We examined the relationship between environmental factors and the richness of submerged macrophytes

species in 73 Danish lakes, which are mainly small, shallow, and have mesotrophic to hypertrophic conditions. We

found that mean species richness per lake was only 4.5 in acid lakes of low alkalinity but 12.3 in lakes of high alkalinity

due to a greater occurrence of the species-rich group of elodeids. Mean species richness per lake also increased significantly

with increasing Secchi depth. No significant relationship between species richness and lake surface area was

observed among the entire group of lakes or a subset of eutrophic lakes, as the growth of submerged macrophytes in

large lakes may be restricted by wave action in shallow water and light restriction in deep water. In contrast,

macrophyte species richness increased with lake surface area in transparent lakes, presumably due to expansion of the

area colonised by submerged macrophytes. Thus, the size of the colonised area is a better predictor of species richness

than lake surface area. The strong increase in species richness accompanying greater transparency can be accounted for

by the combined effect of higher colonised area and higher habitat richness along gradients of deeper macrophyte

growth.