Inorganic Nitrogen Uptake in Oligotrophic Lake Taupo, New Zealand

Abstract

To determine nitrogen and carbon uptake in the surface waters of oligotrophic Lake Taupo, New Zealand, we used isotope tracer methods. Time-course measurements of NH4+ and NO3− uptake at near saturating concentration revealed about a fourfold greater uptake during the first hour of incubation relative to averaged 1–6 h uptake rates. This enhanced uptake was most apparent during the first 5–10 min of incubation. Uptake kinetics experiments indicated that the N assimilation rate was generally limited by low ambient concentrations of inorganic N. The half-saturation constants for N uptake were low [Formula: see text] and comparable with ambient levels of NH4+ and NO3−; NO3− uptake was always lower than NH4+ uptake and was inhibited strongly (~60%) in the presence of ~5 μg NH4+-N∙L−1. Carbon to N uptake ratios averaged (±SD) 20.9 ± 11.7 (grams C to grams N) and were correlated with seston compositional ratios. Polysaccharide was the major end product of photosynthesis (as measured by 14C incorporation) in measurements made at 100 μE∙m−2∙s−1 whereas protein was the primary end product during dark incubations. Samples enriched with NH4+ and incubated at 100 μE∙m−2∙s−1 showed a significant increase in the 14C incorporated into protein, at the expense of that incorporated into polysaccharide and low molecular weight metabolites. No differences in the pathway of C flow were observed in NH4+-enriched, dark-incubated samples. Nitrogen uptake estimates based on the percent of 14C in the protein fraction were similar to those calculated by the 15N technique.