A manipulative mesocosm experiment in Danish coastal waters tested the effect on plankton biodiversity and function of adding nitrate, phosphate and glucose. A comprehensive set of measurements was made over a 6 d period; these included phytoplankton biomass and production in 3 size fractions (>10, 10-2 and < 2 µm), bacterial biomass and production, nitrate and ammonium uptake, and pigment taxonomy. Addition of nitrate and phosphate resulted in increases of biomass and production of all size fractions of phytoplankton. Inorganic nutrients alone had only a minor effect on bacterial abundance and production, with slight increases relative to the control. The largest changes occurred in mesocosms to which glucose was added in excess with nitrate and phosphate. Pigment composition indicated little change in phytoplankton assemblage composition in any treatment. A large increase in bacterial activity in the presence of added glucose had a negative effect on the phytoplankton assemblage and resulted in a decline in phytoplankton biomass. Data on nutrient uptake and size-fractionated carbon fixation suggest that the mechanism of this phytoplankton suppression was the ability of heterotrophic bacteria to out-compete for available inorganic nutrients, resulting in nutrient limitation of the phytoplankton assemblage. KEY WORDS: Phytoplankton/bacterial competition · Nutrient uptake · Pigment taxonomy · MesocosmResale or republication not permitted without written consent of the publisher