SUMMARY 1. Unialgal cultures of three species common in the freshwater phytoplankton were used to test limitation of specific growth rate and final yield in defined media of low K+ concentration (range <0.3–6 μmol L−1 or mmol m−3).
2. Growth rate of the diatom Asterionella formosa was independent of K+ concentration above 0.7 μmol L−1. Final yield was dependent on initial concentration when accompanied by K+ depletion below this concentration, but not by lesser depletion with more residual K+. Analyses of particulate K in the biomass indicated a mean final cell content of 2.8 μmol K 10−8 cells, approximately 1.0% of the organic dry weight.
3. Less detailed work with the diatom Diatoma elongatum showed no dependence of growth rate or final yield upon the initial K+ concentration in the range 0.8–3.2 μmol L−1. The phytoflagellate Plagioselmis nannoplanctica suffered net mortality in the lowest concentration tested, 0.8 μmol L−1.
4. Comparison with the range of K+ concentration in natural fresh waters, including a depletion induced by an aquatic macrophyte, suggests that K+ is unlikely to limit growth of phytoplankton. Nevertheless, there can be correlation of K+ with lake trophy.