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We monitored the biomass and distribution of submerged macrophyte communities in five lakes in the Eastern Townships of Quebec, Canada, in two climatically different growing seasons. Early season warm temperatures allowed for much deeper macrophyte colonization (25–170%), an average of 300 g m−2 greater wet weight biomass (g m−2) and a 45–1160% increase in whole lake biomass in warm 1998 compared to a cooler 1997. In contrast, the inter-year variation in underwater light climate had no effect on macrophyte colonization as water column turbidity was generally higher in the warmer year, although irradiance did affect the distribution and biomass within seasons. The positive relationship between phytoplankton and whole system macrophyte biomass observed in this study cautions that the negative relationship in highly eutrophic shallow systems does not appear applicable to deeper systems characterized by lower macrophyte biomass to lake volume ratios. Earlier growing seasons, as predicted by climate change models, would result in greater biomass and distribution of submerged macrophyte communities, thereby modifying the structure and functioning of north temperate lakes. The effect will be a function of lake morphometry and most pronounced in shallow systems.

Maximum summer biomass and morphology of Myriophyllum spicatum L. were influenced by both sediment composition and irradiance. Plants were grown in Lake Memphremagog, Quebec-Vermont along a natural depth gradient (1.5, 2 and 3 m) in buckets containing 100% fertile marsh sediment, 10% marsh sediment: 90% sand, or 100% sand. Shoot biomass responded to changes in sediment composition regardless of water depth. However, irradiance responses occurred only in the 100% marsh sediments. While sediment composition, irradiance and their interaction explained 91% of the total experimental variation, approximately 75% of the shoot biomass variation was attributable to sediment composition alone. Depth and the interaction term were each responsible for only 8%. These results appear consistent with both the morphology and natural distribution of M. spicatum .