The aim of this paper was to hydrochemically characterize Ibiraquera Lagoon (SC) and verify the changes in water column related to the opening dynamics of its mouth. The monthly monitoring of five physical-chemical properties and dissolved inorganic nutrients from march 2003 to april 2004 demonstrated the mixohaline nature of this lagoon, which presents well oxygenated waters, low turbidity and low inorganic dissolved nutrients concentrations. This lagoon is isolated from coastal waters influence in the most part of the year since its sand bar opens only under severe rainy events or by human intervention. On these situations changes in the water level and physical-chemical properties can be observed along all this water body. Among the monitored variables, the most significant alterations were observed in salinity, silica and dissolved oxygen. An increase in salinity and silica values was observed after the sand bar opening in all lagoonal sectors. In contrast, dissolved oxygen concentrations decreased. These changes were associated to the mixture of brackish and marine waters and to nutrient releases from surface bottom sediments to the water column as a consequence of the intensification of circulation processes.