The uptake of nitrogenous nutrients by phytoplankton in the waters of the fram strait during spring 1989

Three different aspects of the nitrogenous nutrition of phytoplankton in the Fram Strait were examined: the effect of irradiance on nitrate and ammonium uptake rates, the effect of nutrient (nitrate, ammonium) concentration on nutrient (nitrate, ammonium) uptake rates, and the effect of ammonium concentration on nitrate uptake rates. All experiments were performed with natural assemblages of phytoplankton. Labelled nutrients were added in tracer amounts, and incubations were carried out under natural light conditions. Both nitrate and ammonium uptake rates were strongly dependent on irradiance, and the relationship followed the model used by Priscu (1989). Ammonium uptake rates were always inhibited by surface irradiance levels. The phytoplankton present at this time of year were able to utilize nitrate at lower irradiance levels than ammonium, with the average optimal percentage of surface irradiance for nitrate and ammonium uptake being 18.0 and 77.8%, respectively. The relationship between nutrient concentration and nutrient uptake could be characterized by the Michaelis-Menton model. Lastly, there was a definite negative relationship between ammonium concentration and nitrate uptake rates. The average maximal extent of inhibition of nitrate uptake rates by ammonium was 40%, and occured at ammonium concentrations equal to or greater than ca. 2.5 μM. Ambient ammonium concentrations encountered during this study in the Fram Strait were capable of suppressing in situ nitrate uptake rates.