Abstract:Although the key grazers on Synechococcus and other planktonic marine bacteria are generally thought to be nanoflagellates (both non-pigmented and pigmented) as well as ciliates, we previously found in our western subtropical Pacific coastal study site that ciliates exerted almost no grazing pressure. In this study, we used fluorescently labeled particles (FLP) as Synechococcus-sized mimics to examine the contribution of pigmented (may include autotrophic and mixotrophic spp.) versus non-pigmented (heterotrophic) nanoflagellate grazing to Synechococcus morality. During the warm season from June to September, > 50% of the nanoflagellate population was pigmented (1.8–2 × 103 versus 1.2–1.6 × 103 cells mL−1). Consumption, or clearance rates per pigmented nanoflagellate, varied considerably (0.50–46.90 nL cell−1 h−1), with the highest rates in June. Raw data showed pigmented nanoflagellate grazing to account for 2–94% (mean 43%) of Synechococcus production from May to October. Pigmented nanoflagellates consumed 12.5-fold more Synechococcus than did ciliates. This study provides the first evidence that pigmented nanoflagellates are key grazers of Synechococcus populations in subtropical western Pacific coastal waters.