Inorganic nitrogen uptake by epilithic periphyton in a N‐deficient lake1

Abstract

Seasonal patterns of dissolved inorganic nitrogen and inorganic carbon uptake by the sublittoral epilithic periphyton community in N‐deficient, oligotrophic Lake Tahoe were examined. The biomass dominants of this community, N 2 ‐fixing blue‐green algae (e.g. Calothrix, Tolypothrix, and Nostoc ) were persistent and retained their nitrogenase activity throughout the year. Seasonal rates of N 2 fixation exhibited considerable variation, with a distinct summer maximum and winter minimum. Uptake of both NO 3 − and NH 4 + followed Michaelis‐Menten kinetics. K, values were typically extremely high (>100 µ g N liter −1 ) compared to the ambient concentrations of these forms of nitrogen (<10 µ g N liter −1 ). N 2 fixation was the most important source of inorganic N to the yearly N budget of this benthic community. Low ambient substrate concentrations coupled with a low physiological affinity for these substrates at ambient levels were responsible for the relative unimportance of NO 3 − and NH 4 + uptake. Dark uptake of NO 3 − , NH 4 + , and N 2 fixation were all significant and could not be neglected in determining rates of daily inorganic nitrogen utilization. This blue‐green algal community is not adapted for efficient use of NO 3 − or NH 4 + and can survive in the N‐deficient environment because of its ability to use N 2 . In striking contrast, the phytoplankton in Lake Tahoe has no N 2 ‐fixing organisms and depends on NO 3 − and NH 4 + for its production.