Sinking movements of phytoplankton indicated by a simple trapping method

Recoveries of planktonic algae in a vertical series of small traps are analysed in relation to changes in the standing populations and to the physical and chemical characteristics of a shallow, stratified lake. Many motile or buoyant species were trapped predominantly within restricted depth ranges. The results are interpreted as representing the vertical ranges within which the species were directing their downward movements. Ceratium hirundinella, a species showing more wide-ranging movements, was trapped throughout the epilimnion. Non-motile diatoms were caught at all depths. Deduced activity ranges are similar to vertical distributions observed in earlier years and are invoked to support the view that, in a stratified lake, typical movements may play an important role in algal growth and succession.