FISHERY BULLETIN: VOL. 70, NO. 1 



to be relatively uniform, suggesting that hori- 

 zontal patchiness or local contagion is not a ty- 

 pical feature of Great Central Lake zooplankton. 

 The observations of lake circulation (McAllister, 

 personal communication) (Parsons et al., in 

 press), and the chlorophyll a distribution (Par- 

 sons et al, 1972) confirm that the epilimnion is 

 well mixed, thus assuring a nearly uniform dis- 

 persal of planktonic organisms along the lake. 



The 50-m vertical hauls made at Stations 1 and 

 2 provide further opportunity for examining the 

 variability with respect to different sampling lo- 

 cations. Here, the comparisons of mean counts 

 indicated a high degree of similarity between the 

 two locations with respect to species comj^osition, 

 stage of development, and abundance. However, 

 examination of samples collected on the same day 

 indicated a high degree of variability. For- 

 ty-nine samples were collected from Stations 1 

 and 2 during May through December; species 

 counts for Station 1 were plotted against the re- 

 spective count for Station 2. Values which fell 

 outside of a mean ±- half the expected mean 

 (where the expected mean equals half the counts 

 for Stations 1 and 2 combined) are tabulated 

 in Table 3. On half the sampling dates the 

 counts for a particular species tended to be si- 

 milar at both locations (e.g. in the first column 

 of Table 3, the number of samples with a mean 

 ± m/2 is generally greater than half the total 

 number of samples, N/2). Greater numbers of 

 four species were found at Station 2 than at 

 Station 1. It is noteworthy that three of the four 

 species, Kellicottia, Cyclops, and Daphnia, have 

 their greatest abundance below the epilimnion 

 at depths greater than that sampled on the areal 

 surveys. However, there was no apparent cor- 



Table 3. — Comparison of counts of zooplankton from 

 50-m vertical hauls at Stations 1 and 2, N = 49. 



Species 



Counts = (m : 



2 



Station I 

 >(m + m) 



Station 2 

 <(m — m) 

 2 



relation in the relative abundance of any of these 

 three species with respect to each other or to 

 other species at either station. 



There were, however, periods when three or 

 more species would be more abundant at one po- 

 sition than at the other. For example, from July 

 3 to July 21 (six sets of samples) three to six 

 species were most abundant at Station 1 while 

 during the period August 21 to September 8 

 (six sets of samples) three to seven species were 

 most numerous at Station 2. Similarly, for other 

 periods of 4 to 12 days, one or another species 

 was in greater abundance at one station than the 

 other. 



These data have not been examined further 

 to show if variation in species abundance be- 

 tween sampling positions can be correlated with 

 variations in the lake circulation or other envi- 

 ronmental factors such as fertilization or pre- 

 dation by underyearling sockeye. However, it 

 is apparent that all seasonal changes in species 

 composition and abundance were reflected 

 throughout the near-surface waters of the lake 

 and that no local area of high or low zooplankton 

 concentration could be clearly defined within the 

 main body of the lake. 



VERTICAL DISTRIBUTION 



Horizontal tows made within the upper 60 m 

 revealed marked differences in species compo- 

 sition and abundance with depth during the pe- 

 riod of thermal stratification. As an example 

 the weekly tows made during July were com- 

 bined and the average concentration of each spe- 

 cies at each of 17 depths sampled during daylight 

 is shown in Figure 2. The inset associated with 

 each species distribution shows the relative dis- 

 tribution (25% quartile intervals) of the respec- 

 tive populations sampled during a 24-hr period 

 in August. 



Five of the eight species shown in Figure 2 

 have their maximum concentration within the 

 upper 10 m, while the maximum concentration 

 of the other three species was below 20 m. Thus 

 the species maxima fall either above or below 

 the thermocline. However, it should be noted 

 that the number of organisms per m^ decreased 

 from a maximum of greater than 7,000/m^ be- 



28 



