Synechococcus, a unicellular cyanobacterium of about one micron in size, is one of the most prolific and abundant primary producers worldwide and, hence, has an important role in the phytoplankton community. This study sought to determine 1) the distribution and abundance of Synechococcus in the eastern San Juan Archipelago; 2) the environmental variables related most closely to abundance; and 3) the key grazers of Synechococcus in this ecosystem. Two stations were chosen, East Sound near Orcas Island, WA and Rosario Strait near Lopez Pass, for their differing hydrographic conditions. Sampling was conducted from June to September 2012. Water samples were taken at three depths at both stations twice a month June through August, and then approximately every three days for three weeks in September. A CTD (Conductivity, Temperature, and Depth) was lowered at each station to obtain environmental data from the water column. Water samples were used for nutrient analysis, size-fractionated chlorophyll a analysis, and for the enumeration of Synechococcus and the protist grazer community. Synechococcus abundance rose as high as 1.5 x 104 cells ml-1 at both East Sound and Rosario Strait in August. Synechococcus abundance and depth distribution were nearly the same at both stations despite the well-mixed environment at Rosario and the more frequently stratified environment at East Sound. Both stations were abundant in nitrate+nitrite and phosphate throughout the sampling period. However, chlorophyll a concentrations were unusually low July through August, a season that usually exhibits variable and episodically high concentrations. Of all the environmental variables analyzed, only salinity was correlated with Synechococcus abundance at both stations, and that correlation was negative. The importance of salinity as a predictor of abundance may be due to a physiological effect of fresher water that allows for increased biomass production, or simply to the dominant effect of salinity on water column stratification, which may provide a preferable growth environment for Synechococcus. Ciliates, heterotrophic nanoflagellates, and dinoflagellates were observed with ingested Synechococcus. Surprisingly, nanoflagellates were rarely observed with ingested cells. Dinoflagellates seemed to be the key grazers of Synechococcus in the eastern San Juan Archipelago, but there was no clear temporal pattern to the level of Synechococcus ingestion by any of the aforementioned grazers
