LeBRASSEUR and KENNEDY: LAKE FERTILIZATION. II. 



CofiOcMji colonel 



400 

 'III 



CfClops (.9001 No/m^ 



200 400 



D'<v>'omus ()900) No/m' 



Epischura (i 900 1 No/m^ 



300 600 



 J \ 1_ 



Boiiofw al thetmoclnw *6*C 



Figure 2. — Vertical distribution of common zooplankton species in Great Central Lake mean no./m^ for July 1970. 

 (Horizontal lines indicates the top and bottom of the thermocline, McAllister (personal communication). Inset 

 shows the vertical distribution at Ih'/V quartile intervals over a 24-hr period. Note: the scale indicating the 

 quantity of organisms varies for each species.) 



tween 3 m and 5 m to a minimum of approxi- 

 mately SOO/m'' at depths below 40 m. The max- 

 imum concentration of individual species ranged 

 from 180/m^ for Daphnia to greater than 

 3,000/m^ for Holopedium. The tendency for 

 some species to show an increase in abundance in 

 deep samples was likely due to contamination 

 from shallower depths since in the process of 

 setting and hauling with nonclosing nets the 

 deeper nets actually sample for a slightly longer 

 time than the shallower nets. Variations in abun- 

 dance with respect to time of sampling was noted 

 for all species (Figure 2 inset) . The effect was 

 generally most pronounced just after sunset 

 when the maximum concentration per m^ of a 



species might be increased by 30' 



Rotifers, 



which were presumably the least motile of the 

 zooplankton, exhibited the largest shift in 

 abundance towards the surface with the onset of 

 darkness. Some species, notably Holopedium 

 and Eplschura, returned to their daylight depth 

 of maximum abundance within 2.5 hr after sun- 

 set. Other species, such as rotifers, Bosm'ma, 

 and Daphhia exhibited relatively little movement 

 during darkness. It is apparent from Figure 2 



that the shift in species abundance were all with- 

 in the daylight range occupied by the bulk of the 

 respective populations. Furthermore, more than 

 75 Sr of the zooplankton populations were at all 

 times within the euphotic zone (i.e. surface to 

 30-40 m). 



SEASONAL ABUNDANCE 



In Figure 3 the mean monthly numbers of 

 zooplankton are shown for the 50-m vertical haul 

 samples. Three species, Cyclops, Bosmina, and 

 KelUcottia, were relatively abundant through- 

 out the year, whereas the other species were 

 present in numbers which exceeded 1,000/m' 

 for periods of 4 to 5 months. Co7iochilus 

 were the only species present during 1970 to 

 appear subsequent to the initiation of nutrient 

 addition. (They were present in 1969 samples.) 

 Cyclops ranged from a winter minimum of 

 2,000/m' to a maximum in September and Oc- 

 tober of 30,000/m-. Eplschura were never nu- 

 merically dominant but ranged in numbers from 

 2,000 to 4,000/m- from May through September. 

 Counts of Diaptomus did not exceed l.OOO/m^ 

 until August, but by September there was a 



29 



