INTERACTIVE EFFECTS OF ENVIRONMENTAL-CONDITIONS ON THE GROWTH OF SUBMERSED AQUATIC MACROPHYTES

Variations in the growth of dioecious Hydrilla verticillata (L.f.) Royle and Vallisneria americana Michx. were examined in a greenhouse under each of two levels (low and high) of light availability, sediment fertility, and inorganic carbon supply. Under low light (ca. 125 uE/m2/s at midday), effects of sediment fertility and inorganic carbon availability on biomass production were relatively minor. Maximum biomass production in both species was achieved under high light (ca. 550 uE/m2/s at midday) on fertile sediment with a ten-fold increase in ambient CO2 supply. However, with other treatment combinations, biomass and morphological responses differed considerably between species. Biomass production in Hydrilla was most responsive to CO2 supply, while in Vallisneria, production was most responsive to sediment fertility. Shoot elongation in response to low light was much more pronounced in Hydrilla than in Vallisneria. Under high light, Hydrilla maximized shoot density (number) with an accompanying increase in biomass production. In Vallisneria, however, shoot (plant) density diminished with increased biomass production under both light levels. In nature, the outcome of competition between these two important submersed macrophyte species may be influenced significantly by interactions among the studied variables.