Predictability of Phosphorus Load, Hydrological Load and Lake Total Phosphorus Concentration

Abstract

ABSTRACT One goal for eutrophication research is to predict lake trophic state without extensive field programs. However, existing comparisons of predicted:observed TP (total phosphorus) concentration calculate the former from measured watershed characteristics that require field sampling. Predictions of lake TP based on estimated rather than measured components are probably much less powerful, but this possibility should be quantitatively assessed. To assess the predictive power of current mass-balance models of eutrophication, measured estimates of volumetric water discharge (Qobs: m3·yr−1, n=110) and phosphorus load (Lobs: mg·m−2·yr−1, n=96) from lakes in the OECD (Organization for Economic and Cooperative Development) eutrophication data set were compared to calculated values of Q (Qcal) and 16 different variants of calculated phosphorus loading (Lcal, respectively. For the entire data set, Qcal was unbiased and strongly correlated with Qobs. In regional comparisons, some Lcal variants correlated well with Lobs, but others did not, and no single variant was significantly better than all of the others. In these comparisons, up to 95% of the variance in Lobs was explained by a single regional Lcal variant, but the best regression of a single variant against Lobs for the entire data set explained only 58% of its variance. Stepwise multiple regression to improve prediction of Lobs produced the following model; Log Lobs = 1.23 + 0.81 (log Lbest) + 0.19(G) − 0.42(%F) − 0.38(%U)− 0.11 (log Ad); R2 = 0.75, S xy = 0.37, P < 0.001 where; Lbest – a composite variable consisting of the best performing Lcal variables in geographically limited regressions, G – watershed geology (1-igneous, 1.5-mixed igneous-sedimentary, 2-sedimentary), %F and %U – percentage of the watershed covered by forest and urban areas, respectively, and Ad – drainage basin area (m2). Finally, when a phosphorus budget model was used to predict lake TP concentration with (A) estimated components, (B) estimated components where Lcal, was adjusted for bias with the above equation, and (C) measured components, predictions of lake TP from all three estimates were of equal accuracy indicating that no estimate was better than the others.