We examined weekly changes in the bacterioplankton community over a three-year period (January 2011-December 2013) in relation to a suite of environmental variables to identify key drivers of microbial community changes over annual cycles. Although a number of variables influence microbial communities, we found that temperature is most strongly linked to the observed community- and taxa-level seasonal changes. We hypothesized that temperature alone could induce transitions between seasonally-associated microbial communities. In order to test this, we incubated seawater poised at the predicted transition point at ambient and artificially-warmed (+5 °C) conditions.