The Depletion of Carbon Dioxide from Lake Water by Phytoplankton

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Abstract
Photosynthetic activity of several planktonic algae can bring about markedly alkaline and CO2-depleted conditions in the surface waters of 2 English lakes with low alkalinity (0.2-0.5 meq l-1). Examples are described, chiefly connected with the late spring growth of Fragilaria crotonensis in Windermere South Basin and the late summer maxima of Ceratium hirundinella and Microcystis aeruginosa in Esthwaite Water. In both lakes the pH may rise at least 2 units above the air-equilibrium level (to > 10 in Esthwaite Water), and estimated concentrations of free CO2 [H2CO3] fall below 0.5 .mu.M. In Esthwaite Water, more than half of the total CO2 (CT) may be consumed. Rates of photosynthesis of 5 spp. (of Asterionella, Melosira, Fragilaria, Ceratium and Microcystis) in CO2-depleted and CO2-enriched media are described and compared. Uptake of CO2 by the same species was followed with pH-drift and pH-stat experiments, in lake water media at various levels of alkalinity, natural or modified. Nearly complete depletion could be achieved by populations of Ceratium, Microcystis and Fragilaria, sampled from alkaline lake phases. The final pH could then be raised by increase of alkalinity, but above pH 11 there was evidence of limitation by pH per se and hence more incomplete CO2 utilization. The relationship between uptake rate and the concentration of CO2-forms strongly indicated the direct utilization of HCO3 ions in these species, although at low pH the uptake of free (gaseous) CO2 might enhance rates, and was probably predominant for F. crotonensis. Asterionella and Melosira were capable of less complete CO2 depletion, and the kinetic data indicate a dependence upon free CO2 only, with a higher half-saturation constant (Kt) for the Melosira. Values of Kt are also influenced by the uptake capacity at saturation (Vmax); relationships with the total CO2 flux resistance and the CO2 compensation point are considered. The compension points for Asterionella and Melosira were generally much higher than the concentrations reached during maxima of algae (Microcystis and Ceratium) presumed capable of direct HCO3 utilization. The different responses may therefore be of significance for species succession and community composition. Some wider ecological implications are discussed.