Since its introduction in South America around 1990, the freshwater Asian mussel Limnoperna fortunei has been shown to strongly interact with several components of the local biota. However, investigation of its ecosystem-wide effects was hindered by (1) difficulties associated with evaluation of its densities over large spatial scales and (2) scarcity of preinvasion environmental data. The present survey overcomes these shortcomings and addresses the question whether Limnoperna's impact on the ecosystem-wide scale is measurable and significant. On the basis of divercollected bottom samples, we estimated the overall density of this mussel in a reservoir (Embalse de Río Tercero, Argentina), where Limnoperna is present since 1998 and analyzed changes in several water-column properties before and after the invasion. The 47 km 2 reservoir hosts around 45 billion mussels; at these densities, a volume equivalent to that of this water body can potentially be filtered by the bivalves every 2-3 days. Data collected regularly since 1996 indicate that after the invasion water transparency increased, and suspended matter, chlorophyll a, and primary production decreased significantly, with strong changes occurring in the area with highest mussel densities. Our results indicate that the ecosystem-wide impacts of Limnoperna are generally comparable to those described in Europe and North America for another invasive musselDreissena polymorpha. However, given Limnoperna's wider tolerance limits, its influence on newly invaded water bodies, potentially including Europe and North America, will probably be stronger.

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Dissolved nutrient concentrations were rapidly reduced in slow moving waters by the periphyton growing in shallow experimental streams. Nutrients added after an initial decrease were removed at a faster rate by the well developed periphyton community. Biomass of 36.2 g D W .~-~, 23.7 g AFDW.~-' 2nd 100 mg ~h l . m -~ grew in a high phosphorous loaded stream, while 3 1.7 g D W .~-~, 16.6 g A F D W .~-~ and 42 mg ~h l . m -~ were determined in a low loaded stream. The two different nutrient concentrations also generated quite different qualitatively biotic communities £?om identical origin. Stigeoclonium tenue dominated in both systems, but Cyclotella spp., Epithemia spp., and Synedra spp. occurred in different proportions. Nutrient uptakes of 83 mg ~. m -~ .d-' and 203 mg ~. m -~. d -' were calculated based on concentration reduction. Light, temperature, and pH levels in the semi-natural greenhouse environment suggested little effect upon the periphyton community uptake of nutrients.