Nitrogen fixation in Lake Mendota, Madison, Wisconsin1

The effects of various environmental conditions, and of cell composition, heterocyst content, and nitrogen content of algal samples, on fixation of N2 by colonial and filamentous algae in Lake Mendota were investigated. Heterocyst content and temperature were significantly and positively related to acetylene reduction activity (N2 fixation); depth of sample collection was negatively related. Available data do not show whether statistical correlations of acetylene reduction activity with dissolved O2 and pH represent specific effects of O2 and pH on this activity, or simply reflect chemical changes caused by algal photosynthesis. During summer stratification, when surface-water content of combined inorganic nitrogen was severely depleted, N2 fixation associated with heterocystous blue-green algae contributed 85% of the total N2 fixed. Bacterial N2 fixation was low compared to that of algae. Despite its small overall contribution to the annual nitrogen budget (38,000 kg of nitrogen; ca. 7% of the total input), N2 fixation is significant in maintaining blue-green algal nuisances in surface water when non-N2-fixing phytoplankton cannot compete effectively.

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