Clearance of bacteria-sized particles by natural populations of nanoplankton in the Chesapeake Bay outflow plume

The uptake of bacteria-sized particles by non-pigmented nanoplankton was measured in late winter (February) and spring (June) in the Chesapeake Bay outflow plume. Small flagellates (11 to 35 !1m3 average cell volume) numerically dominated the grazer community in both seasons, although experiments in June showed that ciliates could also b e important bacterivores. Rates of ingestion ranged from 1 8 to 25.0 bacteria flagellate-' h-' and were significantly correlated with bacterial abundance. Specific clearance rates (body volumes swept clear per hour) of these natural populat~ons of flagellates were similar to those of forms which have been studied in culture. Flagellate community clearance rates were similar to bacterial growth rates in February. In June, however, grazing was much lower than growth, and flagellates removed, on average, only 23 % of dally bacterial production. Reasons for this shortfall possibly include other sources of bacterial mortality, discrimination by grazers against the particles, the omission from the grazing measurement of larger, rarer bacterivores (e.g. ciliates), and artifacts resulting from particle egestion.