Growth and morphology of submersed freshwater macrophytes in relation to light and temperature

Variations in the growth and morphology of three North American freshwater macrophyte species, Elodea canadensis Michx., Potamogeton nodosus Poiret (P. americanus), and Vallisneria americana Michx., were investigated in large white fiberglass tanks over broad experimental ranges of greenhouse light (ca. 100–1500 μE∙m 2 ∙s −1 as midday maximum photosynthetically active radiation (PAR)) and temperature (12–32 °C). Response variables considered in the study included biomass production, shoot density, shoot length, and various characteristics of leaf form.Total biomass production in all species generally increased with both increasing light and increasing temperature (to at least 28 °C). However, variations in biomass and other parameters as well were highly influenced by the interactive relationship between light and temperature. Shoot density correlated directly with biomass production in these species under all experimental conditions. In contrast, light and temperature elicited opposing responses in shoot length, which decreased with increasing light but increased with increasing temperature. The extent of variations in leaf morphology differed among species, apparently reflecting intrinsic differences in heterophyllous capabilities.Light and temperature appear to interact in influencing lower limits of macrophyte depth distribution and related variations in the duration of seasonal growth. Temperature is likely to assert a strong influence on the latitudinal distribution of submersed freshwater macrophytes, dependent upon species-specific differences in response to temperature and basic differences in macrophyte life cycle.