Biomass and production of Pontederia cordata and Potamogeton epihydrus in three Connecticut rivers.

Maximum biomass of Pontederia, averaging 1212g m^-2 dry weight (524g m^-2 above-ground, 688g m^-2 below-ground), occurred 100-150 days after major spring growth began. Peak biomass of Potamogeton averaged 94g m^-2 (81g m^-2 above-ground, 14g m^-2 below-ground) and was attained in 45-85 days. New growth of Pontederia in spring arose from, and was heavily subsidized by, the large biomass of living overwintered rhizomes and roots, which averaged 497g m^-2 in early June. This new growth appeared to have been produced in only one season, but it contained energy fixed the current season, plus energy carried over from previous years. Net production of Pontederia calculated for only one growing season averaged 1049g m^-2. Potamogeton also perennated from rhizomes, but the biomass of these organs in spring was low, averaging 11g m^-2 in late May. Biomass of Potamogeton in summer consisted primarily of tissue produced during the current season. Rhizomes and roots comprised a much greater proportion of the plant in Pontederia. The ratio of new living below- ground/above ground biomass of Pontederia rose from zero in spring to an average of 1.71 in autumn. For Potamogeton, the below-ground/above-ground ratio averaged 0.37 in late spring, 0.20 in midsummer, and 0.41 in autumn. The overwintered below-ground biomass of Pontederia alive in spring was 42-79% iof the new living below-ground biomass the previous autumn. Net photosynthetic efficiency during the period between initiation of major growth in spring and attainment of peak biomass averaged 1.3% for Pontederia and 0.3% for Potamogeton.- from Authors