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Articles

Diurnal Variation in Algal Acetylene Reduction (Nitrogen Fixation) in Situ¹

Robert E. Fiehweg^b, Jeffery W. Williams^b, Donald L. Wackwitz^b and Kevin T. Mason^b

^a Department of Botany, University of Illinois, Urbana, Illinois 61801, Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706

Diurnal variation in algal nitrogen fixation was studied in Lake Mendota, Wisconsin, during the summers of 1971 to 1973. Approximately two-thirds of the daily acetylene reduction in the surface decimeter occurred before noon. The decline in acetylene reduction (nmoles/liter·hr) near midday was partially because the algae relocated themselves at greater depths. However, acetylene reducing activity (nmoles per A₆₆₃ unit chlorophyll a per hour) also decreased as midday approached. Occasionally algae would resurface near the end of the day. On average, acetylene reduction (nmoles per liter per hour) was maximum at about 0900 Central standard time in the top decimeter, and acetylene reduction between 0830 and 0930 Central standard time represented 13% of the total daily acetylene reduction. Furthermore, acetylene reduction in the top decimeter, on average, represented 3.6% of the total acetylene reduction in the column. Calculation of the contribution by nitrogen fixation to a lake's fixed nitrogen budget is discussed.

¹ This investigation was supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison; by the University of Wisconsin Sea Grant Program maintained by the National Oceanographic and Atmospheric Administration, United States Department of Commerce (SP 144-B188); by a Health, Education and Welfare Biomedical Sciences support grant to the University of Illinois; by the Graduate Research Board, University of Illinois; by the Water Resources Center, University of Illinois (Project S-038-Ill).