Monthly chemical and plankton samples were taken from three shallow freshwater dune waters on the westcoast of the Netherlands. In the Quackjeswater (origin ± 1500) no decalcification could be found. In the Breede Water (origin ± 1920) and the Vogelmeer (origin 1952) decalcification was heavy. The difference is related to a difference in submerged vegetation. In the Quackjeswater there is plenty of organic matter and a vegetation of aquatic mosses ( Fontinalis antipyretica); in the Breede Water and the Vogelmeer a vegetation of Chara sp., Myriophyllum and Elodea is found. There are certain differences in the composition of the plankton in the three dune waters, caused by a difference in density of submerged vegetation, difference in exposition to wind, light conditions, situation near the seashore, salinity and age.