The kinetics of nitrogen utilization in the oceanic mixed layer: Nitrate and ammonium interactions at nanomolar concentrations

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Abstract
The concentration‐dependent uptake of nitrate (NO3) and its inhibit ion by ammonium (NH4) were surveyed in surface waters of the North Atlantic Ocean. Low‐level NO3 determinations combined with conventional tracer methods provided the first comprehensive data set on NO3 utilization kinetics at nanomolar concentrations. Uptake followed saturation kinetics described by the Michaelis‐Menten equation. The half‐saturation parameter for uptake (KN) ranged 2–3 orders of magnitude, covarying with ambient NO3 concentrations. KN concentrations in oceanic waters averaged ~20–30 nM. NH4 half‐saturation parameters could only be approximated (i.e. KA + A), but observations suggested that KA and KN were of similar magnitude in oceanic waters. Maximum uptake rates of nitrate, pm(N), and ammonium, pm(A), covaried, but pm(A) almost always exceeded p,(N); in oceanic waters, the disparity was an order of magnitude or greater. Most of the variability in pm(N) and pm(A) could be explained by variations in phytoplankton biomass and temperature. The slope of the uptake vs. concentration relationship, a, was also investigated but was highly variable and could not be related to any of the oceanographic properties observed; αA was generally greater than αN. Kinetics analysis showed that NH4 is preferentially utilized over NO3 over the full spectrum of nitrogen concentrations, nanomolar to micromolar.The inhibition of NO3 uptake by NH4 was also parameterized using the Michaelis‐Menten expression. The inhibition half‐saturation parameter (Ki) covaried with KN, but Ki concentrations in oceanic waters (~40– 50 nM) always exceeded Kn. Maximum inhibition (Im) was rarely complete (i.e. Imn < 1), even at 2,000 nM ammonium. Overall, results suggest that nitrogen utilization parameters currently used in ecosystem models of the open ocean should be re‐examined.

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