Ranking the effects of site exposure, plant growth form, water depth, and transparency on aquatic plant biomass

The maximum depth of macrophyte colonization and depth distribution of macrophyte biomass were as- sessed over 3 years, in late summer, at six sites in the St. Lawrence River and two sites in the Ottawa River (Lake des Deux Montagnes). Maximum depth of submerged plant colonization could be predicted from the light extinction coeffi- cient (r 2 = 0.82) and Secchi disk depth (r 2 = 0.80). The aboveground and total biomass of macrophytes were related to a variety of environmental variables as follows in descending order of importance: exposure to wind and waves, plant growth forms, water depth, and light intensity. Together, these variables accounted for 67 and 74% of sampling vari- ability of aboveground and total biomass, respectively. These environmental variables were used to elaborate hierarchi - cal predictive models of aboveground and total biomass of emergent and submerged macrophytes. The empirical relationship that links St. Lawrence River and Ottawa River aquatic plants to environmental variables may eventually allow us to forecast wetland response to changes in water levels and water clarity resulting from climate variability and (or) discharge regulation.