The horizontal heterogeneity of nitrogen fixation in Lake Valencia, Venezuela 1

Spatial and temporal variability of nitrogen fixation in Lake Valencia, Venezuela, were quantified on the basis of duplicate water samples collected from a depth of 0.5 m at 16 sites on 10 dates. The concentration of heterocysts in samples was determined and the samples were incubated with acetylene in situ. Two-way ANOVA was used to separate the variance associated with site (fixed spatial patchiness), date (temporal variation), the interaction between site and date (ephemeral spatial patchiness), and sampling error. The nitrogen fixers in Lake Valencia are arranged in large (40-200 km super(2)), ephemeral patches with distinctive fixation rates per heterocyst. Both variability in fixation per heterocyst and variability in heterocyst concentration contribute significantly to variation in fixation per unit volume of lake water, but the variability attributable to heterocyst abundance is greater. Spatial variation in fixation and heterocyst concentration exceeds temporal variation in these parameters, and the ephemeral component of patchiness is much greater than the fixed component.