Distribution of phosphorus species in below-cage sediments at the tuna farms in the middle Adriatic Sea (Croatia)

Abstract

        The phosphorus species in sediments from two tuna farms located in semi-enclosed bays at island of Brač, (Croatia), were examined. Investigations were performed at two sites inside the breeding area on each farm and at control site in adjacent bay with no tuna farm. Measurements included concentrations of organic phosphorus (OP) and inorganic P species: P in biogenic apatite (P-FD), P adsorbed onto iron-oxy/hydroxides (P-Fe), P in authigenic apatite (P-AUT) and P in detrital apatite (P-DET). Regarding the previous analyses of phosphorus in middle Adriatic sediment, additional extraction step in determination of OP was applied. Granulometric composition, organic matter (OM), organic carbon (C-ORG), total nitrogen (N-TOT), carbonate content (CA) and sediment redox potential were also determined to investigate interactions between P species and specified physico-chemical characteristics of sediment.
         Results proved total phosphorus (TP) sediment concentrations below the tuna cages enhanced up to 6 times in relation to the sites inside the farm area and to the control site. Sequential analyses of inorganic and organic P fractions indicated elevated concentrations of P-FD, P-AUT and P-Fe in below-cage sediments compared to other investigated sediments. Calculated portions of each fraction in the TP of all stations were as follows: P-FD (41%), P-Fe (32%), P-AUT (25%), P-ORG (1%) and P-DET (1%). In relation to previously obtained results for the middle Adriatic sediments under the anthropogenic impact, higher P-FD and P-AUT portion, as well as lower portion of P-ORG was revealed. Altered percentages between P-AUT and P-ORG are result of applying the different technique in sediment extraction regarding the earlier analyses. Sediment vertical profiles of P-FD, P-Fe and P-AUT pointed to elevated concentrations in surface layer (0-1cm) in below-cage sediments that can be assigned to fish farming influence. P-DET vertical profiles reflected the most inert nature of detrital apatite P species, while P-ORG profiles probably indicated fast degradation of P in organic matter. Redox potential in below-cage sediments was permanently more negative than at referential site indicating enhanced input of organic material from fish farm and its decay in anoxic conditions. Accordingly, molar C-ORG/TP and C-ORG/N-TOT ratios were significantly lower in below-cage sediments compared to referential sediments due to increased input of phosphorus and nitrogen compounds as well as to fast degradation of organic material settled in sediments.
         Spearman correlation analysis of all investigated parameters provided linkage between P forms, N-TOT, fine-grained particles and OM that arises from increased nitrogen and phosphorus input in the organic material from the water column. The absence of usual correlation between C-ORG and OM is due to relatively rapid removal of carbon from sediments under the cages.