To evaluate the quantitative and qualitative changes in amino acids related to internal nitrogen content and growth rate of Ulva ohnoi, the supply of nitrogen to outdoor cultures of the seaweed was manipulated by simultaneously varying water nitrogen concentrations and renewal rate. Both internal nitrogen content and growth rate varied substantially, and the quantitative and qualitative changes in amino acids were described in the context of three internal nitrogen states: nitrogen-limited, metabolic, and luxury. The nitrogen limited state was defined by increases in all amino acids with increasing nitrogen content and growth up until 1.2% internal nitrogen. The metabolic nitrogen state was defined by increases in all amino acids with increasing internal nitrogen content up to 2.6%, with no increases in growth rate. Luxury state was defined by internal nitrogen content above 2.6%, which occurred only when nitrogen availability was high but growth rates were reduced. In this luxury circumstance, excess nitrogen was accumulated as free amino acids, in two phases. The first phase was distinguished by a small increase in the majority of amino acids up to ≈3.3% internal nitrogen, and the second by a large increase in glutamic acid, glutamine, and arginine up to 4.2% internal nitrogen. These results demonstrate that the relationship between internal nitrogen content and amino acid quality is dynamic but predictable, and could be used for the selective culture of seaweeds.