Fish assemblage changes along a trophic gradient induced by agricultural activities (Santa Lucيa, Uruguay)

The input of nutrients into rivers and other aquatic ecosystems is one of the most common causes of damage to the integrity of the ecosystem. The use of biological communities for water quality assessment is a common and effective practice. Among these biological indicators, the fish community is one of the most used in monitoring programs and ecosystems studies and so there is much knowledge about their response to the process of eutrophication. In this study we analyzed the fish composition, diversity and evenness in fourteen reaches, of low stream orders within a basin with differing land use intensity. ructure of the fish community was related with physiochemical water composition by canonical correspondence analysis, which enabled us to group the fish species according to their specific tolerance to eutrophication. Oligotrophic habitats were characterized by a higher evenness, larger individuals, a fish composition of at least 40% of sensitive species (Crenicichla scotti, Gymnogeophagus gymnogenis, Gymnogeophagus sp., Heptapterus mustelinus, Hoplias malabaricus and Rhineloricaria sp.) and less than 20% of very tolerant ones (Astyanax fasciatus, Cnesterodon decenmaculatus, Corydoras paleatus, Cyphocharax voga, Hisonotus sp., Pimelodella australis). Eutrophic reaches showed the opposite community features and a fish composition with more than 40% of very tolerant species and less than 20% of sensitive ones. Among the fourteen study reaches, four were classified as oligotrophic, six as mesotrophic and four as eutrophic.