ABSTRACT: Experiments to measure phytoplankton growth rates and microzooplankton grazing rates, both with and without additions of nutrients, were conducted over a 5 yr span to determine the relative roles of physical factors, nutrients, and microzooplanktongrazing in regulating phytoplankton pigment biomass in the Rhode River estuary (Maryland, USA). Microzooplankton grazing rate was overall only weakly correlated with phytoplankton growth rate, although fluctuations in the rates appeared to parallel oneanother for restricted periods. Grazing was about 73% of growth rate at ambient nutrients during 1995, a year in which the two were correlated. In winter and early spring, phytoplankton growth rates were low and unaffected by nutrient additions. In latespring, stimulation of growth by nutrient addition was more common, but microzooplankton grazing was partially effective in relieving nutrient limitation. Growth enhancement by nutrient addition was most common in summer, and microzooplankton grazing wasless effective at relieving nutrient limitation. P or N addition stimulated phytoplankton growth in late spring and autumn, but only N addition stimulated growth in summer. Temperature constrained upper bounds of observed growth rates throughout the year,but many observed growth rates were far below the temperature-dependent maximum, even with nutrients added. Considered as a single group, temperature-normalized growth rates were poorly related to in situ daily photon flux; but the rates appearedto fall into species-dependent higher and lower groups. The higher growth rates were statistically associated with the diatoms Thalassiosira pseudonana, Thalassiosira sp., and Nitzschia longissima. The lower growth rates were associated withthe dinoflagellates Gyrodinium uncatenum and Prorocentrum minimum. Results suggest that interpretation of bioassay experiments requires consideration of both grazing effects and species composition of the phytoplankton community. Nutrientstimulation of some slow growing assemblages may be difficult to detect in 1 d incubation experiments.