When acid precipitation impacts freshwater systems, littoral blooms of the filamentous green alga Mougeotia (Zygnematales, Charophyceae) frequently develop. Field observations of its development in Little Rock Lake, an experimentally acidified seepage lake in north-central Wisconsin, indicated that the species of Mougeotia present there may have an optimum pH for growth of-5.2. Because a number of metals increase in concentration in acidified waters, we also proposed that this species of Mougeotia may be particularly tolerant of metals. To test these hypotheses, we grew Mougeotia from Little Rock Lake in culture at eight levels of pH (3 through 10) and measured net photosynthesis and respiration at each level. We also grew this isolate over a range of Zn concentrations from 10 to 2,400 pg liter- ’ and Al concentrations from 10 to 3,600 pg liter- I. Little Rock Lake Mougeotia grew over a broad range of pH levels from 3 to 9 with an optimum at 8, in contrast to our expectation. Mougeotia, however, was tolerant to levels of Zn and Al considerably higher than the elevated levels to which it had been exposed in the acidified basin of the lake. Mougeotia may grow luxuriously in acidified waters because of an efficient uptake of CO, and dissolved organic C and a lack of grazing pressure. The acidification of freshwater aquatic systems by aci
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