We took advantage of the spatial architecture of water masses distribution in the East China Sea (ECS) to examine how the relative importance of environmental filtering and dispersal processes shapes bacterial metacommunity across spatial scales. The ECS consists of two major water masses with distinct hydrographic characteristics from inshore (river runoff) to offshore (Kuroshio). Additionally, the strength of river runoff varies temporally in the inshore area. To examine the effects of spatial scale on assembly processes, we collected bacterial metacommunity in summer (during 2010-2012) and winter (in 2011), and analyzed the variance partitioning for different spatial scales. Our results showed that dispersal effect always dominated at the largest scale (across inshore and offshore) where multiple water masses existed. While, environmental filtering was important when a relatively small scale (inshore region) was considered. Specifically in the inshore scale, increased river runoff critically strengthened dispersal capacity (i.e. mass effect), which reduced the influences of environmental filtering. Our findings suggest that the scale dependence of dominant processes varied depending on the spatial structure of water masses and advection.