Ria Formosa is the major harvesting area of bivalves in Portugal (90%). Nevertheless, the lagoon suffers stress from UWWTPs from the main cities. After 8 months starting the operation of Faro-Olhão and the decommissioned of Olhão-Poente UWWTPs (June 2019), this thesis aimed to determine the footprint of the effluents discharge on Faro-Olhão and Olhão-Poente study areas and factors influencing this impact in terms of salinity, dissolved oxygen, nutrients, chlorophyll a and fecal contamination. The changes of the water quality were assessed in both low and high tide, during neap and spring tides. The result confirmed the important tidal renewal to the contaminants decrease along the distance increase showing the maximum impact around 500-750m from the discharging point, at Faro-Olhão, as expressed by the TRIX index that extended to 1250m, particularly in the eastern sector. Regardless that the clam bed areas apparently showed no relevant impact from the effluents discharge. Concerning the Olhão-Poente study area the water quality after the decommissioning was high as expressed by the TRIX index for any tidal condition as well as for E.coli concentration. Seasonally the worst period revealed in the winter, while in summer E.coli and nutrients were reduced by phytoplankton consumption in both areas.
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