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Spatial variation of communities composition (metacommunities) results from multiple assembly mechanisms, including environmental filtering and dispersal; however, whether and why the relative importance of the assembly mechanisms in shaping bacterial metacommunity changes through time in marine pelagic systems remains poorly studied. Here, we applied the elements of metacommunity structure framework and the variation partitioning framework to examine whether temporal variation of hydrographic conditions influences bacterioplankton metacommunity dynamics in the southern East China Sea (ECS). The spatiotemporal variation of bacterial communities composition was revealed using 454 pyrosequencing of 16S rDNA. In addition to the whole bacterial community, we analyzed four dominant taxonomic groups (Cyanobacteria, Alphaproteobacteria, Gammaproteobacteria, and Actinobacteria) separately. Our analyses indicate that, considering the whole community level, the determinism of metacommunity structure varied among seasons. When the degree of connectivity was low (December), the metacommunity exhibited random distribution and was explained mainly by the environmental component. However, Clementsian metacommunity was found at intermediate connectivity (May), during which the environmental and spatial predictors were both significant. When connectivity was high (August), a random distribution pattern was found and no significant effect of environmental filtering or dispersal limitation was detected. Nevertheless, when considering different taxonomic groups, the differences in metacommunity dynamics among groups were found. Our results suggest that the driving forces of metacommunity dynamics varied depending on hydrography, as the degrees of environmental heterogeneity and connectivity among habitat patches were determined by circulation pattern. Moreover, mechanisms varied among different taxonomic groups, suggesting that differential dispersal capacity among taxonomic groups should be integrated into community assembly studies. [ABSTRACT FROM AUTHOR]