Total phosphorus-chlorophyll a size fraction relationships in southern Quebec lakes

The study is a first attempt to generate a series of Total Phosphorus-Chlor ophyll a (TP-Chl a) predictive models relating the quantitative responses o f four algal size fractions to phosphorus gradients. The study was carried out in 27 glacial lakes from two regions in southern Quebec, the Laurentian s and the Eastern Townships, and covered a relatively modest range of troph ic conditions CTP, 3-34 mu g P L-1; Chl a, 0.3-7.6 mu g L-1). Algal biomass was estimated using measurements of Chl a, and the total Chl a was divided into four operational size fractions: picophytoplankton <3 mu m, nanophyto plankton 3-20 mu m, nanophytoplankton plus picophytoplankton <20 mu m (edib le fraction), and microphytoplankton >20 mu m (inedible fraction). We teste d the hypothesis that the slopes of the TP-Chl a regression models develope d for algal size fractions would increase consistently from the smallest to the largest algal size fraction, as suggested by the first half of the sig moidal TP-Chl a models. Although there was no consistent trend in the magni tudes of the slopes of TP-Chl a relationships for picophytoplankton (slope = 1.14), nanophytoplankton (0.93), and microphytoplankton (1.22), CN a conc entrations in the largest size fraction increased more rapidly with phospho rus enrichment than in either of the smaller fractions. When included as an additional variable, lake water alkalinity improved the prediction of Chl a and presented differential effects on size fractions. The effect of TP en richment on microphytoplankton is more pronounced in well-buffered lakes, w hereas TP enrichment has a stronger effect on nanophytoplankton in low-alka linity lakes. The effects of alkalinity may be the result of either a pH in fluence on phytoplankton carbon uptake or a stronger top-down grazing effec t on small algae in well-buffered lakes.