ABSTRACT: Variability in size structure and composition of the microbial plankton community in the Ría de Vigo (NW Iberian coastal upwelling system) was studied as a function of the prevailing oceanographic conditions during 4 seasonal sampling periods (autumn, winter, spring and summer). The impact of mussel culture on this microbial plankton community was also evaluated by comparing the results obtained at a reference station (ReS) located outside the farming area with those found inside the farming area (raft station, RaS). Integrated microbial plankton biomass remained relatively constant (2.5 ± 0.4 g C m^–2) at ReS during autumn, spring and summer, when microplankton clearly dominated, accounting for 64 ± 13% of the total microbial plankton biomass. Pico- and nanoplankton were present in the microbial community all year round, with mean biomass values of 0.32 ± 0.09 and 0.42 ± 0.23 g C m^–2, respectively. These 2 fractions became more relevant during winter, when the contribution of microplankton to total microbial plankton biomass decreased (to 23 ± 9%), and a balanced trophic structure between autotrophs and heterotrophs was established. At RaS, a significantly lower biomass of microplankton (by 46 ± 32%) and nanoplankton (by 35 ± 22%) was observed compared to ReS, regardless of their trophic nature. Picoplankton biomass did not differ between sites. These results suggest that mussel farming exerts a top-down control over the microbial plankton community by consuming micro- and nanoplankton without affecting picoplankton. An excess of ammonium, probably excreted by mussels, and a lower autotrophic carbon:chlorophyll ratio at RaS suggest that mussel culture could also exert a bottom-up-like control on the phytoplankton that escape mussel consumption in farming zones.