THE ZOOPLANKTON IN RELATION TO THE 

 METABOLISM OF LAKES 



By WILLIS L. TRESSLER 



UNIVERSITY OF Bl'FFALO, BUFFALO, X. Y. 



Introduction 



The important part played by the ani- 

 mal 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 be- 

 tween the available organic matter, and the 

 larger aquatic animals. The organisms 

 comprising the zooplankton include the 

 Copepoda, Cladocera, Rotifera, and Proto- 

 zoa, 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 interrelatioiiships of these 

 animals. This knowledge is of interest for 

 its OAvn sake and it is also important in a 

 practical way as a tool in the increased pro- 

 duction of fish life. Our information re- 

 garding 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 ex- 

 perimentation 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 man>- 

 cases we are able to effect a cure. 



Before going into the well-known facts 

 concerning the life and habits of the zo- 

 oplankton, 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, plank- 

 ton could have migrated from the relatively 

 fresh-water seas into the inland waters with- 

 out 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 fresh- 

 water 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 mi- 

 grated from the Arctic regions. Others 

 liold 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 forms are de- 

 pendent upon the plankton for food. The 

 question of the routes taken by plankton 

 in entering fresh water has caused con- 

 siderable 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 

 tlie theory put forth by Wesenberg-Lund 

 (1926) that modern pelagic forms have 

 been developed and are still being de- 

 veloped from bottom dwelling and littoral 

 organisms. 



Distribution of Zooplankton 



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. 



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