In land-based integrated aquaculture of marine finfish and macroalgae, rearing space is a large expense. Increasing algal stocking density can increase efficiency, but this may require greater irradiance because of self-shading. To determine the irradiance needed, experiments were conducted in one-litre flasks with enriched seawater under natural and artificial illumination. Under natural illumination, a Daily Photon Dose of 17 mol m-2 d-1 at 10 and 14 ºC, Palmaria palmata cultured at 10 g L-1 grew 100 % faster and absorbed 20 % more nutrients than Chondrus crispus. However, Atlantic halibut farm effluent can reach up to 19 ºC in summer, too high for P. palmata. Under artificial illumination, C. crispus performed better than under natural illumination. Light saturation curves indicated nutrient uptake by C. crispus at 10 g L-1 and 10 ºC was highest at 23 mol m-2 d-1 irradiance, equivalent to 400 µmol m-2 s-1 for 16 hours