LeBRASSEUR and KENNEDY; LAKE FERTILIZATION. II. 



Table 2. — Zooplankton species found in Great Central 



Lake, 1970. 



Rotifera 



*Kellicottia spp. 



Ktratella cochlearis 



K. quadrata 

 *Conockilus unicornis 



Cladocera 



*Bosmina coregoni 

 *Holopedium gibberum 

 "Daphnia longiremis 



D. pulex 



Scapholeberis kingi 



Polyphemus pediculus 



Alona affinis 



Copepoda 



*Cyclops bicuspidatus thomasi 



C. vernalis 

 *Epischura nevadensis 

 *Diaptomus oregonensis 



D. kenai 



Unknown 



Actinopoda 



Pollen 



Egg clusters 



Arachnoidea (mites— 2 spp.) 

 Chironomid larvae 

 Fish larvae (cottid) 



* Indicates the most common species. 



ents of the zooplankton and it is also possible 

 that new species are being introduced into the 

 lake through a hydroelectric installation which 

 discharges water from an adjacent watershed 

 into the lake. It will be noted from Table 2 

 that the common zooplankton constituents con- 

 sisted of two rotifer species, three species of 

 cladocera, and three species of copepods; these 

 species are identified throughout the text by their 

 generic names. There has been no change in 

 the species composition during the course of the 

 experiment, i.e. the common species have re- 

 mained numerically abundant while the rare spe- 

 cies have continued to occupy a minor role. 



PATCHINESS 



It was anticipated that the zooplankton would 

 exhibit contagious distributions reflecting local 

 circulation patterns, species preferences, and 

 predation. Accordingly, oblique samples from 

 20 m were collected at weekly intervals at 18 

 positions along the lake, both near the shore 

 and in midlake. In general, with the exception 

 of the area near the inlet and outlet of the lake 

 where the abundance of organisms was some- 

 times low, there was greater variability found 

 with respect to the date of sampling than the lo- 

 cation of sampling. Weekly means and standard 

 deviations computed for each species showed 

 that Cyclops was the only species in which the 

 standard deviation exceeded the weekly count 

 for more than half the surveys (11 out of 17). 

 The apparent variability in Cyclops abundance 



might be due contagion or, more likely, to the 

 fact that sampling was limited to depths (20 m) 

 where Cyclops were seldom abundant (see sec- 

 tion on vertical distribution) . The major source 

 of variability in weekly mean counts appears to 

 be associated with the number of organisms 

 counted, i.e. the number of organisms in a sample 

 and the size of the aliquot counted. The weekly 

 mean number of organisms for each species were 

 grouped together with their respective standard 

 deviations as follows: 1-50, 51-250, 251-500, 

 501-1,000, 1,001-2,000, 2,001-5,000. The mean 

 coefficient of variation (C.V.), the range, the 

 number of means present in each group, and 

 the number of times a standard deviation ex- 

 ceeded its respective mean are shown in Figure 1 

 (e.g. for 50 or fewer organisms counted, the 

 standard deviation in 17 out of 23 samples ex- 

 ceeded the mean). The magnitude of C.V., or 

 the relative variation about a mean, is closely 

 associated with the number of organisms 

 counted. The high degree of variability about 

 a mean of 50 or fewer organisms reflects counting 

 errors due to the subsampling technique used in 

 the initial analyses of the samples. However, 

 counts of organisms of 250 or more per m^ tend 



5100 



c 50 



o 

 o 



1 1 r 



2 000 



Meon (No/m3) 



Figure 1. — Coefficient of variation computed for mean 

 counts of species sampled in oblique (20 m to surface) 

 tows, where N is the number of weekly means, m and 

 cr is the standard deviation. 



27 



