The effects of 16 different combinations of nutrient load and agitation on yield, nutrient uptake and proximate chemical composition of the seaweed Ulva lactuca cultured in tanks were evaluated. Intensive fishpond outflow passed through seaweed tanks at four nutrient loading levels and four water agitation combinations of water exchange, bottom aeration and frequently changing water levels (an accelerated tide regime). Specific results from these outdoor experiments were examined further under controlled conditions in laboratory experiments. Agitation treatments affected the performance of U. lactuca only under TAN (NH3 þ NHþ 4 ) load levels below 4 g N m−2 day−1; biofiltration of TAN was the parameter most affected. Biomass yields at each of the four nutrient loading levels were not significantly different between the agitation treatments. Protein content increased significantly with increasing nutrient loading. The agitation treatments had a slight effect on seaweed protein content only at the lowest nutrient loading levels. There were no significant differences in dissolved oxygen concentration, pH, and temperature among the agitation treatments at all nutrient loading levels. Under laboratory conditions, growth rates, protein content, and photosynthetic and biomass yield of the seaweed were affected by water velocity under low nutrient concentrations. It is concluded that the effect of air agitation under the conditions of these experiments was not directly related to photosynthesis, excess dissolved oxygen, or carbon limitation, but to the diffusion of macro nutrients from the water to the seaweed. Therefore, once nutrient concentrations are high enough (above about 4 μM of TAN with the other nutrients in their corresponding proportions), aeration per se is not essential for effective growth and biofiltration by seaweeds.