Inorganic Nitrogen Uptake in Oligotrophic Lake Taupo, New Zealand

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Abstract
To determine N and C uptake in the surface waters of oligotrophic Lake Taupo, New Zealand, isotope tracer methods were used. Time-course measurements of NH4+ and NO3- uptake at near saturating concentration revealed .apprx. a 4-fold greater uptake during the 1st 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 (.ltoreq. 10 .mu.g N .cntdot. l-1) and and comparable with ambient levels of NH4- and NO3-; NO3- uptake was always lower than NH4 uptake and was inhibited strongly (.apprx.60%) in the presence of .apprx. 5 .mu.g NH4+-N .cntdot. l-1. C 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 .mu.E .cntdot. m-2 .cntdot. s-1; protein was the primary end product during dark incubations. Samples enriched with NH4+ and incubated at 100 mE .cntdot. m-2 .cntdot. 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. N uptake estimates based on the percent of 14C in the protein fraction were similar to those calculated by the 15N technique.