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Summary
This paper is an attempt to answer the question as to what degree of stability is shown by the plankton populations of a lake, over a rather long period of time.
A comparison has been made between the biocoenotic pattern of the plankton populations of Entomostraca in the Lago Maggiore, caught during the years 1909 to 1913 and the years 1948 to 1950. Supplementary data are furnished by catches of 1940.
Only very small pattern changes can be observed over the period of nearly forty years. The general features of the population seem to remain the same during this time. Only a small increase was found in the numbers of Diaptomida and of the most frequent Cladocera (Daphnia and Diaphanosoma). It seems probable, on the contrary, that Heterocope and Bytotrephes have undergone a decrease.
The author thinks that the stability of biocoenotic pattern in a lake, within a short lapse of time (of the order of a man's life) is a function of the stability of its ecological conditions and more generally of the hydrographic stability of the lake.