Orthophosphate Concentrations in Lake Water: Analysis of Rigler's Radiobioassay Method

Abstract

Riger's radiobioassay method is frequently used to estimate maximum possible orthophosphate (P) concentrations in natural waters. An evaluation of the method, based on simulated P uptake by hypothetical phytoplankton communities, reveals that the Rigler value is not a consistent estimator of true maximum possible P concentration. Analyses show that all members of that family of curves for which the difference between true and assumed (or estimated) values of P is below the minimum half-saturation constant of a component species will pass through the plot's origin. A new upper bound, termed [Formula: see text] (Rigler), which is the sum of the true ambient P concentration and the lowest half-saturation constant of a component species, is identified as a consistently distinguishable bound on maximum possible P concentrations in lake water. The [Formula: see text] curve cannot be distinguished in a lake water experiment because of the complex behavior of uptake curves in the unobservable substrate region. A theoretical procedure, based on comparing uptake parameters for lake water samples and multispecific hypothetical communities, offers potential for calculating upper and lower limits on [Formula: see text] in P-limited lake water samples.