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 me asured during two upwelling seasons and one winter off Oregon. Althoug h PN-specific and absolute uptake rates showed no dependence on nitrat e concentration, Michaelis-Menten kinetics applied when the uptake rat es were normalized to Chl a. Chl a-specific nitrate uptake rates were saturated when nitrate concentrations were > 5 mu M. Uptake rates decr eased in response to either low nitrate concentrations or when extreme ly high phytoplankton biomass caused shading. PN- and Chl a-specific u ptake rates were similar when Chl a concentrations were greater than o r equal to 4 mu g liter(-1) and phytoplankton N comprised most of the PN (particulate organic N) pool. When Chl a was <4 mu g liter(-1), how ever, phytoplankton N accounted for only 20-30% of the PN, and estimat ed phytoplankton-specific uptake was 5-fold greater than PN-specific u ptake rates. These results suggest that observed temporal changes in P N-specific nitrate uptake rates reflect variations in phytoplankton bi omass rather than changes in phytoplankton-specific activity.