Mangueira Bay is located south of Patos Lagoon. It receives poorly treated official and clandestine household and industrial effluents from the city of Rio Grande. This study analyzes parameters that characterize the water quality of Mangueira Bay at various sites influenced by effluent discharge from different activities carried out in the surrounding area. These data are also useful for water-quality models. Water was sampled under various salinity conditions in October 2002 (spring), June 2003 (winter), December 2003 (summer) and April 2004 (fall). Because sewage discharge is relatively constant, water contamination is highly dependent on the association between industrial activity and hydrodynamics. The simultaneous combination of nitrogenous and phosphatic nutrient inputs causes serious eutrophication problems and an excessive increase in vegetal biomass in Mangueira Bay. An assessment of bacteria, greases and oils indicates that the water contamination near the city of Rio Grande comes not only from domestic sources but also from wastewater from fish plants and a local petroleum refinery. In order for Mangueira Bay to meet the requirements based on its water classification under federal law and enforced by environmental agencies, the discharge of clandestine domestic and industrial effluents with no or insufficient treatment must be avoided because the survival of artisanal fishing and natural species depends on the proper preservation of this complex aquatic environment.