Spatial variability overwhelms seasonal patterns in bacterioplankton communities across a river to ocean gradient

Few studies of microbial biogeography address variability across both multiple habitats and multiple seasons. Here we examine the spatial and temporal variability of bacterioplankton community composition of the Columbia River coastal margin using 16S rRNA gene tag pyrosequencing of 300 water samples collected in 2007 and 2008. Communities separated into seven groups (ANOSIM, p<0.001): river, estuary, plume, epipelagic, mesopelagic, shelf bottom (depth<350 m), and slope bottom (depth>850 m). The spatial differentiation among samples was correlated with salinity (r2 =0.87) and depth (r2 =0.46). Temporal patterns were obscured by spatial variability among the coastal environments, and could only be detected within individual groups. Thus, structuring environmental factors (e.g. salinity, depth) dominate over seasonal changes in determining community composition. Seasonal variability was detected across an annual cycle in the river, estuary, and plume where communities separated into two groups, early year (April-July) and late year (August-Nov), demonstrating annual reassembly of communities over time. Determining both the spatial and temporal variability of bacterioplankton communities provides a framework for modeling these communities across environmental gradients from the river to the deep ocean.