Nitrogen uptake and phytoplankton growth in coastal upwelling regions1

Uptake of nitrogenous nutrients by microplankton off the Washington and Oregon coasts was measured during the 1985 upwelling period. Nitrogen uptake rates in low‐NO3− waters (<5 µM) were 0.020–0.258 µmol N liter−1 h−1 and were primarily supported by regenerated nitrogen (71% of total uptake). Nitrogen uptake rates in high‐NO3 waters(≥20 µM) were 0.281–1.480 µmol N liter−1 h−1 and new (NO3−) nitrogen supported 83% of total uptake. Phytoplankton N was estimated by assuming a constant Chl a : PN for phytoplankton and was used to calculate phytoplankton‐specific uptake rates. Despite differences in nutrient concentrations, PN and Chl a at three upwelling sites (Oregon, Benguela, and Peru), NO3 uptake normalized to Chl a, and estimates of NO3−‐supported phytoplankton growth rates are remarkably similar. Nitrate uptake supports growth rates on the order of 1–2 d−1. Estimates of phytoplankton growth from rates of NH4+ + NO3− uptake range from 1.36 to 4.79 d−1 and are negatively correlated with concentrations of NO3− and phytoplankton N. The apparent increase in phytoplankton growth rates for waters with lower nitrogen availability may result from heterotrophic bacterial utilization of NH4+ nitrogen.