LAMINARIA CULTURE FOR REDUCTION OF DISSOLVED INORGANIC NITROGEN IN SALMON FARM EFFLUENT

Finfish culture is a growing industry, and it causes a nutrient loading problem. To investigate the feasibility of an integrated culture of kelp and salmon, 15-cm long kelp (Laminaria saccharina) was grown in salmon culture effluent. The objectives were to test the effects of flow rate and kelp density on dissolved inorganic nitrogen removal (DIN), and DIN uptake and growth by the kelp. NH4+, NO3- and DIN (NH4+ + NO3-) loadings were in the ranges 6.2-25.4, 12.9-40.0, 19.7-52.7 mumol l-1, respectively, over the experimental period. Surplus uptake of nitrogen was not evident, because the C:N ratio (10-11) was constant in all experiments. During light periods, the kelp removed from 170-339 mumol l-1 h-1, and approximately 26-40% of the incoming DIN. The DIN uptake rate, based on daylight sampling periods, ranged between 6.1-22.5 mumol g-1 dry mass h-1. The highest-flow rate, lowest-density tank had the highest DIN uptake rate. Debris from the fish effluent settling on the kelp thalli in the low-flow rate tanks affected uptake. Mean DIN uptake rate based on 3 days of growth for all flow-density combinations ranged between 5.4-8.3 mumol g-1 dry mass h-1. The kelp utilized NH4+ and NO3- equally. The growth ranged between 6.5-9% d-1. The biomass production ranged from 1-2 g per sampling period. The highest growth rate and biomass production were achieved by kelp in the highest-flow rate, lowest-density tank. Lower DIN concentrations due to higher DIN removal rates in the other tanks and light limitation due to self-shading in the high-density tanks were probably responsible for the reduced growth rate in these tanks.