Quantitative studies on the phytoplankton of the Rivers Thames and Kennet at Reading

Summary The phytoplankton of the River Thames and its tributary the River Kennet was studied weekly from May 1966 to May 1968. Samples were concentrated by sedimen‐tation and counted in a haemocytometer. Data on discharge, temperature and silica concentration are followed by descriptions of the variations in the number and composition of the phytoplankton. In both rivers there were spring and autumn peaks of the centric diatom. Stephanodiscus hantzschii. Chlorophyceae were most abundant during summer. Two cryptophyceans, Cryptomonas and Rhodomonas were sometimes numerous. In the Thames, the size of the population was closely correlated with the discharge, highest numbers always occurring during periods of low discharge. In the Kennet, increases in discharge often brought about increases in cell number due to the infiux of benthic forms. It appeared that nutrients were always present in sufficient quantity and did not limit algal growth. Comparisons were made with earlier studies on the Thames and differences were noted in the composition of the phytoplankton which were almost certainly due to the process of eutrophication.