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Abstract:
The extent of nutrient limitation of phytoplankton in eutrophic Pluβsee was studied by enrichment bioassays and by analysing the cellular stoichiometry of monospecific fractions obtained by size fractionation and density-gradient separation. In this lake silicate and nitrogen, but not phosphorus, at times limit the reproductive rates of phytoplankton. The dependence of nutrient-limited reproductive rates on the cellular content of the limiting nutrient (cell quota) could well be described by the Droop model. Biomass specific minimal cell quotas of nitrogen ranged from 0.014 to 0.061 mol N mol-1 C, minimal cell quotas of silicon ranged from 0.055 to 0.127 mol Si mol-1 C. The cell quotas of the non-limiting nutrients usually increased with the cell quotas of the limiting nutrient. In contrast to the Droop model, the Monod model which relies on ambient concentrations of limiting nutrients was a much poorer predictor of growth rates.
