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

Abstract

The concentration‐dependent uptake of nitrate (NO 3 ) and its inhibit ion by ammonium (NH 4 ) were surveyed in surface waters of the North Atlantic Ocean. Low‐level NO 3 determinations combined with conventional tracer methods provided the first comprehensive data set on NO 3 utilization kinetics at nanomolar concentrations. Uptake followed saturation kinetics described by the Michaelis‐Menten equation. The half‐saturation parameter for uptake ( K N ) ranged 2–3 orders of magnitude, covarying with ambient NO 3 concentrations. K N concentrations in oceanic waters averaged ~20–30 nM. NH 4 half‐saturation parameters could only be approximated (i.e. K A + A ), but observations suggested that K A and K N were of similar magnitude in oceanic waters. Maximum uptake rates of nitrate, p m( N ) , and ammonium, p m( A ) , covaried, but p m( A ) almost always exceeded p , ( N ) ; in oceanic waters, the disparity was an order of magnitude or greater. Most of the variability in p m( N ) and p m( 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; α A was generally greater than α N . Kinetics analysis showed that NH 4 is preferentially utilized over NO 3 over the full spectrum of nitrogen concentrations, nanomolar to micromolar. The inhibition of NO 3 uptake by NH 4 was also parameterized using the Michaelis‐Menten expression. The inhibition half‐saturation parameter (K i ) covaried with K N , but K i concentrations in oceanic waters (~40– 50 nM) always exceeded K n . Maximum inhibition ( I m ) was rarely complete (i.e. I mn < 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.