Nitrogen fixation in an oligotrophic, saline desert lake: Pyramid Lake, Nevada

Open Access
Publisher Website
Google Scholar
Abstract
High rates of nitrogen fixation by a short‐lived but dense unialgal bloom of the planktonic blue‐green Nodularia spumigena provided 99.5% of the alga’s needs and 81% of Pyramid Lake’s annual total combined nitrogen input in 1979. The bloom was spatially very heterogeneous. Bloom size, duration, and presumably N2 fixation vary from year to year, but in 1979 about 900 t of nitrogen were fixed in 2 months in this large deep lake. The annual rate of N2 fixation was about 2 g m−2. In this year of low inflow the Truckee River provided 54 t of inorganic nitrogen and 83 t of organic nitrogen. Planktonic N2 fixation has not been measured during high inflow years and may have been small relative to river input. Lakewide average heterocyst to vegetative cell (h : c) ratios followed seasonal trends in N2 fixation, but synoptic samples showed only a weak relation between h : c and N2 fixation. N2 fixation was induced by low epilimnetic levels of inorganic nitrogen and ended before lake overturn in the fall. High rates of N2 fixation were confined to the upper 5% of the epilimnetic volume and thus occurred only in calm weather when Nodularia colonies floated to the lake surface. Access to freshly dissolved atmospheric CO2 may account for the near‐surface dependence, since the lake pH is normally about 9.2. We predict that Nodularia will not show the same degree of near‐surface dependence in near‐neutral lakes or in the ocean.