Nutrient pollution in Pagassitikos Gulf arises mainly from the disposal of Volos untreated domestic wastewater inside Volos Bay, and also the agricultural run-off from the surrounding the Gulf cultivated areas. Volos Bay's limited circulation provides the conditions for the Bay's eutrophication by the discharge of untreated wastewater. In July 1987 this surface discharge was replaced by a new deep outfall system. The latter was located outside Volos Bay discharging primary treated effluents offshore at a depth of 55 m. To monitor any ecological changes, pre-operational (1986-1987) and operational (1988-1989) oceanographic data have been compared. The results showed a decrease in nutrient and phytoplankton concentrations between pre-operational and operational periods. Furthermore, in the vicinity of the present discharge a significant increase in the abundance of benthic communities occurred, due presumably to the continuous supply of food, adequate water circulation and high dissolved oxygen levels.