Rapid physiological assays for nutrient demand by the plankton. I. Nitrogen

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Abstract
Three assays for nitrogen demand were compared on samples of natural plankton and on green and blue-green algal species in culture. The most reliable guide was selective luxury uptake of nitrogen by the plankton after enrichment with a 10-to-1 (wt/wt) mixture of inorganic N and P. Ammonium transport capacity, measured either directly (cultures) or by Vmax for uptake of the NH4+ analogue methylammonium (lakewaters), was generally high in lakewaters with low dissolved inorganic N-to-P ratios and in N-deficient cultures of green algae. By contrast, transport capacity was much reduced in both natural and cultured populations of heterocystous blue-green algae growing under conditions of low combined inorganic nitrogen. Ammonium enhancement assays (heterotrophic CO2, fixation after NH4+ enrichment) were conducted at monthly intervals on eutrophic Lake Rotorua. There was a strong, negative correlation between this indicator of N deficiency and dissolved inorganic N-to-P ratios below a threshold of 6:1. Ammonium enhancement was not, however, a reliable indicator of combined inorganic N-demand by populations of heterocystous blue-green algae. All three assays provided strong evidence of a persistent shortage of nitrogen relative to phosphorus for algal production in Lake Rotorua.
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