Lake Winnipeg, remnant of glacial Lake Agassiz, an efficient dispersal route of planktonic crustaceans through central and northern Canada

Zooplankton in five lake regions once inundated by Lake Agassiz were compared with zooplankton of 10 other lake regions and in 5 Great Lakes in south—central Canada. The highest resemblance to a hypothetical composite of Lake Agassiz plankton, from 94 to 100 percent, was found in lakes of the Northern Manitoba, Northern Saskatchewan and Upper Mackenzie regions. Eighty nine to 94 percent of zooplankton from the Laurentian Great Lakes came from the Agassiz Lake fauna. The only other species found in the Great Lakes in 1969 were Eurytemora affinis, and Eubosmina coregoni, newcomers to the North American fauna from Europe. The number of species shared with the hypothetical fauna of Lake Agassiz declines with the distance from this glacial lake. From 91 to 93 percent of Lake Agassiz species were found in Manitoba Western, Southern Manitoba and Alberta, 85 to 86 percent in New Brunswick and Newfoundland, 80 to 84 percent in Nova Scotia and Southern British Columbia and about 74 percent in the lakes of the Mackenzie Delta region. Lake Agassiz appeared to be a very efficient south-north dispersion route. Of the 35 species originating from the Mississippi refugium, 24 species penetrated as far as the Upper Mackenzie region and 16 species reached the Mackenzie Delta. The north to south dispersion route was not as effective. Of the 15 species originating from the Beringia refugium, seven species reached the Upper Mackenzie area and 5 species penetrated only to Northern Manitoba and Northern Saskatchewan. The fact that they did reach so far south could be explained by two different hypotheses. In the first, Lake Agassiz did not extend as far north as suggested by Teller or Fisher and Smith but rather the northern boundary corresponded to that indicated by Elson, and Northern Manitoba and Northern Saskatchewan were part of Lake McConnell. The second hypothesis states that the most northern portion of Lake Agassiz according to the boundary suggested by Teller or by Fisher and Smith served in the very late stage as a wide connecting channel with Lake McConnell and allowed some Beringia species to penetrate south, but no further than to Reindeer Lake, or perhaps to Southern Indian Lake.

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