Hydrological disconnection from the Yangtze River triggered rapid environmental degradation in a riverine lake

Abstract

Interactive River-Flood Ecosystem (RFE) is an important feature characterizing the dynamics of the hydrological connectivity between lake and river worldwide. Any disruption in the RFE can impose significant effects on the functioning of floodplain ecosystems. However, due to the lack of long-term monitoring data, the understanding of the process and mechanism behind limnological responses to the changes in hydrological alterations and nutrient loading in floodplain lakes is restricted. The middle and lower reaches of the Yangtze river floodplain lake system of China is one of the significant food bowls of the nation, and makes important contribution to ecosystem services including drinking water and food to the society for generations. The interactive RFE of the Yangtze floodplain lakes is profoundly disrupted by anthropogenic activities over the recent decades. However, the response of such changes in river-lake connectivity is not comprehensively investigated yet. Here, we studied Cehu Lake in the lower reaches of the Yangtze floodplain system to examine the long-term environmental changes with altered river-lake connectivity and increased nutrient loading using high-resolution paleolimnological multi-proxy analyses including geochemistry, grain size and diatom assemblages of a 210Pb/137Cs dated core. Our results suggest that Cehu Lake have experienced two pronounced shifts: immediately after the damming in Yangtze river (1964) completely ceasing the lake-river connectivity, and in 1994. The disruption in the hydrological connectivity in 1964 led to an abrupt decrease in the abundance of Aulacoseira granulata, and an increase in the organic matter (loss-on-ignition), epiphytic and benthic diatoms, while the disruption in 1994 led to increased total phosphorus (TP), lead (Pb), dry mass accumulation rate, and a decline in Fe/Mn, and consequently enhanced lake eutrophication with emergence of enriched nutrient tolerant diatoms (Cyclotella meneghiniana, Cyclostephanos tholiformis). The redundancy analysis (RDA) showed increased TP loads in lake sediment after damming in the Yangtze river. The condition stimulated macrophyte growth with, but altered diatom flora. Reconnecting Cehu lake with Yangtze river together with reducing nutrient loading to the lake from the incoming river may be an effective management practice to relieve the ecological stress.