Diel patterns of grazing by pigmented nanoflagellates on Synechococcus spp. in the coastal ecosystem of subtropical western Pacific

Abstract

Natural populations of planktonic Synechococcus spp. exhibit diel variations in abundance and frequency of dividing cells (FDC) during the warm seasons (>25°C) in coastal water of the western subtropical Pacific ocean. We hypothesized that differences in grazing rates during the day/dark cycle were a major cause of the observed diel variations in Synechococcus spp. abundance. We used fluorescently labeled particles (FLP) as tracers of feeding on Synechococcus spp. in June and August 2007. Our results showed that FDC of Synechococcus spp. were highest at dusk (50%) and lowest (<10%) between midnight and early morning. Synechococcus spp. had three abundance phases, accrual (I), peak (II), and diminished (III). Moreover, FDC values gradually increased in phase I, declined to the lowest values during phase II, and remained at low levels throughout phase III. Our results strongly indicated that pigmented nanoflagellate (PNF) grazing was the underlying biological factor regulating diel variations in Synechococcus spp. abundance. And, the rate of PNF ingestion peaked during the night on smaller non-dividing cells following a peak of Synechococcus spp. abundance in phase II. These findings provide evidence that diel variations in ingestion rates are affected by non-dividing cells of Synechococcus spp. and imply that the impacts of PNF grazing on Synechococcus spp. is based on food selectivity by size.