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, N2‐fixing blue‐green algae (e.g. Calothrix, Tolypothrix, and Nostoc) were persistent and retained their nitrogenase activity throughout the year. Seasonal rates of N2 fixation exhibited considerable variation, with a distinct summer maximum and winter minimum. Uptake of both NO3− and NH4+ followed Michaelis‐Menten kinetics. K, values were typically extremely high (>100 µg N liter−1) compared to the ambient concentrations of these forms of nitrogen (<10 µg N liter−1). N2 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 NO3− and NH4+ uptake. Dark uptake of NO3−, NH4+, and N2 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 NO3− or NH4+ and can survive in the N‐deficient environment because of its ability to use N2. In striking contrast, the phytoplankton in Lake Tahoe has no N2‐fixing organisms and depends on NO3− and NH4+ for its production.