Uptake of New and Regenerated Forms of Nitrogen in Primary Productivity1

The use of 15 N‐labeled compounds to obtain specific uptake rates for the various nitrogen sources available to the phytoplankton makes it possible to separate the fractions of primary productivity corresponding to new and regenerated nitrogen in the euphotic zone of the ocean. Measurements of nitrate uptake as a fraction of ammonia plus nitrate uptake have been obtained from the northwest Atlantic and the northeast Pacific oceans. Mean values range from 8.3 to 39.5%, the former being characteristic of subtropical regions and the latter of northern temperate regions or coastal and inland waters. Nitrogen fixation is also a source of new nitrogen. Rates of nitrogen fixation are found to be as high or higher than nitrate uptake, in some cases suggesting an important role for nitrogen‐fixing phytoplankton. The role of zooplankton in regenerating nitrogen as ammonia in the Sargasso Sea is examined theoretically. Probably only about 10% of the daily ammonia uptake by phytoplankton is contributed by the zooplankton living in the upper 100 m.