The relationship between chemically analyzed phosphorus fractions and bioavailable phosphorus1

To identify available phosphorus, phosphorus was measured by 2‐week bioassay in water from 39 sites in Ontario and Québec and regressed against total phosphorus and the phosphorus (P) in 11 analytically measured fractions (total soluble, total reactive, particulate, and soluble reactive P, both before and after autoclaving; and soluble reactive P after exposure to enzymes, to UV radiation, and to both). Covariance analysis showed that low‐P (<30 µg TP liter−1) lakes and rivers, P‐rich lakes, and P‐rich rivers required separate analysis. Total P was a consistently poor correlate of available P (r2 = 0.44–0.72). A best estimator for various subsets of the data was identified in terms of descriptive power (r2, F), normality, and analytical ease. At low concentrations of P, the fraction of choice is total reactive phosphorus, which explained 73% of the total variation in bioassay P. Total soluble phosphorus was the most useful correlate of bioassay P in eutrophic lakes (r2 = 0.98) and rivers (r2 = 0.95). The applicability of these regressions to waters other than those sampled or as predictors of lake eutrophication remains to be evaluated.