Changes in flora down a stream showing a zinc gradient

The results are summarized of a study of the chemistry and flora of Gillgill Burn, Cumbria, a stream showing a steady decrease in zinc levels from the source. There was a tendency for the number of species to increase on passing down the zinc gradient, but the effect was not very marked. The change in species composition on passing downstream was more marked. The maximum number of species recorded in any reach with high zinc levels (about 3 mg per litre) was 41, while a reach of an unpolluted tributary had 61 species . The lowest number of species in any reach was 25. The extent to which the changes in flora are due to the zinc levels, or to other variations, such as the substratum and the flow regime, is discussed.