Effects of nitrate and ammonium enrichment on growth and nitrogen physiology in Fucus spiralis1

Growth and physiological responses of whole specimens of Fucus spiralis to nitrate and ammonium enrichment were measured in a continuous‐flow system using seawater from the Massachusetts coast. As physiological indicators of nitrogen status, total C and N, dark ammonium uptake, and respiratory stimulation by ammonium were examined in plant apices. Total nitrogen content and growth rate increased with the level of nitrate or ammonium addition, both supporting similar growth. Dark ammonium uptake was not as sensitive an indicator of nitrogen status as the respiratory stimulation by ammonium. Although the growth of F. spiralis may become nitrogen limited, plants did not exhibit severe nitrogen deficiency responses.