Submersed macrophyte growth at low pH : II. CO2 × sediment interactions.

The submersed macrophyte Vallisneria americana was grown for seven weeks in a greenhouse to test for differences in the ability of three different sediments to support growth stimulation in response to CO2 enrichment at low pH. Plants accumulated 21- to 24-fold greater biomass at 10 × ambient CO2 concentrations than at ambient CO2 on all sediments. At both CO2 levels, plants grown on sediment from an acidified lake accumulated ca. 81%, and those grown on oligotrophic lake sediment ca. 47% as much biomass as plants grown on alkaline lake sediment. Despite striking CO2 and sediment effects on biomass accumulation, there was no significant interaction (using log-transformed data) between CO2 and sediment effects, indicating that all sediments allowed similar proportionate growth responses to CO2 enrichment. Plants grown on the less fertile sediments showed greater relative allocation to horizontal versus vertical growth by producing more rosette-bearing stolons in relation to plant height (leaf length) than plants grown on relatively fertile, alkaline lake sediment. Tissue analysis suggested that sediment effects on Vallisneria growth could be attributed neither to mineral putrient (nitrogen and phosphorus) limitation nor to aluminum toxicity in these low pH treatments. In any case, CO2 availability can be an important regulator of submersed macrophyte growth at low pH on a variety of sediment types, including those from oligotrophic and acidic lakes.