The role of carbon dioxide in the initiation and maintenance of blue‐green dominance in lakes

1. In 1993 the south basin of Squaw Lake, Wisconsin, U.S.A., was artificially circulated and injected with CO 2 in an attempt to eliminate the massive quantities of blue‐greens normally present during summer. The unmixed, uninjected north basin was the control. Despite a great difference in CO 2 concentration and in pH between the two basins, their blue‐green maxima began simultaneously and eventually reached the same size. The predominant algae in both basins were Aphanizomenon flos‐aquae and Anabaena flos‐aquae . 2. From 13 May to 9 September, experiments were done to determine the responses of photosynthesis (oxygen production) of the phytoplankton communities in both basins to changes in pH over the range 7–9. Both the north and south basins underwent two distinct shifts in the slopes of their photosynthetic responses to pH. These shifts paralleled changes in the proportion of blue‐greens in their phytoplankton. 3. Experiments with additions of KHCO 3 or NaHCO 3 showed that the responses to pH were really responses to CO 2 concentration. Therefore, the data obtained in the photosynthesis experiments were used to calculate the kinetic parameters K s * and V max *. The K s * values varied in relation to the proportion of blue‐greens, with lower values during the period when blue‐greens predominated. The V max *values showed no such systematic changes. 4. CO 2 compensation and zero photosynthesis concentrations confirm that when the blue‐greens dominated, CO 2 was taken up much more efficiently than when the phytoplankton comprised mostly non‐blue‐greens. No evidence appeared suggesting direct use of bicarbonate even at pH values of 10.0. 5. Experiments with water collected seasonally from four other lakes show that Squaw Lake is not unique. In each case the response slopes (and therefore K s * values) were high during spring when non‐blue‐greens were dominant, but decreased when the blue‐green maximum began. 6. Experiments with individual species of algae confirm that blue‐greens generally have better CO 2 kinetics than do greens. 7. It is concluded that initiation of the blue‐green maximum does not depend upon conditions of low CO 2 concentration or high pH. However, once the blue‐greens become abundant they ensure their dominance by reducing concentrations of CO 2 to levels available only to themselves.