A study of an intertidal mytilid community in the Southwestern Atlantic shore was carried out in order to characterize the mytilid bed’s response to an intertidal sewage discharge. Four Stations were sampled around the outfall, with a Control site. Total Weight and Organic Carbon content of sediments among mussels, as well as water quality variables were measured. All macrofauna were counted, and species number, Shannon’s diversity index and evenness were calculated. The mytilid Brachidontes rodriguezi increases it density inversely to distance to the effluent. The polychaetes Capitella capitata and Boccardia polybranchia characterized the most impacted Stations, while crustaceans (Corophium insidiosum, Jassa falcata and Caprella sp.) were dominant at medial distances. Both environmental and biological data show significative differences between Stations. The most impacted Station has the lowest species number, but the highest values of both diversity and evenness; while control Station has the lowest values of diversity and evenness. The study reject the use of univariate indices to assess organic pollution in this community, being the presence of indicator species with mytilid proportion the best tool for sewage impact assessment.
