Today the inputs of anthropogenic nitrogen into aquatic environments increase globally, which result from the mass production of chemical fertilizers and their consumptions. The variations in chemical compositions of phytoplankton potentially affect their nutritional values for secondary producers as well as potential impacts on the sea bottom environments via their depositions and oxygen consumptions. We experimentally cultured a diatom species at six levels of inorganic nitrogen concentrations, and examined the response of the algae in terms of their chemical compositions and oxygen consumptions. As a result, the content of essential fatty acids in the diatom cells increased with the nitrogen concentration in ≤3.0 mgN/L, but decreased in >3.0 mg-N/L. The optimum ratio of inorganic nitrogen to phosphorus, at which the diatoms efficiently produce the essential fatty acids, was estimated at 12:1. Furthremore, the oxygen consumption rate of the diatoms increased with the nitrogen concentration, whereas we found no significant effects of the experimental settings of seawater nitrogen cocenration on the settling velocity of the cultured diatoms.