Active ingestion of fluorescently labeled bacteria by mesopelagic heterotrophic nanoflagellates in the East Sea, Korea

To determine whether heterotrophic nanoflagellates (HNF) in the mesopelagial actively ingest bacteria and whether HNF grazing rates comprise a significant fraction of the mesopelagic bacterial production, we measured uptake rates of fluorescently labeled bacteria (FLB) by mesopelagic HNF and bacterial production in the East Sea, 5 times from April 1996 to September 1997. Although bacterial abundance and production were much lower in the mesopelagic than in the epipelagic zone, HNF clearance rates and volume-specific HNF clearance rates in both layers were usually within the same range, 1 to 11 nl HNF -1 h -1 and 0.1 to 1 10 6 h -1 , respectively. Two samples in the mesopelagial showed somewhat increased clearance rates (14 to 16 nl HNF -1 h -1 ) and volumespecific HNF clearance rates (up to 2.6 10 6 h -1 ). Fractions of actively grazing HNF were not statistically different in the 2 layers (epipelagial: 33 to 98%, mesopelagial: 22 to 79%) in most cruises. HNF grazing rates in the mesopelagial (0.1 to 1.1 10 6 cells l -1 h -1 ) were either major fractions of or often greater than bacterial production, suggesting that HNF grazing on bacteria was the major fate of mesopelagic bacterial production at the study site.