Grazing loss-rates in pico- and nanoplankton in the Faroe-Shetland Channel and their different relations with prey density

In a number of dilution experiments performed in different surface water masses of the Faroe-Shetland Channel loss-rates due to grazing were determined for heterotrophic bacteria and three size categories of small phytoplankton between 1 and 10 μm. The prey abundance and loss-rates varied considerably among stations; grazing on picoplankton was strongly correlated with prey abundance while no correlation between grazing and abundance was detectable for the larger prey. Grazing control, assumed to be exerted by unicellular grazers in general, seems thus to be restricted to the smaller microbial components of the food web. The absence of density-dependent grazing of nanophytoplankton (2–5 μm) and larger algae suggests that the microzooplankton, dominated by ciliates, are not as tightly linked to their prey populations as heterotrophic nanoflagellates.