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

The concentration-dependent uptake of nitrate (NO3) and its inhibition 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 (K-N) ranged 2-3 orders of magnitude, covarying with ambient NO3 concentrations. K-N concentrations in oceanic waters averaged similar to 20-30 nM. NH4 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, rho(m(N)), and ammonium, rho(m(A)), covaried, but rho(m(A)) almost always exceeded rho(m(N)); in oceanic waters, the disparity was an order of magnitude or greater. Most of the variability in rho(m(N)) and rho(m(A)) could be explained by variations in phytoplankton biomass and temperature. The slope of the uptake vs. concentration relationship, alpha, was also investigated but was highly variable and could not be related to any of the oceanographic properties observed; alpha(A) was generally greater than alpha(N). 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 (K-i) covaried with K-N, but K-i concentrations in oceanic waters (similar to 40-50 nM) always exceeded K-N. Maximum inhibition (I-m) was rarely complete (i.e. I-m < 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.