Responses of phytoplankton functional groups to the hydrologic regime in the Daning River, a tributary of Three Gorges Reservoir, China.

Daning River is a deep tributary of Three Gorges Reservoir (TGR) in China, with water level fluctuations of 30 m annually. It was assumed that the hydrologic regime would be the main driving force in the self-assembling of the phytoplankton community in the river. In order to test this hypothesis, limnological study was performed monthly in the estuary, midstream and upstream of this tributary from May 2008 to April 2009. We identified 17 phytoplankton functional groups among 63 genera. These phytoplankton functional groups varied significantly, both seasonally and longitudinally. During the flood season (March-September), low water level and high inflows caused a marked increase in the turbidity, especially in the estuary and upstream, allowing functional group MP (the meroplanktonic diatoms) to dominate the phytoplankton community. Meanwhile, constant water level and high temperature led to the stability and thermal stratification in the midstream. These conditions resulted in a high phytoplankton biomass and the dominance of phytoplankton functional groups Y (Cryptomonas spp.) and Lo (motile Peridiniopsis niei and Peridinium) that were adapted to water stratification. During the dry season (October-February), although the inflows were low and water retention time was long, the thermal stratification was disrupted by the disturbance due to the impoundment of TGR, and the water column was deeply mixed. The phytoplankton biomass reduced and functional groups changed: group Lo declined, and group C (small diatom Cyclotella meneghiniana) increased in the estuary and midstream. Group Y replaced group MP to dominate the phytoplankton community in the upstream with the water becoming clear and stagnant. It could be deduced that the dynamics of phytoplankton in the Daning River were mainly influenced by hydrologic regime.