Hydrological isolation accelerates algal blooms in floodplain lakes: Biomarker evidence from Dongting Lake, China and its satellite lake.

Hydrological disconnection from main channels (either via natural siltation or due to construction of hydrological infrastructures) is modifying biogeochemical cycling in river-floodplain systems. Knowledge on how this process influences phytoplankton composition and harmful algal blooms (HABs) in floodplain lakes is quite scant due to the lack of long-term water quality monitoring and the concurrent influence of multiple drivers of change. Here, chlorophyll and carotenoid pigment biomarkers from dated sediment cores were analyzed from Dongting Lake (China's second largest freshwater lake) and one of its satellite lakes (Donghu) in the Yangtze floodplain, to evaluate the long-term influence of hydrological isolation on algal community composition and HABs. The results showed that pigment concentrations and the ratio of canthaxanthin/diatoxanthin (which reflects the relative abundance of cyanobacteria to diatoms) increased after the 1910s in Donghu Lake, when it was separated from Dongting Lake due to siltation. In contrast, significant increases in pigments started from the 1980s in Dongting Lake. Variance partitioning analysis revealed that the combined influence of hydrology, temperature and anthropogenic pollutants explained the largest proportion of variance (33.4%) in the pigment assemblages in Donghu Lake, followed by the joint effects of anthropogeny pollutants and hydrology (23.6%) and the sole effects of anthropogenic pollutants (14.9%) and hydrology (11.2%). In Dongting Lake, anthropogenic pollutants explained 24.5% of the variance in pigment assemblages solely, followed by the additive effects of anthropogenic pollutants and temperature (17.8%). These long-term analyses therefore demonstrate that, in combination with anthropogenic pollutants and warming, hydrological isolation from the main channel may stimulate algal production and the prevalence of cyanobacteria, whereas free hydrological connection with the Yangtze main channel seems to alleviate such HABs in these Yangtze floodplain lakes.