Aquatic macrophyte distribution in relation to water and sediment conditions in the Itaipu Reservoir, Brazil

Aquatic macrophyte community distribution along the eastern shoreline of th e Itaipu Reservoir (one of the South America's largest impoundments) is des cribed in relation to limnological and sedimentological factors. The centra l body of the reservoir is mesotrophic, while the arms (flooded influent ri ver valleys) along the eastern shore may be oligo-mesotrophic to eutrophic, depending on time of year and sub-catchment characteristics. Macrophyte co mmunity composition and species cover were surveyed at 30 sites in four arm s, in relation to sediment total P and organic matter; underwater light reg ime; and water total P and Kjeldahl N concentration, alkalinity, conductivi ty, depth and pH. Seventeen euhydrophyte and six emergent macrophyte specie s were recorded. Large stands of Egeria najas dominated the euhydrophyte ve getation, together with free-floating weed species (Pistia stratiotes Linn. , Salvinia auriculata Aublet and Eichhornia crassipes (Mart.) Solms.). Cano nical Correspondence Analysis of the data showed that two sets of variables were important predictors of aquatic macrophyte community structure. Float ing macrophyte assemblage was closely related to concentration of nutrients in both water and sediment, while light penetration was the strongest pred ictor of submerged species occurrence. Although a large number of potential nuisance species were present, dense growths were restricted to shallower areas of the Itaipu Reservoir, causing localised problems. The possibility of increasing interference by these plants with fisheries, recreational use , transport and hydroelectricity generation suggests a need for continued m onitoring of weed distribution and abundance, and investigation of appropri ate management measures.