Abstract
Abstract Measurements of acetylene reduction by the plankton of Lake Rotongaio (New Zealand) were carried out approximately monthly from November 1979 through 1980, and were calculated in terms of N,‐fixation. Parallel photosynthetic 14C‐fixation assays were also made. As well as the seasonal cycle of N2‐fixation dependent upon the summertime presence of the heterocystous cyano‐phyte Anabaena oscillarioides there were other fluctuations in fixation rate. For each assay occasion the N2‐fixation rates through the water column were strongly correlated with light attenuation and less so with heterocyst numbers. Inorganic nutrient nitrogen and temperature had little influence. The relationship between N2‐fixation, light, and heter‐ocysts was variable with time, and was interpreted as being influenced by small scale vertical changes in physiological environment, particularly of the light field. Approximate estimations suggested that N2‐fixation was not a major contributor to the lake's daily nitrogen requirement (0.03–2.0%) nor to the total annual nitrogen budget (23–24% of the supply from the surface influent), in spite of the occasional occurrence of high fixation rates at some water depths.
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