THE FERTILIZATION OF GREAT CENTRAL LAKE 

 11. ZOOPLANKTON STANDING STOCK 



R. J. LeBrasseur and 0. D. Kennedy^ 



ABSTRACT 



The regional, vertical, and seasonal abundance of the dominant zooplankton species were studied in con- 

 junction with a series of nutrient additions to Great Central Lake. Two rotifer species, Kellicottia spp., 

 Conochilus unicornis, three cladocera species, Bosmina coregoni, Holopedium gibherum, and Daphnia 

 longiremis, and three copepod species, Cyclops bicuspidutns thomasi, Epischura nevadensis, and Diap- 

 tomus oregonensis were the most numerically abundant zooplankton species. The introduction of the 

 fertilizer and the consequent higher rate of primary production produced no changes in the species com- 

 position. The zooplankton exhibited a relatively uniform horizontal distribution within the upper 20 m 

 along the lake, a factor which was attributed to the lake circulation. All eight species were concentrated 

 in the euphotic zone (upper 40 m), and five were most abundant in the upper 10 m. The center of abun- 

 dance for the remaining three species was between 20 and 30 m depth. The respective depths of maximum 

 abundance for the various species showed little variation between daylight and darkness. Seasonally, 

 there were two periods, June to July and September to October, of maximum abundance for most spe- 

 cies. The cause for somewhat lower levels of abundance in August is not known. The average zoo- 

 plankton biomass showed a similar seasonal pattern with a maximum weight in July which exceeded 

 8 g/m2. The average biomass over a 6-month period, May through October, exceeded 5 g/m^ (more 

 than 10 times greater than for the comparable period prior to fertilization in 1969). In contrast to 

 the high standing stock of zooplankton, the estimated growth rate for underyearling sockeye salmon, 

 the principal predator species in the lake, was only slightly improved over 1969 (1.2 vs. 0.99^ /day). 

 In comparison with other lakes producing young salmon the growth rates appear low with respect to 

 the zooplankton stock. It was suggested that the temperature structure of the lake, 14° to 23°C above 

 the thermocline and 4° to 6°C below the thermocline, may reduce availability and prevent the efficient 

 utilization of the zooplankton by the underyearling sockeye salmon. 



The following account is the second in a series 

 of papers which report on the effects of sustained 

 nutrient additions to an oligotrophic lake. In 

 the first report, Parsons et al. (1972) showed 

 that an increased primary productivity resulted 

 from nutrient additions made to Great Central 

 Lake, B.C.; the objective of this report is to de- 

 termine if nutrient additions affected the stand- 

 ing stock and diversity of secondary produc- 

 ers. 



The overall purpose of these studies has been 

 to determine if nutrient additions will increase 

 sockeye salmon (Oncorhynchus nerka) produc- 

 tion; zooplankton, as the principal food of under- 

 yearling sockeye salmon, occupy a central posi- 

 tion in the food chain of young sockeye during 

 lake residence. Previous studies (Ivlev, 1961; 



^ Fisheries Research Board of Canada, Biological Sta- 

 tion, Nanaimo, B.C., Canada. 



Johnson, 1965; Brocksen et al., 1970) have sug- 

 gested that prey density and availability may 

 limit the predator biomass. The latter authors 

 compiled data for several sockeye nursery lakes 

 with which they were able to demonstrate a 

 direct relationship between mean zooplankton 

 biomass (prey) and the mean growth rate and 

 biomass of underyearling sockeye salmon (pred- 

 ator). Other studies (Ricker, 1962) have indi- 

 cated that the ocean survival, i.e. the return to 

 coastal waters of adult sockeye salmon, can be 

 directly correlated in many instances with the 

 size at which the sockeye as year-old migrants 

 leave the nursery lakes to enter the ocean for 

 2 or more years. While the above studies 

 rely heavily upon circumstantial data, as well 

 as data which were collected for other purposes, 

 they serve as a rational basis for attempting to 

 increase the available zooplankton biomass for 



the enhancement of salmon growth. 



Manuscript accepted September 1971. 

 FISHERY BULLETIN: VOL. 70, NO. 



1, 1972. 



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