Nitrogen fixation in Clear Lake, California. 4. Diel studies on Aphanizomenon and Anabaena blooms1

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Abstract
Day and night measurements of N2 fixation (as acetylene reduction) were made during spring blooms of Aphanizomenon flos‐aquae and two autumn blooms of Anabaena spp. From 9 to 23% of the 24‐h fixation occurred between 1100 and 1300 hours. Nitrogen fixation in spring showed complex, physically shallow but optically deep and mobile subsurface peaks of nitrogenase activity, which were totally unrelated to Aphanizomenon biomass but may have been due to diel changes in light penetrating the relatively clear water. Nocturnal fixation was uniformly distributed with depth and accounted for ⅕ to of daylight fixation. In more turbid autumn waters, the pattern of N2 fixation for Anabaena blooms was simpler, with a surface (or near‐surface) peak decreasing with depth. Nocturnal fixation was more uniformly distributed with depth. The difference in fixation patterns between the two species is attributable to the interactions of oxygen with the nitrogenase enzyme system. The diel changes in nitrogenase activity suggest a need to establish whether the precursors of nitrogenase accumulate in an oxygen‐stable form.