FISHERY BULLETIN; VOL. 70, NO. 1 



after the weights declined at an average rate of 

 1.5^'c per day to the December biomass of 0.9 

 g/m-. The actual rate at which the mean weight 

 of zooplankton declined in any one month fol- 

 lowing July was greater than the average com- 

 puted above due to the increase in biomass in 

 October. Inspection of Figures 3 and 4 indicates 

 that the decline in biomass seen in August was 

 a result of fewer numbers of Epischura, Bosmi- 

 na, and Holopedium. Epischura never reached 

 its earlier level of abundance after August and 

 was virtually absent from the samples by Octo- 

 ber whereas most other species, Daphnia excep- 

 ted, showed an increase in abundance in October 

 which gave rise to the October increase in bio- 

 mass. 



Dry weights of Great Central Lake zooplank- 

 ton (determined by the freeze-dry method) 

 ranged from 14 '/f to 26 % of the wet weight M'ith 

 a mean of 19 '^r . The variation in the percentage 

 dry weight was directly attributable to the spe- 

 cies composition of a sample. For example, the 

 dry weight of Holopedium was 14 '"r of their wet 

 weight, whereas the dry weight of Cyclops was 

 approximately 26 ^V of the wet weight. The av- 

 erage dry weights of the summer zooplankton 

 (May through October) integrated over a 25-m 

 column, i.e. the depth range in which most zoo- 

 plankton were concentrated, for 1969 and 1970 

 was 4 mg and 40 mg/m^ respectively (from Fig- 

 ures 2 and 6). 



Table 5. 



Species 



-Length- weight measurements of adult 

 crustaceans. 



Mean length 

 (Microns) 



Wet weight 

 (Micrograms) 



Cyclops 



Diaptomus 



Epischura 



Bosmina 



Holopedium 



Daphnia 



Length-wet weight determinations were made 

 for different sizes and stages of the common 

 crustacean species and the data are summarized 

 in Table 5. The data in Table 6 were obtained 

 by multiplying the maximum concentration 

 (no./m^) of a species within particular depth 

 intervals (Figure 2) by their respective weight 

 from Table 5, thereby providing a measure of 

 biomass with depth. Included in Table 6 are the 

 mean July temperatures within the respective 

 depth intervals. Nearly 60 ^r of the total bio- 

 mass occurs in the upper 10 m where the mean 

 temperature was about 18°C. In the thermo- 

 cline, from 10 to 20 m, with a temperature range 

 from 12° to 6°C (mean temperature, 9°C) 

 the biomass was about 50 mg/m^ or approxi- 

 mately 30 Sr of the total. From 20 to 30 m depth 

 the biomass was about 89r of the total. The re- 

 maining 3 to A*^ of the total biomass occurred 

 below 30 m (30 to 60 m). While these data 

 were derived from July sampling it should be 

 noted that the general distribution of the biomass 

 with depth was similar throughout the period of 

 thermal stratification, i.e. June to October. 



DISCUSSION 



The zooplankton standing stock in 1970 shows 

 a phenomenal increase over 1969. This can be 

 largely attributed to the affect of the nutrient 

 additions upon the rate of primary production. 

 The results of Parsons et al. (1972) demonstrate 

 a marked increase in the rate of primary pro- 

 duction within the upper 5 m; at the same time 

 there was little or no change in the standing 

 stock of primary producers. While experiments 

 and observations of a direct relationship between 

 particular species of primary and secondary pro- 

 ducers have not been attempted, the obvious in- 

 ference is that the zooplankton through increas- 



34 



