Nitrogen Metabolism in Lakes Iii. Tracer Studies of the Assimilation of Inorganic Nitrogen Sources1

Using a stable isotope technique, the uptake rates of three inorganic nitrogen sources were followed during a summer season in Sanctuary Lake, Pennsylvania, and compared with the amount of the dissolved inorganic nitrogen in the water. Variations in the uptake regime during the season were considerable and seemed to be connected with the seasonal changes in the phytoplankton community. In the early part of the season, high uptake rates of all three forms were noted, whereas the second main uptake pulse in nitrogen utilization involved predominantly the assimilation of molecular nitrogen at the end of the summer.