REPRODUCTION IN 2 SUBMERSED MACROPHYTES DECLINES PROGRESSIVELY AT LOWPH

1. Greenhouse experiments tested the effect of pH 5 v pH 7.5 on reprod uctive success for the freshwater macrophytes Najas flexilis, an annua l, and Vallisneria americana, which perennates as a tuber. 2. Seed pro duction by the small Najas plants grown at pH 5 averaged 0.25 seeds/pl ant, in contrast to 95.5 seeds/plant at pH 7.5. At low pH, Vallisneria grown from seeds produced no flowers and too few tubers to replace th emselves, so that sexual reproduction failed nearly completely for bot h species. 3. Vallisneria grown from tubers produced 97% less total tu ber mass at pH 5 (0.4 v 14.9 g), the compounded result of producing, o n average, 89% fewer and 82% smaller tubers. The smaller tubers develo ped at pH 5 were less likely to overwinter in the field, and those sur viving tubers subsequently grew into smaller plants. 4. These findings generated the hypothesis of a closing spiral: growth at low pH (and r elatively low CO2 concentrations) results in small plants that produce a smaller number of progressively smaller tubers each autumn, which i n turn develop into progressively smaller plants each summer. This hyp othesis was supported by field transplant experiments in two acidic Ad irondack Mountain (NY) lakes.