediating nutrient release process. As a consequence, multiple stresses in concert have caused the lake switch to a further eutrophic state indicated by prominent increases in more eutrophic species (e.g., Stephanodiscus parvus) since 2000. This study provided information on complex trajectories of aquatic ecosystem shifts driven by increasing nutrient loading, hydrological alteration and climate warming in the Yangtze floodplain lake.