Chemical fluxes and mass balances in a marine fishcagefarm. IV Nitrogen

Abstract

Fluxes of nitrogen in a trout cage farm in open coastal water of the Gullmar Fjord, western Sweden, were measured to examine and describe the fate and major pathways of nitrogen supplied to the farm. The flux measurements included sedimentation from the cages, and sediment-water solute exchange measured with flux chambers operated by divers in situ. Benthic flux chambers were also utilized to measure denitrification rates in the sediment using the acetylene inhibition technique in situ. The nitrogen input to the f arm with fish food and juveniles, and the removal through harvest and fish loss (death and escape) were also quantified. Two approaches were used to construct nitrogen mass balances for the farm. One (the flux method) was based on the measured fluxes and constructed for each of 2 consecutive cultivation seasons. The other (the accumulation method) was based on the total nitrogen input with food and juveniles to the farm since it was started, the removal of nitrogen with harvested fish and fish loss, and the recovery of nitrogen in the farm-derived sediment after 7 cultivation seasons. Of total nitrogen input to the farm, 27 to 28 % was recovered in harvest, fish loss constituted 2 to 5 and 67 to 71 % (or 95 to 102 kg N per tonne produced fish) was lost to the aquatic environment. Of the nitrogen input to the farm and on a seasonal basis, solute release from the cages (presumably mostly excreted urea and ammonia) removed 48 particle sedimentation (excess feed and faeces) constituted 23 benthic nitrogen net flux dissolved organic nitrogen (DON), ammonium, nitrate, urea and denitrification products transferred 1 to 3 % back to the water column, and 20 % was accumulated in the sediment. On a longer time-scale (7 seasons), sediment nitrogen accumulation amounted to 12 % of the nitrogen input to the farm. A net fraction of 11 % of the nitrogen input to the farm sediment was released back in dissolved form to the overlying water on a seasonal basis. DON dominated the seasonal benthic flux. Denitrification constituted d very minor pathway for the transfer of nitrogen from the farm sediment to the water column (and ultimately to the atmosphere). The benthic flux removed seasonally 1.2 to 3.8 % of the nitrogen present in the farm-derived sediment. Loss to the environment of dissolved nitrogen (the sum of benthic flux and solute release from the cages) amounted to 51 % of the nitrogen input to the farm (or 72 % of the total environmental loss) on a seasonal basis, and 58 % (or 83 % of the total environmental loss) on a long-term basis. In this last paper in the series on the fish farm, the input and fate of carbon, phosphorus and silicon are compared with those of nitrogen.