WILLIS L. TRESSLER
UNIVERSITY OF BUFFALO, BUFFALO, N. Y.
THE important part played by the animal members of the plankton is difficult to overestimate. It is they which form an intermediate link between the producing green plants of the water, or perhaps between the available organic matter, and the larger aquatic animals. The organisms comprising the zooplankton include the Copepoda, Cladocera, Rotifera, and Protozoa, together with other groups of somewhat lesser importance such as the Ostracoda and Phyllopoda. The problem with which limnologists are concerned is the attainment of a complete knowledge of the structure, behavior and interrelationships of these animals. This knowledge is of interest for its own sake and it is also important in a practical way as a tool in the increased production of fish life. Our information regarding the zooplankton was until recently of little practical importance, since nothing could be done about a lake or pond poor in plankton. Within the past few years experimentation with fertilizers has shown great increases in plankton production, and as a direct result in added rate of growth of fish, so that now we may say that not only are we familiar with the disease but in many cases we are able to effect a cure.
Before going into the well-known facts concerning the life and habits of the zooplankton, let us concern ourselves briefly with the ancient origin of fresh-water plankton in general. According to Welch ( 1935), there are two possibilities: plankton originated in ancient geologic ages in the ocean, or, as some believe, it may have had a fresh-water origin in shallow pools. We know little about the composition of the ancient seas and it is possible, according to one theory, that these waters were quite fresh and have gradually become saline in the course of time. If this were true, plankton could have migrated from the relatively fresh-water seas into the inland waters without the barrier imposed by a difference in salinity. Since then the two realms have developed along diverging lines, each with its own plankton. As to the age of freshwater plankton, there is a great difference of opinion; some authorities contend that all fresh-water plankton in temperate regions goes back only to the ice age, when it migrated from the Arctic regions. Others hold that fresh-water plankton is of much greater antiquity than the ice age and that while it may have been forced to leave certain areas, it reentered the old regions after the end of the ice age. In any event, fresh-water plankton must have come into fresh water before the arrival of the higher aquatic animals, since these form's are dependent upon the plankton for food. The question of the routes taken by plankton in entering fresh water has caused considerable speculation. Three theories in general have been advanced to explain the path of the migrants. The tropical origin theory has few adherents. The other two possibilities are that of a polar origin under conditions of greatly varying salinity and temperature, where the organisms became accustomed to a varying environment, and the theory put forth by Wesenberg-Lund ( 1926) that modern pelagic forms have been developed and are still being developed from bottom dwelling, and littoral organisms.
While plankton is found in almost all natural waters, its distribution is subject to wide variation. Geographical differences, differences due to the depth of the water, variations at different locations on the lake, and seasonal changes are all very marked.