English freshwater records of Actinocyclus normanii (Greg.) Hustedt (Bacillariophyceae)

Abstract

The occurrence of Actinocyclus normanii in reservoirs and in the River Thames is recorded and the morphology of the cells is described. Attention is drawn to differences in the mean and maximum sizes of cells from different habitats, those from the river (including the estuary) being on average larger than those in reservoirs. It is suggested that factors apart from mean salinity levels such as calcium content, coupled with the occasional replenishment of populations by the introduction of fresh cells from brackish waters through artificial water transfers, may be concerned in controlling the English freshwater distribution of this species.