Nitrate and ammonium uptake by phytoplankton populations during the spring bloom in Auke Bay, Alaska

Abstract

The importance of nitrate and ammonium as phytoplankton nitrogen sources was examined by 15N tracer experiments during the spring bloom in Auke Bay, Alaska. Nitrate and ammonium uptake rates as functions of substrate concentration and light intensity were experimentally determined. During the early phase of biomass increase, nitrate contributed nearly all of the nitrogen uptake. Ammonium turnover time was several hours, and the high contribution of nitrate reflected the limited ammonium supply. The contribution of ammonium increased during the later period of biomass increase. After the biomass maximum, ammonium contributed the majority of nitrogen, with occasional and sporadic supplies of nitrate causing a biomass increase. Under nutrient-repleted conditions during the early part of the bloom, the nitrate uptake rate was similar to that of ammonium under subsaturating light intensities (<200 μE m −2 s −1) and nitrate concentrations (< 1·5 μM), but higher under saturating light intensities and nitrate concentrations. After the bloom peak, under nutrientdepleted conditions, the nitrate uptake rate was lower than that of ammonium for all light intensities and nitrate concentrations. Time-course analysis of 15N uptake and partitioning into macromolecule fraction indicated a physiological nitrogen deficiency during this period. Cumulative nitrate uptake estimated from 15N tracer experiments was comparable to the net decrease in nitrate in the water column; new production defined as nitrate uptake was greater than the sedimentation loss, which accounted for 77% of the former.