Nitrate uptake rates in a coastal upwelling regime: A comparison of PN‐specific, absolute, and Chl a‐specific rates

PN‐specific, absolute, and Chl a‐specific nitrate uptake rates were measured during two upwelling seasons and one winter off Oregon. Although PN‐specific and absolute uptake rates showed no dependence on nitrate concentration, Michaelis‐Menten kinetics applied when the uptake rates were normalized to Chl a. Chl a‐specific nitrate uptake rates were saturated when nitrate concentrations were > 5 M. Uptake rates decreased in response to either low nitrate concentrations or when extremely high phytoplankton biomass caused shading. PN‐ and Chl a‐specific uptake rates were similar when Chl a concentrations were ≥ 4 µg liter−1 and phytoplankton N comprised most of the PN (particulate organic N) pool. When Chl a was <4 µg liter−1, however, phytoplankton N accounted for only 20–30% of the PN, and estimated phytoplankton‐specific uptake was 5‐fold greater than PN‐specific uptake rates. These results suggest that observed temporal changes in PN‐specific nitrate uptake rates reflect variations in phytoplankton biomass rather than changes in phytoplankton‐specific activity.