Uptake of ammonium and urea in the northeast Pacific: comparison between netplankton and nanoplankton

Abstract

The stable isotope 15N was used to measure nitrogen uptake in the coastal upwelling region off Oregon and Washington, USA, where nitrate concentrations in the surface water ranged from 0.7 to 49.1 μM. Nanoplankton biomass (1 to 10 pm) was relatively more abundant at low-nitrate stations, while netplankton biomass (10 to 200 μm) was dominant at high-nitrate stations. Ammonium was more important than urea as a regenerated nitrogen source for both size classes of plankton, and ambient ammonium concentrations appeared to inhibit urea assimilation. Nanoplankton and netplankton were equally important in use of regenerated nitrogen in low-nitrate waters, while netplankton dominated nitrogen use in high-nitrate waters. In high-nitrate waters, the percent uptake in each size fraction was proportional to the percent particulate nitrogen (PN) and chlorophyll a (chl a) in that fraction. At low-nitrate stations, uptake in each fraction was proportional to PN but not to chl a. Nitrogen-specific (mass N taken up/mass particulate N) uptake rates ranged from 0.2 to 2.6 d-l, and were generally higher for the netplankton than for the nanoplankton fraction. However, these rates are not directly proportional to phytoplankton use as a result of varying amounts of non-phytoplankton N. Phytoplankton N was estimated by assuming a constant chl a/PN ratio for each size class, and used to calculate phytoplankton-specific uptake rates. Phytoplankton growth rates were estimated by extrapolating the short-term uptake rates to daily rates. Since calculated growth rates exceeded the expected maxima for phytoplankton In the low-nitrate coastal water, we postulate heterotrophic utilization of nitrogen