Conditions stimulating planktonic N₂‐fixation in Lake Rotongaio

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 14 Cfixation assays were also made. As well as the seasonal cycle of N 2 -fixation dependent upon the summertime presence of the heterocystous cyanophyte Anabaena oscillarioides there were other fluctuations in fixation rate. For each assay occasion the N 2 -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 N 2 -fixation, light, and heterocysts 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 N 2 -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.

Control of Lacustrine Phytoplankton by Nutrients: Erosion of the Phosphorus Paradigm

“…For example, if a hypothetical lake had an N : P molar ratio of 10 : 1, implying nitrogen deficiency, a 25% boost in the N component would raise the ratio only to 12.5 : 1, which is unlikely to bring the lake into phosphorus deficiency. The reasons for the low compensatory potential of phytoplankton nitrogen fixers when phosphorus is abundant are unclear, but micronutrient limitation (WURTSBAUGH and HORNE, 1983), light limitation (LEWIS and LEVINE, 1984;VINER, 1985), and limitation related to the maximum sustainable ratio of heterocysts to vegetative cells (LEWIS and LEVINE, 1984) have been suggested. Nitrogen fixation by lake and stream periphyton appears to be less constrained than is fixation by phytoplankton (BERGMANN and WELCH, 1990;LEVINE and SCHINDLER, 1992;MARCARELLI et al, 2008), but the underlying reasons are not well understood.…”