ABSTRACT: The short-term effects of inorganic N and P (nitrate, ammonium, phosphate) and organic C and N (glucose, amino acids) inputs, added separately as well as jointly, on phytoplankton and bacterioplankton community composition were studied in 6 microcosm experiments conducted in a eutrophic coastal embayment under contrasting hydrographic conditions. The responses of the different bacterioplankton and phytoplankton groups to the distinct nutrient inputs were highly variable among experiments, which was partially related to changes in the initial environmental conditions. Gammaproteobacteria and nanoflagellates were the most responsive groups to nutrient additions. Inorganic nutrients did not promote important changes in the microbial plankton community structure but did significantly reduce diatom diversity. In contrast, organic additions promoted changes mainly in bacterioplankton groups, whilst mixed additions provoked changes in both bacterial and phytoplankton groups. While nanoflagellates increased equally in abundance after inorganic and mixed additions (2.9-fold), dinoflagellates and diatoms increased their abundances more in the mixed treatment (2.3-fold and 2.2-fold, respectively) than in the inorganic treatment. Organic and mixed additions did not provoke changes in diatom or dinoflagellate diversity. The magnitude of response of Gammaproteobacteria largely explained changes in bulk bacterial biomass and activity, whereas changes in bulk phytoplankton biomass and primary production associated to nutrient enrichment were mostly explained by the response of diatoms and large picoeukaryotes. Our results demonstrate that the type of nitrogen inputs (inorganic and/or organic) strongly affects the microbial plankton community composition and functioning in this coastal ecosystem.