Benthic fluxes of nitrogen and phosphorus at southern bluefin tuna Thunnus maccoyii sea-cages

To assess the effects of southern bluefin tuna farming on benthic nutrient cycling, we measured sedimentation rates, porewater nutrients, sedimentary total nitrogen (N) and phosphorus (P) content, and benthic fluxes at 2 tuna sea-cages and associated control (non-farm) sites in South Australia over the course of a farming season. Sedimentation rates exceeded 40 g dry weight m–2 d–1 only at the sea-cage sites, and porewater concentrations of ammonium and phosphate were up to 10 and 100 times that measured at control sites, respectively. The highest ammonium and phosphate fluxes from the sediments into the water column were recorded at sea-cage sites (9962 and 2177 µmol m–2 h–1, respectively) towards the end of the season, and these were in excess of 10 times those recorded at control sites in any month. The annual average of the sediment fluxes associated with southern bluefin tuna farming produced over 7 and 40 times the daily requirements of N and P, respectively, for calculated primary productivity. The high sedimentation rates, porewater concentrations and benthic flux rates recorded at sea-cages at the end of the farming season were not observed 4 mo after the southern bluefin tuna were harvested. These data show that sedimentation rates and sediment geochemistry respond rapidly to farming activities. The risks associated with farming southern bluefin tuna include enrichment of pelagic nutrients from benthic fluxes that can lead to increased primary production. Thus continuation of site fallowing on an annual basis is strongly recommended. Sustainability of the coastal ecosystem supporting southern bluefin tuna farming must consider the regional consequences from these inputs of N and P, occurring not only through increased sedimentation but also direct nutrient availability in the water column.