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In reviewing the ecology of planktonic ctenophores, Walter (1976) pointed out how laboratory estimations of such parameters as growth, feeding and fecundity had been utilized to interpret population dynamics of ctenophores in the natural environment. In organisms with short life-cycles, which inhabit the water column, it is usually impossible to follow the growth and mortality characteristics of cohorts by the use of such techniques as size/frequency histograms. The observer can never be sure he is sampling the same population, over a period of time, and even if this were so, continuous breeding and rapid growth tend to quickly obscure the identification of distinct cohorts. It is also not possible to experimentally manipulate the population. This report describes an experiment in which plankton populations were studied in three containers for six weeks following empoundment of a water column and its natural plankton assemblage.