Potassium - a non-limiting nutrient in fresh waters?

摘要

Possible limitation of growth and distribution of freshwater organisms by the availability of potassium, an essential major bio-constituent and plant nutrient, 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 themedium. The range of K+ concentrations in inland waters is surveyed. Most concentrations exceed 10 pmol L1, and are greatly in excess of those (under 1 pmol L1) found limiting specific growth rates of test species. They are also in excess of the contentin 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 innature are therefore probably rare. They, and yieldlimitations, 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 pmol L1 and 5 umol L1. 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 yield-limitation of species of the chrysophyte Dinobryon byhigher and naturally occurring K+ concentrations.