Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

Abstract

The isotope 15 N was used to examine nitrogen dynamics in Lakes Fryxell and Vanda, two permanently ice-covered Antarctic lakes. Half-saturation constants for NH 4+ . uptake in the shallow waters of both lakes were <10 μg N l −1 ; uptake kinetic experiments on populations forming the deep-chlorophyll layers of these lakes showed zero-order kinetics and could not be fitted with the Michaelis-Menten equation. Elevated uptake within the first few minutes following pulses of NH 4+ . and NO 3− occurred in both lakes. NH 4+ regeneration, determined from isotope dilution experiments, exceeded uptake at 4.6 m in Lake Fryxell, was less than uptake at 9 m in Lake Fryxell and was equal to uptake at 10 m in Lake Vanda under the experimental conditions. NO 3− uptake was suppressed by NH 4+ levels as low as 2 μg NH 4+ -N l −1 in Lake Fryxell; the suppression was strongest in the near-surface populations. Substrate-saturated C:N uptake ratios (g:g) in Lake Fryxell decreased from 8.4 near the surface to 1.8 at the bottom of the trophogenic zone. Overall, the nitrogen dynamics in these lakes are similar to other lakes and the open ocean in that biological productivity during the austral summer is supported by regenerated nutrients.