Water Quality Trends in a Coastal Lagoon Impacted by Non-point Source Pollution after Implementation of Protective Measures

Abstract

Water quality data from two different monitoring periods are used to evaluate the trophic state and effectiveness of various protective measures on the restoration of a eutrophic, coastal Mediterranean lagoon. Main protective measures included elimination of municipal/industrial raw wastewater discharges in the rivers outflowing to the lagoon, sediment/erosion control practices in the lagoon’s drainage basin (i.e., construction of sediment/debris dams and grade control structures, reforestation and ban on livestock grazing), and reduction of fertilizer application quantities as a result of changes in crops. Water quality data include, among others, chlorophyll a, dissolved oxygen and nutrient concentrations, various physicochemical parameters, and transparency, measured during two monitoring periods, i.e., before (1983–84) and after (1998–99) implementation of protective measures. Rainfall depth for the two time periods was also available. Empirical equations were developed, from statistical analyses of the data, relating the water quality parameters during the two monitoring periods. These models help identify water quality trends. Based on the analyses, it seems that measures were effective in reducing sediments transported into the lagoon. However, the lagoon remains eutrophic to hypereutrophic, mostly due to phosphorus released in the water column from bottom sediments. Therefore, future restoration efforts should be directed towards the management of bottom sediments.