In order to evaluate the effects of the golden mussel Limnoperna fortunei on phytoplankton density and composition and nutrient recycling we conducted a 24 h filtration experiment in Rio Tercero Reservoir (Argentina) using four 400 L mesocosms, two of them stocked with 1700-1800 adult mussels each, and two controls (without mussels). Nutrient concentrations and phytoplankton composition and density were evaluated at 0, 3, 6, 12, and 24 h. Estimated filtration rates were 1.48-3.14 mL mg DW -1 h -1 . Grazing pressure by the mussel was not associated with algal taxonomy or cell size. After 24 h, L. fortunei removed 84% of the particulate nitrogen, and 49% of the particulate phosphorus. Nutrient regeneration was very significant as well: ammonium was produced at a rate of 3 μM NH3g DW -1 h -1 , whereas production of phosphates was 0.42 μM PO4g DW -1 h -1 . It is concluded that the impact of L. fortunei on phytoplankton and nutrient cycling can be as significant as that reported for another invasive bivalve - the zebra mussel Dreissena polymorpha in Europe and North America, but the overall effect of this impact on the biota may differ strongly under different environmental settings.