Ecosystem deterioration in the middle Yangtze floodplain lakes over the last two centuries: Evidence from sedimentary pigments

Water quality of floodplain lakes in the Yangtze region which supports ca. 450 million people is being severely compromised by nutrient pollution, climate change and dam installation resulting from intensive socio-economic development. However, due to a lack of long-term monitoring data, the onset and causes of ecosystem degradation are unclear. Here, we used chlorophyll and carotenoid pigments in dated sediment cores from six lakes spanning the region to reconstruct changes in algae and cyano-bacterial HAB (harmful algal bloom) taxa alongside sedimentary nutrient flux measurements and his-torical archives. Sedimentary N fluxes are linked to changes in agriculture, while urbanization has had greater influences on P fluxes. Over the last 70 years algal and N2-fixing HAB pigments increased markedly in lakes (Luhu, Wanghu) that are strongly influenced by urbanization/industrialization. Algal assemblages in two other lakes (Futou, Honghu) changed gradually and responded primarily to agri-culture and associated N fluxes; diazotrophic HAB pigments were absent and the lakes retained macrophyte cover. Local dam installation had no discernible effect on pigment assemblages in three of the four lakes in the past 70 years, but in the two hydrologically-open lakes (Poyang, Dongting), increasing algal production was significantly related to the upstream installation of the Three Gorges Dam (TGD) and to urban/industrial and agricultural stressors. Temperature only influenced phototrophs in the most degraded lakes (Luhu, Wanghu). This spatial and temporal overview identifies that nutrient pollution is the primary regional driver of lake phototrophs, but that diazotrophic HABs are stimulated by P-enriched urban wastewater pollution, and agriculturally-derived N pollution favors non-N2-fixing cyanobacteria. Despite negative effects of the Three Gorges project, free connection to the river appears to help mitigate excess HABs in freely connected lakes. Management thus needs to be tailored appro-priately to specific lake conditions and palaeolimnology can be valuable in identifying appropriate strategies.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

Water quality of floodplain lakes in the Yangtze region is being compromised by nutrient pollution, climate change, and dam installation. The lack of long-term monitoring data makes it difficult to understand the onset and causes of ecosystem degradation. This study used sediment cores from six lakes to reconstruct changes in algae and harmful algal blooms (HABs). Nutrient fluxes in the sediment were linked to agriculture and urbanization, while dam installation had varying effects on pigment assemblages. Nutrient pollution was identified as the primary driver of lake phototrophs, with different types of HABs favored by different pollution sources.