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

Stephane Masson, Bernadette Pinel-Alloul, Val H. Smith

Limnol. Oceanogr., 45(3), 2000, 732-740 | DOI: 10.4319/lo.2000.45.3.0732

ABSTRACT: The study is a first attempt to generate a series of Total Phosphorus-Chlorophyll a (TP-Chl a) predictiveOC models relating the quantitative responses of four algal size fractions to phosphorus gradients. The study was carried out in 27 glacial lakes from two regions in southern Quebec, the Laurentians and the Eastern Townships, and covered a relatively modest range of trophic conditions (TP, 3-34 µg P L-1; Chl a, 0.3-7.6 µg 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 mm, nanophytoplankton 3-20 mm, nanophytoplankton plus picophytoplankton <20 mm (edible fraction), and microphyto-plankton >20 mm (inedible fraction). We tested the hypothesis that the slopes of the TP-Chl a regression models developed for algal size fractions would increase consistently from the smallest to the largest algal size fraction, as suggested by the first half of the sigmoidal TP-Chl a models. Although there was no consistent trend in the magnitudes of the slopes of TP- Chl a relationships for picophytoplankton (slope = 1.14), nanophytoplankton (0.93), and microphytoplankton (1.22), Chl a concentrations in the largest size fraction increased more rapidly with phosphorus 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 enrichment on microphytoplankton is more pronounced in well-buffered lakes, whereas TP enrichment has a stronger effect on nanophytoplankton in low-alkalinity lakes. The effects of alkalinity may be the result of either a pH influence on phytoplankton carbon uptake or a stronger top-down grazing effect on small algae in well-buffered lakes.

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