Possible limitation of growth and distribution of freshwater organisms by the availability of potassium, an essential major bio-constituent and plantnutrient, is considered for inland waters. It is interpreted in relation to the range of concentrations normally encountered and experimental work on specific growth rates of algae at low concentrations, minimum cell and biomass quotas of K, biomass yields under graded additions, inhibition at higher concentrations, and response to cation-ratios in the medium.

The range of K concentrations in inland waters is surveyed. Most concentrations exceed 10 µmol L^-1, and are greatly in excess of those (under 1 µmol L^-1) found limiting specific growth rates of test species. They are also in excess of the content in most natural populations of phytoplankton when yields have the experimental minimum or limiting cell/biomass quota, of the order of 1% dry weight. Limiting concentrations for specific growth rate in nature are therefore probably rare. They, and yield limitations, might possibly be reached after extreme depletion by dense stands of aquatic macrophytes; some depletions are recorded for a charophyte and water-cress (Rorippa nasturtium-aquaticum). Retention by seasonal vegetation in the catchment can more than offset delivery in rainfall and result in minimal concentrations of between 1 µmol L^-1 and 5 µmol L^-1. There is evidence that these concentrations, combined with poor nutrition, can limit the distribution of larger Crustacea in some upland streams. Other less substantiated sources of limitation in nature relate to ratios of cationic concentrations, in part for function of algal flagella. There is some experimental evidence for growth- and yieldlimitation of species of the chrysophyte Dinobryon by higher and naturally occurring K concentrations.