Importance of bacterivory by pigmented and heterotrophic nanoflagellates during the warm season in a subtropical western Pacific coastal ecosystem

We investigated temporal variations in the effects of bacterivory by different sizes of heterotrophic nanoflagellates (HNF) and pigmented nanoflagellates (PNF) during a warm period (May to September) in oligotrophic coastal waters of the subtropical western Pacific. Short-term experiments with fluorescently labeled bacteria (FLB) demonstrated ingestion rates of 0.3 to 5.8 bacteria HNF-1 h(-1) by HNF in the size range 3-6 mu m-rates that were higher than observed for other sizes of HNF. Rates of ingestion by PNF ranged between 0.9 and 15.5 cells PNF-1 h(-1), and, as for HNF, were greatest for PNF in the 3-6 mu m size group. Nanoflagellates of size < 6 mu m removed about 98% of the total amount of bacteria consumed. The 3-6 mu m PNF, 2-3 mu m HNF, and 3-6 mu m HNF were major consumers in the nanoflagellate community and were responsible for an average of 52, 28 and 16% of the total consumption of bacteria, respectively. The smallest PNF (2-3 mu m) consumed only about 2% of the total and were considered to be primarily autotrophic. Despite ingestion rates in the range of those reported elsewhere, the low abundance of nanoflagellates observed resulted in relatively low grazing impacts (< 10% of bacterial standing stock). We found a significant negative correlation between PO4 concentrations and ingestion rates of the 3-6 mu m PNF, suggesting that the PNF ingestion rate increased under nutrient-deficient conditions.