Environmental heterogeneity and productivity drive the assemblage of benthic diatom: a case study of the Three Gorges Reservoir

Dam construction has a critical effect on river ecosystems in the world, resulting in a rapid decline in biodiversity and ecosystem health. However, the effect of dam construction on the assemblage pattern of benthic diatoms remains unclear. The benthic diatoms assemblages in the tributary backwater area of the Three Gorges Reservoir (TGR) were investigated under low water level (LWL) and high water level (HWL) conditions. Results show that Discostella stelligera, Nitzschia palea, and Craticula subminuscula were dominant species in LWL, while Achnanthidium minutissimum and Nitzschia dissipata were dominant in HWL. Furthermore, environmental variables, productivity parameters, diversity indices, and community similarity were found significantly different between LWL and HWL periods. The Mantel test indicated that both environmental and spatial factors had significant effects on diatom communities in backwater area, while productivity was also a key driving force in LWL. The variation partitioning analysis (VPA) further demonstrated that environment factors could explain the largest variance of diatom assemblages in HWL (9%) and LWL (11%), followed by spatial factors. The relative importance of productivity to diatom community was significantly enhanced in LWL, but only a slight effect was found in HWL. These results indicate that the diatom assemblages were shaped by both environmental filtering and spatial factors. The relative importance of spatial factors depended on the degree of productivity. Therefore, the homogenization of diatom communities due to environmental disturbance, e.g. eutrophication and dam building, is the underlying mechanism in assembling the benthic diatoms.