Interactions of light and inorganic nitrogen in controlling nitrogen uptake in the sea*

-The uptake of ammonium and nitrate in natural populations of marine phytoplankton is light dependent and often can be described adequately by the Michaelis-Menten equation. The halfsaturation constant for light intensity in nitrate and ammonium uptake is found to be from 1 to 14 ~o of surface light intensity, an intensity range occurring near the bottom of the euphotic zone. Since the uptake of these compounds as a function of substrate concentration is also described by the same equation, many vertical profiles of nitrate and ammonium uptake obtained with the 1 ~N technique can be interpreted from the interaction of light and nutrient concentration at different depths. In oligotrophic waters, nutrient uptake is severely limited by nutrient concentrations; but in eutrophic areas, light intensity very often is the limiting factor. In the former, maximal uptake may occur deep in the euphotic zone, while in the latter, the maximum occurs near the surface. Ammonium is taken up more readily than nitrate by populations in all areas, and in oligotrophic regions may account for 80 % of the inorganic nitrogen uptake. In eutrophic areas, most of the inorganic nitrogen uptake is based on nitrate. Ratios of carbon to nitrogen taken up by phytoplankton vary over a wide range (12-76), with some indication that the lower values are associated with regions having a certain degree of environmental stability--either very rich or very poor waters. I N T R O D U C T I O N THE CONSIDERABLE geographic and vertical variability in the rates and relative importance of nitrate and ammonium uptake by natural populations of marine phytoplankton have been described by DUGDALE and GOERIN6 (1967). Certain variability in natural uptake can be attributed to nutrient concentration variations (MACISAAC and DUGDALE, 1969), but light intensity also is important in controlling uptake, either through photosynthesis or directly. A light requirement for nitrogen uptake is suggested by the similarities in shape between vertical profiles of nitrogen and carbon uptake by marine phytoplankton (DuGDALE and GOERIN6, 1970), and in certain freshwater algae the relation of nitrate assimilation to light intensity is known to be described by a saturation curve (BONGERS, 1956; HATTORI, 1962). A computation model for the uptake of nitrate described by DUGDALE and MACISAAC (1971) had variables of light intensity, and nitrate and ammonium concentration. The model was based upon the kinetics of nitrogen uptake as a function of ambient concentration (MACISAAC and DUGDALE, 1969) and upon unpublished experiments measuring inorganic nitrogen uptake as a function of light intensity. These experiments with light are described in this paper and nutrient uptake profiles are discussed and classified in accordance with currently available theory. *Contribution No. 605 from the Department of Oceanography, University of Washington. tDepartmcnt of Oceanography, University of Washington, Seattle 98195.