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 μM. 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.