Copper is commonly used as an algicide and plant herbicide in the aquatic environment. Despite of its frequent application in rivers, ponds and lakes, few studies have been performed at the ecosystem level on primary producers' communities. A long-term lotic mesocosms study was carried on, in 20 m long channels, under environmentally realistic concentrations of copper (0, 5, 25 and 75 mu g L-1) delivered continuously for 18 months. Results showed significant effects at the 25 and 75 mu g, L-1 copper treatment levels on the phytoplankton, periphyton and macrophyte community structure. Effects on biomass and abundance were dissimilar depending on the studied community. Phytoplankton showed no abundance variation with treatment, whereas periphyton had higher biomass and macrophytes had lower coverage at 25 and 75 mu g L-(1). Taxa richness was significantly affected for all primary producers communities at 25 and 75 mu g L-1. Some taxa seemed to be sensitive to copper, i.e. Cocconeis spp., Epithemia sp., Gomphonema spp., Lemna minor, Nasturtium officinale and filamentous macroalgae. Other taxa appeared to be tolerant or facilitated by indirect effects, i.e. Leptolyngbya sp., Mougeotia sp., Nitzschia palea, Pseudanabaena sp. and Ulothrix sp. and therefore increased in the 25 and 75 mu g L-1 treatments. Important indirect factors influenced the community responses, such as trophic interactions like decrease of grazing pressure, competition for light and nutrients and temperature favouring blue-green algae development. After 18 months of copper exposure, the no observed effect concentration at community level (NOECcommunity) for primary producers was set up at 5 or 4 mu g L-1, if we consider the average effective concentration, with a lowest observed effect concentration (LOECcommunity) at 25 or 20 mu g L-1 (AEC).