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Abstract
Daphnia populations from the region of Yellowknife (N. W. T.) on the Great Slave Lake, from Tuktoyaktuk (N. W. T.) on the coast of the Beaufort Sea east of the Mackenzie-Delta, and from Shingle Point (Y. T.) west of the Makenzie-Delta were described. Population diagrams were given for the beginning of September 1974. The populations found at the above mentioned sites were compared with Daphnia pulex from Spitsbergen and the German island of Spiekeroog. Though some characteristics of Daphnia middendorffiana FISCHER occurred, mainly in individuals from very northern ponds, these were always found in individuals which also possessed some of the characteristics of Daphnia pulex. In every pond, specimens with characteristics of Daphnia middendorffiana lived together with other individuals resembling Daphnia pulex. Males were found in three out of the four visited ponds on the shore of the Arctic Sea, and were apparently Daphnia pulex. AIl the material sampled in northern Canada was regarded as belonging to Daphnia pulex LEYDIG 1860 emend. RICHARD 1896. There was evidence, that characteristics of Daphnia middendorffiana develop under the influence of environmental conditions, such as food supply. Daphnia magna STRAUS was found far north of the arctic circle.
