Effects of fish farm effluents on the periphyton of an Andean stream

In this study we analyse the structure and composition of the periphyton, u pstream and downstream of a fish farm located at an Andean stream. The effl uents discharged large quantities of phosphorus and organic matter from the feed remnants and the faeces. The input of organic matter caused a reducti on in algal biomass and a change in the relative abundance of some species. The ratio between chlorophyll a and total organic matter concentrations in dicated that the periphyton consisted of a high photosynthetic fraction at upstream sites, while downstream heterotrophic conditions were favoured. Th e community, which was composed mainly of diatoms, was dominated by Achannt hes minutissima at every site, except at: the mouth of an effluent channel of the fish farm, where Nitzschia supralitorea dominated. When there are hi gh organic matter inputs, the prostrate motile form acquires advantages ove r the rosette, adnate and filamentous forms, with a consequent change in co mmunity structure.