Exchange of nutrients and oxygen across the sediment–water interface below a Sparus aurata marine fish farm in the north-western Mediterranean Sea

PURPOSE: This study analyzes the effects of aquaculture activities in open seawater in the north-western coastal waters of the Mediterranean Sea. It is the first of its kind to be based on benthic flux data gathered in situ below fish farms for this particular area. MATERIALS AND METHODS: Samples were collected on four sampling campaigns over a 1-year cycle under a Sparus aurata fish farm facility where benthic fluxes were measured in situ using light and dark benthic chambers. Bottom water and sediment samples were also collected. Data were compared to those for a nearby control station. RESULTS AND DISCUSSION: Significant differences were found (ANOVA, p < 0.05) between concentrations of organic matter (OM), total phosphorus and redox potentials in sediments located under the cages and those of the control station. The consumption of dissolved oxygen (DO) by sediment and positive ammonium (NH ₄ ⁺ ) fluxes was stimulated by OM content, with correlations of r = −0.60 (p < 0.01) and r = 0.70 (p < 0.01), respectively. The OM content of sediments was found to be consistently higher under the cages than at the control station, with the highest value (1.8 ± 0.7 %) under the cages observed during the early summer; values of DO and NH ₄ ⁺ fluxes were −64 ± 17 and 12.7 ± 1.0 mmol m⁻² day⁻¹, respectively. PO ₄ ³– fluxes were consistently higher in the fish farm sediments (between 0.58 and 0.98 mmol m⁻² day⁻¹) than those observed at the control station. Nitrate (NO ₃ ⁻ ) fluxes were found to be consistently negative due to denitrification occurring in the sediments and were related to the concentration of NO ₃ ⁻ in bottom waters (r = 0.92, p < 0.01). Si fluxes were shown to be associated with water temperature (r = 0.59, p < 0.05). CONCLUSIONS: The results imply that sediments located below cages accumulate organic matter originating from aquaculture activities, especially during summer months when this activity increases. Sediments undergo biogeochemical changes that mainly affect fluxes of DO, NH ₄ ⁺ and soluble reactive phosphorus, although these do not seem to have a significant impact on the quality of the water column due to the hydrodynamic characteristics of the area.