Grafham Water

Abstract

SUMMARY. The phytoplankton and zooplankton of Grafham Water reservoir, Cambridgeshire, are described and discussed for the 5 years 1974–78 together with the main physical variables and macro‐nutrients believed to affect them. This follows on from the Water Pollution Research Laboratory study of 1969–74. The rationale for the work was to provide short‐term warning of treatment and management problems caused by algae and to gain insight into the factors controlling them.During the period reviewed the 1976 drought occurred with a massive draw‐down. Afterwards replenishment of the lake from the River Great Ouse resulted in its enrichment with high concentrations of nitrate, orthophosphate and silica. The algal cycles changed to some extent over this period with a few species becoming even more dominant and others diminishing in importance. The effect of the drought and post‐drought pumping is discussed; a return to former nutrient levels seems likely. The effects of weather, particularly radiation and wind, on algae may be of equal importance to nutrient levels in Grafham and will be further investigated.