Some effects of artificial mixing on the dynamics of phytoplankton populations in large limnetic enclosures

Abstract

Effects of a series of artificial mixings of part of the water column of a limnetic enclosure (‘Lund Tube’ C, Blelham Tarn, Cumbria) upon the rates of net change in the standing populations of selected species of phytoplankton are described. Increase of Sphaerocystis and Anabaena was relatively faster under the ‘quiescent’, stratified conditions of unmixed columns whereas mixing generally favoured the net increase of diatoms (e.g., Fragilaria) or, at times, of Oscillatoria. The sequences corresponded closely to supposed successional pathways, which are thus suggested to be naturally regulated by seasonally-changing physical characteristics of the water column. Performance characteristics of the mixing apparatus and a mathematical expression of the net rate of change in natural Fragilaria populations are appended.