CHEMICAL FLUXES AND MASS BALANCES IN A MARINE FISH CAGE FARM .3. SILICON

Fluxes and pathways of silicon in a marine rainbow trout cage farm wer e studied. The measured fluxes included those carried by fish food, ju veniles, harvest, fish loss (death and escape), sedimentation from the cages, and benthic release measured with flux chambers in-situ. Two d ifferent types of Si mass balances for the farm were constructed. The mass balance according to the flux method was based on the measured fl uxes and constructed for each of two consecutive growing seasons. The mass balance constructed according to the accumulation method was base d on the total input and removal of Si to the cages of the farm since it was started, and the recovery of Si in the sediment originating fro m the farm. In both types of mass balances external input was the majo r source of Si, contributing a minimum of 55-80% of the total biogenic silica in the material collected in sediment traps below the farm and of the material accumulated in the farm-derived sediment. The Si remo ved from the farm with harvest contributed a very small part, approxim ately 0.3%, of the amount of Si supplied to the farm with fish food an d juveniles. The loss of Si to the environment was 2.4 kg (1985), 2.5 kg (1986) and 2.5 kg (1980-86) for each ton of fish produced. On a sea sonal basis, about 4.5% of the sedimented biogenic Si was returned in dissolved form from the sediment to the overlying water. This constitu ted about 0.3% of the biogenic Si content in the farm-derived sediment . The flux of reactive silicate from sediment below the fish farm was enhanced about 2.5 times compared to nearby sediments unaffected by th e farm.