Physical and chemical properties of the water column and sediments at sea bass/sea bream farm in the middle Adriatic (Maslinova Bay)

Physical and chemical characteristics of the water column and sediments at sea bass/sea bream farm located on the middle Adriatic island of Brač were investigated during the 2001-2006 period. Measurements and samplings were performed at stations inside the farm area and at reference station, approximately 1 km distant from the farm. Investigated parameters in the water column were temperature, salinity, transparency, suspended matter concentration, oxygen saturation and nutrient concentrations. Parameters measured in the sediments were granulometric composition, redox-potential, organic matter, organic carbon and total nitrogen content, phosphorus (P) species concentration (biogenic apatite P, authigenic apatite P, detrital apatite P, P bound to iron oxides and hydroxides and organic P). Transparency in the water column at all stations followed the natural seasonal course, although constantly lower values at the farm stations relative to the reference site indicate a certain impact of fish farming on this parameter. Impact on chemical parameters was moderately expressed in the surface and middle layer of the water column through the deficiency of oxygen and increased concentrations of dissolved inorganic nitrogen, orthophosphate and urea due to fish excretion and degradation of their metabolic products. Sediment inorganic P concentrations were enhanced under the cages compared to the reference site, while organic P was mainly within the range of “natural” middle Adriatic concentrations. Sequential analysis of inorganic P species showed an increased portion of biogenic apatite P and decreased detrital apatite P at the farm stations with respect to the inorganic P pool at the reference station as a direct consequence of fish farming. Sediment redox-potential, granulometric composition, organic matter content, carbonates as well as organic carbon and total nitrogen content reflected natural sediment characteristics rather than fish farm influence.