Also occasionally occurring in the samples 

 were: Polyarthra vulgaris Carlin, an unidenti- 

 fied ostracod, and an unidentified hydracarina. 

 Photographs of seven of the species are shown 

 in figure 3. 



RESULTS AND DISCUSSION 



Depth Distribution 



Although Bare Lake is relatively shallow 

 (7.5 m. ), a difference was found among the 

 vertical distribution patterns of individual 

 species of zooplankton organisms. The vertical 

 distribution of zooplankton as a whole was 

 fairly uniform however (table 1, fig. 4). Tem- 

 perature apparently has little effect on vertical 

 distribution at Bare Lake, as there was rarely 

 more than 1.50C. of variation from surface to 

 bottom throughout the study period. The water 

 was found to be saturated with oxygen at all 

 depths throughout the season. Samples were 

 collected about the same time each sample 

 day. Trophic relations of zooplankton orga- 

 nisms, although little studied in fresh-water 

 species, might offer a possible explanation of 

 the distribution pattern exhibited at Bare Lake. 



Although the problem was not studied at 

 Bare Lake, it is of interest to note that 

 Pennak (1944) found a diurnal vertical migra- 

 tion of 12 zooplankters in a study of five 

 shallow lakes in Colorado. At least five of the 

 species exhibiting this phenomenon in the 

 Colorado lakes studied by Pennak are present 

 in Bare Lake. 



Seasonal Variations in Abundance 



The rapid embryonic development and brief 

 egg production cycle of many of the rotifers 

 enable them to respond rapidly to environmental 

 changes. Hence, seasonal peaks of abundance 

 for some zooplankton organisms may follow 

 quite closely optimal seasonal conditions. 

 According to Edmondson (1957, pp. 238-9): 

 "The rotifer Keratella cochlearis (Gosse) at- 

 taches a newly laid egg to the lorica and 

 carries it until hatched. Ordinarily the length 

 of embryonic period is shorter than the inter- 

 val between the production of eggs .... In 

 Bare Lake during one of the years of investi- 

 gation, the egg : female ratio for Keratella 



cochlearis was 0.33 before fertilization and 

 0.73 one week after fertilization. By this time 

 there had been a distinct increase in the 

 quantity of phytoplankton, and the implication 

 is that the rotifer population was responding 

 to increased food supply by the increasing 

 reproductive rate." 



Conversely, entomostracans have been ob- 

 served to have a more extended life cycle 

 and their response may be much more delayed. 

 It is interesting to note that the similar 

 seasonal variation patterns of Keratella 

 cochlearis and K. canadensis, as shown in figure 

 5, indicated a possible similarity in the 

 ecology of these two closely related species. 

 It was noted that, while Kellicottia longispina 

 was most abundant a.S Ceratium hirundinella 

 was declining in numbers, the Keratella species 

 reached apeak earlier than Ceratium. Ploesoma 

 hudsoni followed a similar pattern in relation 

 to that of the Keratella species (fig. 5), pos- 

 sibly as a result of a trophic association. 



The No. 10 net is admittedly not adequate 

 to assess accurately the abundance of small 

 organisms such as Ceratium, immature cla- 

 docerans, nauplii, and small rotifers. Still 

 the seasonal variations in abundance found 

 for these organisms as indicated in figure 5 

 were as might be expected. The one exception 

 to this might be Epischura nevadensis whose 

 apparent low numbers, small fluctuations in 

 abundance, and long life cycle, coupled with the 

 increasing ratio of nauplii to adults as the 

 season progressed, cast doubt on the validity 

 of the variations as indicated in figure 5. The 

 average number per liter of the above zoo- 

 plankters should therefore be interpreted as 

 minimal figures rather than as true estimates 

 of abundance. Conochilus unicornis, although 

 a small rotifer, is of a colonial type and was 

 probably sampled adequately by the No. 10 

 net. 



Comparison of Zooplankton Abundance 



The depth of a lake must be considered 

 when comparing zooplankton production among 

 lakes (Rawson, 1953, p. 230-232). Since zoo- 

 plankton are usually most concentrated in the 

 epilimnion, a shallow lake would have a 

 decided advantage when compared to a deep 



