Different effects of natural siltation, damming and pollution on two adjacent lakes of Dongting Lake Wetland: Evidence from paleolimnology

Abstract

Evaluating the influences of hydrology on lotic floodplain lakes has been difficult due to concurrent effects of different human activities in the catchment. This study aims to explore the responses of floodplain lakes to natural siltation and damming by studying grain size and geochemical elements in 210Pb-dated sediment cores from two adjacent lakes with different hydrological conditions in the same catchment, including a hydrologically-isolated lake (Donghu Lake) and a hydrologically-connected lake (East Dongting Lake) in the middle Yangtze floodplain. The results showed that the riverine sediment supply declined in Donghu Lake after its isolation from the Yangtze River due to natural siltation in the 1910s. In East Dongting Lake, the riverine sediment supply remained relatively stable before the operation of the Three Gorges Dam (TGD) in 2003 CE, and then declined rapidly in response to a decrease in riverine sediment supply. Total phosphorus and total organic carbon in Donghu Lake increased after the “reform and opening-up” in the late 1970s, whereas they remained at low concentrations in East Dongting Lake before the TGD operation in 2003 CE. Hydrological isolation from the Yangtze River reduced the influxes of riverine sediments, but promoted nutrient enrichment and primary production in East Dongting Lake through extending water residence times. Stable element composition in Donghu Lake after 2003 CE suggested that the TGD operation had minor effects on hydrologically-isolated lakes. The comparison between hydrologically-closed and hydrologically-free lakes within the same catchment provides an effective method to identify different effects of natural siltation, damming and environmental pollution.