Diel variation of nitrogen fixation in Lake Valencia, Venezuela1

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Abstract
During 1981 we examined the diel variations of nitrogen fixation in Lake Valencia, Venezuela. Four species of heterocyst‐bearing blue‐green algae were common but subdominant in the phytoplankton. In samples taken from and incubated at 0.5 m, the rate of nitrogen fixation per unit volume of water was lowest at night (<6% of the maximum daytime rate), increased from dawn until early afternoon, and then diminished between late afternoon and the first hour of darkness. This pattern was caused partly by diel changes in light intensity and partly by diel migrations of heterocystous blue‐green algae. Heterocyst concentrations at 0.5 m increased between midmorning and early afternoon and then decreased from late afternoon until evening. The heterocyst‐specific nitrogen fixation rates (nitrogen fixed per heterocyst per unit time) at 0.5 m were much less variable than the nitrogen fixation rates per unit volume of water. Heterocyst‐specific rates rose rapidly in early morning and fell slowly in the evening, but were almost constant over much of the day. Heterocyst‐specific nitrogen fixation rates were very close to those predicted by a model based on the light dependency of nitrogen fixation. There was no evidence for temporal changes in the light response of nitrogen fixation.