The Limnology of Nitrogen in an Oklahoma Reservoir: Nitrogenase Activity and Related Limnological Factors

The surface waters of Lake Carl Blackwell, Oklahoma, were assayed, using acetylene reduction techniques, for their N2fixing potential at intervals of 2 to 3 weeks, between November 1969, and July 1971. Data were obtained on limnological factors which affect the rate of N2fixation; temperature, transparency of the water and concentration of nitrate, nitrite and ammonia at meter intervals. The ranges of the rates of ethylene production by concentrated and unconcentrated plankton were 0.1 to 225.6 and 50.6 to 185.4 nmoles (mg N hr)-1, respectively. Rates fluctuated widely during the season and were nil in winter. Acetylene reduction at the surface was associated with the expanding phase of blooms of blue-green algae. Acetylene reduction by the sediments was also observed, and uptake of15N2by the plankton verified that an N2fixing potential exists in the lake. Observations on the seasonal cycle of inorganic nitrogen revealed that nitrification predominated in winter and early spring. Decreases in inorganic N in early summer at the surface were attributed to uptake by autotrophs. Mineralization proceeded after the demise of the autotrophs in autumn. During summer stagnation, nitrate, ammonia and nitrite were stratified, but there was no evidence of denitrification.