As repurification takes place there is a succession of different communities of algae: Stigeo-clonium tenue is dominant in the polysaprobic zone, Nitzschia palea and Gompheonema parvulum in the mesosaprobic, and Cocconeis placentula, Chaemaesiphon spp. and Uvella frequens in the oligosaprobic zone. The last community is the same as that in eutrophic water.