SPATIO-TEMPORAL PATTERNS OF DIATOM COMMUNITIES IN FRONT OF THE THREE GORGES DAM: EVIDENCE OF THE EFFECTS OF HYDRODYNAMICS

The Three Gorges Reservoir (TGR) has a peculiar hydrological regime in China, with water level fluctuations of 30 m annually. In order to investigate the effects of hydrodynamic conditions on the diatom communities, limnological investigations were conducted monthly in front of the Three Gorges Dam (TGD) in 2013. We identified 18 diatom genera from two classes. The spatio-temporal patterns of diatoms showed an apparent heterogeneity. During the flood season (May October), high flows, high velocity, low water level, and high turbidity in the main stream indicated strong mixing and made Melosira sp. the dominant species, while the lower velocity in the bay near the TGD indicated weak mixing and made Cyclotella sp., Melosira sp., and Synedra sp. the dominant species. Meanwhile, the strong mixing and high turbidity resulted in a low diatom abundance. During the dry season (the other months), the water level was stable, water flow and velocity were low in both the bay and the main stream, which led to weak mixing and low turbidity, and Cyclotella sp. dominated in the front region of the TGD. Spring was the transitional season and water level decreased gradually, but in spite of the weak mixing and low turbidity conditions, the diatom abundance increased along with the increase of water temperature and water flow. It could be concluded that the spatio-temporal heterogeneity determined by hydrodynamics was a characteristic of the diatom communities' dynamics in front of the TGD