Aquatic macrophyte richness in Danish lakes in relation to alkalinity, transparency, and lake area

We examined the relationship between environmental factors and the richness of submerged macrophytes species in 73 Danish lakes, which are mainly smal l, 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 sp ecies-rich group of elodeids. Mean species richness per lake also increased significantly with increasing Secchi depth. No significant relationship be tween 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 m acrophytes in large lakes may be restricted by wave action in shallow water and light restriction in deep water. In contrast, macrophyte species richn ess increased with lake surface area in transparent lakes, presumably due t o expansion of the area colonised by submerged macrophytes. Thus, the size of the colonised area is a better predictor of species richness than lake s urface area. The strong increase in species richness accompanying greater t ransparency can be accounted for by the combined effect of higher colonised area and higher habitat richness along gradients of deeper macrophyte grow th.