Diatoms on herbarium macrophytes as indicators for water quality

To describe the reference situation for water quality in the eutrophicated broad area De Nieuwkoopse Plassen (Holland), attached diatoms from aquatic macrophytes, collected between 1934 and 1958 have been used. Recent samples for comparison were taken in 1989.The diatoms do not indicate significant changes in salinity. Eutrophication has reduced the number of rare species and increased the abundance of meso- to hyper-eutraphentic species, particularly Fragilaria berolinensis. Reduction of total phosphorus to concentrations below 0.02 mg 1^–1 might be sufficient to restore the diatom assemblages of the 1930s.