Crowded stands of submerged plants in Québec lakes have a weight-density with a shallower, but not significantly different, slope and an intercept 10 fold lower than those for terrestial stands. The examination of a larger data set including both freshwater and submerged macrophytes supported these differences. This data set suggested that those differences, as well as those among submerged stands, are largely attributable to the light levels incident upon the stand. The differences in weight-density relationships between submerged and terrestrial stands were paralleled by a smaller biomass per unit volume outgrown of the submerged plants, as predicted from the dimensional examination of the weight-density relationship. The variations in biomass per unit volume associated to differences in the species growth form explained deviations about the weightdensity relationship of stands growing under relatively similar light conditions.