Inorganic nitrogen assimilation in a subalpine lake1

Abstract

Isotope tracer techniques were used to determine the seasonal patterns of inorganic nitrogen and carbon assimilation in the epilimnion of meso‐oligotrophic Castle Lake, California. Kinetics experiments with 15N‐labeled ammonium and nitrate indicated that the nitrogen assimilation rate was generally limited by low ambient concentrations of inorganic nitrogen (DIN). The half‐saturation constants for DIN assimilation were low (0.7–9.3 µg · liter−1) and comparable to ambient levels during the nutrient‐depleted, ice‐free growing season. Ammonium assimilation was always higher than nitrate assimilation, even when nitrate concentration greatly exceeded ammonium during early summer, This observation was also supported by the results of 13NO3− uptake experiments.The midsummer proliferation of nonheterocystous blue‐green algae was accompanied by increased carbon and nitrogen assimilation, and higher rates of particulate C and N turnover. Carbon to nitrogen and carbon to maximum nitrogen assimilation ratios were generally high (34.5 ± 13.3 and 15.6 ± 4.5 g C:g N), did not show obvious seasonal differences, and were not correlated significantly with seston compositional ratios.These results integrated with other Castle Lake studies suggest that phytoplankton growth is maintained by efficient use of internally regenerated ammonium, in agreement with many such studies in nitrogen‐deficient marine waters.