Ecosystem Evolutionary Trajectory of Lake Xiliang over the Past Century Driven by Eutrophication

Shallow lakes in the middle and lower reaches of the Yangtze River are important natural resources for the regional sustainability. Eutrophication has transformed many lakes in this region from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state, limiting the supply of lake ecosystem services. Environmental deterioration of these lake ecosystems was the result of long-term pressures from external forces. A full understanding of the lake evolution is the prerequisite for ecological management and protection, but the relevant understanding is limited by the lack of long-term monitoring data. Lake Xiliang was thus selected to clarify the evolutionary trajectory of typical macrophyte-dominated lake ecosystems in the middle and lower reaches of the Yangtze River and better govern lakes in this region. Based on high-resolution radionuclide dating, the evolutionary patterns over the past 150 years of Lake Xiliang were analyzed using sedimentary proxies such as grain size, chemical elements, and diatoms. The results showed that before the 1940s, Lake Xiliang was clear with a low nutrient level and relatively fewer submerged plants. Conversely, after the 1940s, human activities gradually increased and led to the higher nutrient level and higher coverage of macrophytes; the diatom composition gradually evolved from planktonic-dominated assemblages to benthic- and epiphytic-dominated ones, and the nutrient level of sediment also kept increasing, indicating that human activities represented by land use changes in the watershed, polder, and fish farming have profoundly changed the ecosystem structure of the lake. The study showed that although lake nutrients have been maintained at a high level after the 1970s and became eutrophic, the growth of submerged vegetation limited the concentration of algae in the lake and prevented an algae outbreak in Lake Xiliang. It was also found that the increase in exogenous nutrients profoundly affected the geochemical cycle of the lake through primary producers, causing the accumulation of sediment calcium carbonate. This process showed that the succession of submerged plants driven by eutrophication affected the nutrient geochemical cycle of Lake Xiliang, which may increase the risk of endogenous release. The evolutionary processes of a typical macrophyte-dominated lake in the middle and lower reaches of the Yangtze River was thus clarified, and the ecological risks faced by this type of lake under long-term human activities and climate change were also assessed, providing a scientific basis for the management of regional lakes. © 2022, Science Press. All right reserved.