Submersed macrophyte growth at low pH : I. CO2 enrichment effects with fertile sediment.

Vallisneria americana was grown for six weeks in a greenhouse on relatively fertile sediment to test for factors other than nutrient limitation which may slow growth of this submersed macrophyte at pH 5. On the basis of dry mass accumulated, (1) low pH significantly depressed Vallisneria growth at constant free CO2 levels; (2) free CO2 enrichment, however, greatly stimulated Vallisneria growth at pH 5, by 2.8-fold and 10-fold at 3.2 times and 10 times air-equilibrated CO2 levels, respectively; and (3) growth was greater by far at pH 5 than at higher pH with constant total dissolved inorganic carbon (DIC). Free CO2 availability was thus an important controller of growth at low pH by Vallisneria americana on fertile sediment, and low pH was not directly deleterious. Field surveys of acidic lakes in the Adirondack Mountains of New York state revealed that DIC levels in low pH lakes were often well above equilibrium values and could potentially support vigorous macrophyte growth. Aluminum and/or iron toxicity did not appear to impair growth at low pH, and aluminum concentrations in Vallisneria shoots significantly decreased with increasing free CO2 concentrations at pH 5.0, perhaps due to growth dilution. Rosette production (a measure of asexual reproduction), maximum leaf length, and extent of flowering within treatments were positively correlated with plant biomass, rather than with pH or free CO2 levels per se.