Reduction of land based fish-farming impact by phytotreatment pond system in a marginal lagoon area

Abstract

Intensive land-based fish farming facilities discharged wastewaters into an inlet of the Orbetello lagoon (Southern Tuscany, Italian West Coast), producing large blooms of the green macroalga Ulva rigida C. Ag., accompanied by dystrophic crises. Enclosing a part of this critical inlet resulted in a new pond for phytotreatment of the fish-farming effluents. Between June 1998 and May 1999 a trial study was carried out, during the daylight, in order to: a) define the changes in the wastewater physico-chemical parameters after discharge through the phytotreatment, b) quantify the removal of nitrogen and phosphorus, c) estimate the efficiency of U. rigida biomass produced within the pond in reducing wastewater nutrient loads. During the test period the water physico-chemical parameters were recorded; water dissolved nitrogen (N) and phosphorus (P), Ulva thalli total N and P (UTN, UTP) and macroalga biomass were determined. The results indicated that: 1) the phytotreatment system affected greatly pH and DO values and it reduced thermal impact on the lagoon; 2) the wastewater inorganic nutrient loads were reduced significantly while dissolved organic components (DON and DOP) were not meaningfully lowered; 3) the diurnal nutrients removal was not directly due to U. rigida uptake because the biomass produced annually stored only 1.63% and 0.91% of the dissolved inorganic N (DIN) and P (SRP) inflow values, respectively. Inefficiency of phytotreatment action was probably due to the drop of U. rigida nutrient uptake owing to high macroalgae density inside the pond. Therefore, other processes must be taken into account to explain N and P abatements, such as ammonia volatilization and nitrification/denitrification for N, and the precipitation of insoluble compounds for P.