. The Canadian field-naturalist. 1978 HEALEY: SPHAERIID MOLLUSC POPULATIONS, NWT 243. Figure \. Map of the Yellowknife area. Northwest Territories, showing the locations of the eight lakes where abundance of sphaeriid molluscs was studied. however, became anoxic under the ice in winter (G. Brunskill, personal communication). Surface temperature reached 18-20°C in July and epilimnion temperatures were commonly 16-18°C throughout the summer. The littoral zone (as defined by Ruttner 1963) was of limited extent in all the lakes except shallow Long and Frame. Littoral substrates were generally rock
. The Canadian field-naturalist. 1978 HEALEY: SPHAERIID MOLLUSC POPULATIONS, NWT 243. Figure \. Map of the Yellowknife area. Northwest Territories, showing the locations of the eight lakes where abundance of sphaeriid molluscs was studied. however, became anoxic under the ice in winter (G. Brunskill, personal communication). Surface temperature reached 18-20°C in July and epilimnion temperatures were commonly 16-18°C throughout the summer. The littoral zone (as defined by Ruttner 1963) was of limited extent in all the lakes except shallow Long and Frame. Littoral substrates were generally rocky or sandy and there was little rooted vegetation in the deeper lakes. Frame and Long lakes had soft substrates below 1-m water depth. In the other lakes rocky and sandy sediments in the shallow littoral were replaced by clay and moss in the deep littoral and shallow profundal and silt in the deep profundal. Healey and Woodall (1973a, b), Healey and Kling (1975), and Healey (1977), give more details of the physical, chemical, and general biological characteristics of the lakes. Materials and Methods I took all samples with an Ekman grab, 15 X 15 X 22 cm tall. If the jaws or top doors of the grab did not close, or the surface of the sediment was within 5 cm of the top of the grab I repeated the sample. I sieved the samples in the field through a 600-^m mesh screen and preserved the sieve contents in 5% formalin for later sorting by hand under lOX magnification. I sampled all eight lakes in 1971, but only Alexie, Baptiste, Chitty, and Drygeese in sampled the lakes twice in 1971 between June and August, the visits to each lake being about 1 month apart, except for Michel which I sampled only once (July). I took 10 grab samples at four different locations in shallow lakes on each visit. In deep lakes I took five samples from the littoral Table 1—Physical and chemical features of the eight lakes Area, ha Depth (m) Secchi (m) pH Conductivity, Lake Maximum Mean Range Mean /
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