Diatom ecological response to altered hydrological forcing of a shallow lake on the Yangtze floodplain, SE China

Abstract

ABSTRACTThe Yangtze floodplain is characterized by numerous large (surface area > 1 km2; ∼25% are > 10 km2) and shallow lakes (maximum depth mainly < 5 m). Floodplain lakes are inherently dynamic and important sites for biogeochemical cycling, biodiversity as well as human resources. Importantly, the degree of hydrological connectivity between lakes and river channels is affected by climate, geomorphic processes as well as cultural disturbance of the catchment. This study investigated the long‐term (∼200 years) interaction between a shallow floodplain lake (Taibai Lake; area 25 km2, mean depth 3·2 m) and the Yangtze River using high‐resolution multi‐proxy analysis (diatoms, geochemistry, and grain size) of a 210Pb‐dated sediment core. Diatom assemblages in Taibai Lake exhibited a strong temporal succession and were successively dominated by Aulacoseira granulata (planktonic), Gyrosigma acuminatum (periphytic), and the facultatively planktonic Nitzschia palea over the past 200 years. Eutrophication as inferred by diatom inferred‐total phosphorous (DI‐TP), however, only started around 1990 with TP concentration increasing from 65 to 95 µg L−1. Prior to 1950 DI‐TP was constant and detrended correspondence analysis axis 1 sample scores (of the diatom assemblages) were not correlated with DI‐TP concentration suggesting that there were other stressors driving changes in the diatom assemblage besides nutrients. We used sediment grain size as a proxy for hydrological connectivity; median grain size is strongly correlated with the abundance of A. granulata (R2 = 0·67). A comparison of the diatom stratigraphy with physical and geochemical proxies suggested that Taibai Lake shifted from a riverine‐influenced, turbid algae‐macrophyte mixed state to a clearwater macrophyte‐dominated state around 1950 AD, primarily due to the changes in the hydrological regime of the floodplain resulting from land reclamation and building of reservoirs. After 1990 AD, the lake ecosystem deteriorated following continuous input of nutrients from agriculture and aquaculture, shifting to a hyper‐eutrophic algae‐dominated state. These results indicate the dynamic nature of floodplain ecosystems and the complexity of multiple stressors affecting them over decadal timescales and the long‐term interaction between anthropogenic and natural forcing processes. Copyright © 2011 John Wiley & Sons, Ltd.