Diel variation of nitrogen fixation in Lake Valencia, Venezuela’

During 198 1 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. Diel variations of lacustrine nitrogen fixation appear to be common, but the patterns reported are variable: maximum nitrogen fixation rates may occur in the morning (Vanderhoef et al. 1975) at midday (Stewart et al. 1967; Ganf and Horne 1975), or in the afternoon (Paerl 1979; Kellar and Paerl 1980). The inconsistency of its diel pattern indicates that nitrogen fixation is rarely a simple function of light intensity. Acting alone on a cloudless day, the light response of nitrogen fixation should result in midday maxima of nitrogen fixation at subsurface depths (Lewis and Levine 1984). Twin maxima around a midday depression could occur at the surface, where midday light intensities are inhibitingly high. Two phenomena could cause nitrogen fixation to be centered around a morning or afternoon maximum: change in the depth distribution of fixers or change in light response caused by changes in internal or external factors such as oxygen and carbon dioxide concentrations that inhibit or enhance nitrogen fixation. The literature suggests that both phenomena occur. Lake Mendota, Wisconsin, is an example of a lake with diel variations in nitrogen fixation that are strongly influenced by changes in the vertical distribution of heterocystous blue-green algae (Vanderhoef et al. 1975). In Lake Mendota, bluegreen algae migrate downward in midmorning and do not return to the surface until late afternoon. Consequently, nitrogen fixation rates usually peak in the morning. Pearl and Kellar have provided examples of diel variations in nitrogen fixation correlated with certain environmental factors. In studies of three New Zealand lakes (Paerl 1979), one Canadian lake (Kellar and Paerl 1980), and a culture of Anabaena oscillarioides (Paerl 1979), these investigators found that most nitrogen fixation occurred in the afternoon (or in the latter half of the culture’s light cycle) when oxygen concentrations were maximal. Paerl and Kellar postulated that oxygen evolved by photosynthesis severely inhibited carbon fixation, but scarcely affected nitrogen fixation, which was protected by the heterocyst. While carbon fixation was depressed, the flow of photoreductant and carbon skeletons to nitrogen fixation pathways was enhanced. The mechanism proposed by Paerl and Kellar 1 Supported by NSF grants DEB 78-05324 and DEB may not be a general one for lakes, however; 80-03883. Other support was provided by the Funit could not be detected in Wintergreen Lake, dacion Eugenio Mendoza of Venezuela, Michigan (Ward and Wetzel 1980). * Present address: Ecosystems Research Center, CorThe relative importance of physiological nell Univ., Ithaca, N.Y. 14853. change and diel migrations of blue-green al-