An investigation of phytoplankton and environmental conditions in a new Thames Valley reservoir, subject to artificial turbulence

Abstract

Development and decline of phytoplankton populations was investigated in Wraysbury Reservoir between October 1971 and December 1974. This reservoir is the second to be supplied by the River Thames via a choice of jet or low velocity inlets. Substantial periods of artificial mixing have supplemented natural wind-induced turbulence, thus usually preventing thermal and chemical stratification and creating a high mixed depth/euphotic depth ratio. Input of eutrophic River Thames water provides a more or less constant supply of nutrients: a phytoplankton succession characteristic of enriched waters developed although the species were not those typical of the river Diatom growths predominated, in particular the centric diatoms Steuhanodiecus astraea (Ehr) Grun and Melosira granulata Ehr. Blue-green algae were conspicuous during the summer months. A similar seasonal distribution of phytoplankton at different stations resulted from the mixing process. Inverted microscope, counts of Stephanodiscus populations and the use of the Coulter Counter suggested near homogeneity with depth when the diatom cells were healthy: increased heterogeneity through the water column accompanied decline of populations and cell sedimentation. Ageing of Stephanodiscus cells was accompanied by changes in pigment composition; large growths (800 cells per ml) could substantially modify the vertical attenuation coefficient. Consideration of the silica budget has suggested that 100 cells per ml "require" approximately 1.5 pgm S10. Primary productivity experiments in the field suggested that reduced illumination restricted growth at and near the bottom of the reservoir for much of the year. Laboratory cultures of mud samples shoved the presence of numerous algae. Whereas the centric diatom Cyclotella ref. meneghiniana Kutz. was maintained relatively easily in laboratory cultures, similar attempts to grow Stephanodiecus astraea were unsuccessful.