The Bottom Fauna of Great Slave Lake

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Abstract
THE macroscopic bottom organisms of Great Slave Lake were sampled with more than 600 dredgings in the summers of 1944 to 1947. Additional ecological information was obtained from qualitative collections and from the examination of stomach contents of fish. The large area (10,500 sq. mi.) and extreme depth (2,000 ft.) of the lake necessitated special equipment and sampling procedures which are described.The bottom fauna of Great Slave Lake is typical oligotrophic, dominated by the amphipod Pontoporeia affinis and including several species of sphaeriids (Pisidium), oligochaete worms and chironomid larvae (mostly Spaniotoma). In these respects the bottom population resembles those of Lakes Athabaska, Reindeer, Winnipeg and Michigan. In the extreme depths, 200–600 metres, of Great Slave Lake the bottom fauna is made up of single species of amphipod, ostracod, nematode, oligochaete, sphaeriid and chironomid. Attention is drawn to the contrast between this restricted fauna of a recent glacial lake and the great numbers of species found in ancient lakes such as Baikal and Tanganyika.The average bottom population for the whole lake is 1,603 organisms per square metre and 3.7 kg. dry weight per hectare. However, in Yellowknife Bay the population is double this general average, while in McLeod Bay, of the east arm, it drops to one-quarter. Edaphic, climatic and morphometric factors are discussed in explaining these differences between areas. The bottom population of the upper 50 metres is fairly heavy. It declines rather rapidly from 50 to 150 metres, but is relatively constant from 200 to 600 metres.The heavily silted area off the Slave River Delta has a bottom population heavier than that at similar depths elsewhere in the main lake. Oligochaetes and nematodes are more numerous in the silted area while molluscs and ostracods are much reduced. Bottom organisms provide about 40 per cent of the food of the main fish in the lake. Two species of whitefish and two species of suckers obtain more than 90 per cent of their food from bottom organisms.The average weight of bottom organisms in ten large lakes has been compared and found to show an inverse relation to mean depth. A formula has been derived and a curve drawn to describe this relationship. The implication is that productivity is dominated by morphometric conditions in these large, deep lakes.

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