FISHERY BULLETIN: VOL. 70, NO. I 



in bacterial numbers (Figure 8) and the fact 

 that very high dark uptake of ^^C-bicarbonate 

 (up to 50^; of the light bottle uptake) were 

 encountered during the summer at some stations 

 following fertilization. We are at present not 

 sure of the accuracy of this result, however, and 

 it will be reinvestigated during 1971. Nauwerck 

 (1963) has concluded that the heterotrophic for- 

 mation of particulate material is a principal 

 mechanism for supplying food to particle feeders 

 in some lakes and one might expect this mech- 

 anism to be enhanced by the additional avail- 

 ability of nitrogen and phosphorus. 



The most interesting aspect of changes in the 

 species composition of the principal primary 

 producers is that in spite of differences in sur- 

 face primary productivity at Stations 1 and 2 

 during 1970 (Figure 10) the relative abundance 

 of principal species at these two stations (and 

 on several occasions at Station 3) was substan- 

 tially the same (Figure 9) . This was important 

 because it was intended that there should be no 

 change in the species composition of organisms 

 leading up the food chain to young salmon, but 

 only an increase in their productivity. In ad- 

 dition, the occurrence of the Cyclotella-Chroococ- 

 cus association is characteristic of oligotrophic 

 lakes (Hutchinson, 1967) which indicates that 

 the general classification of the lake (based on 

 species association) had not been changed by 

 fertilization. However, some eutrophic species 

 of phytoplankton, such as Ceratium, Peridwium, 

 and Scenedesmtis, were also observed as minor 

 constituents of the plankton, especially during 

 the summer. 



In conclusion, it appears that the fertilization 

 of Great Central Lake resulted in an increased 

 primary production but did not substantially 

 change the standing stock of primary producers, 

 water clarity, or the principal phytoplankton 

 species at locations near and distant from the 

 site of nutrient enrichment. The effect of zoo- 

 plankton on the primary producers was essen- 

 tially to suppress the increase in standing stock 

 of phytoplankton while the standing stock of 

 zooplankton itself increased by almost an order 

 of magnitude. Zooplankton growth and distri- 

 bution are described in the second paper in this 

 series (LeBrasseur and Kennedy, 1972). 



ACKNOWLEDGMENT 



The authors wish to acknowledge the assist- 

 ance of S. Sheehan in carrying out analyses of 

 water samples. 



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