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Growth rates of microphagous ciliates (forms which feed primarily on picoplankton-sized prey) were estimated, along with rates of their consumption by copepods, in shipboard experiments conducted in the mesohaline portion of Chesapeake Bay, USA, under contrasting water column conditions in April, June, and August 1987. Estimates were based on temporal changes in cell densities in size-fractionated water samples incubated under in situ conditions. In April, at low temperatures (7 to 10°C) and with oxygen present throughout the water column, similar generation times of ca. 1 to 1.5 d were estimated for surface and deep water (24 m) ciliate populations. In June, water was anoxic below 12 m and a distinct anoxic microphage community grew at about twice the rate of the surface community with generation times of ca. 7 and 14 h, respectively. In August, bottom water was again anoxic, but the same Strobilidium sp. dominated both surface and deep waters with low or no growth apparent in anoxic waters and a generation time of ca. 8 h in surface waters. Copepod (primarily Acartia tonsa Dana nauplii) clearance rates for microphagous ciliates in surface waters were 0.11, 0.56, and 0.53 ml h copepod for April, June and August, respectively. Calculation of removal rates, based on average densities, indicated that from 34 to 200% of surface waters were cleared d of microphagous ciliates by copepods. [ABSTRACT FROM AUTHOR]