THE FERTILIZATION OF GREAT CENTRAL LAKE. 

 I. EFFECT OF PRIMARY PRODUCTION 



T. R. Parsons/ K. Stephens,- and M. Takahashi^ 



ABSTRACT 



Commercial fertilizer was added at a rate of 5 tons per week to a lake (51 km^, mean depth 200 m) over 

 a period of 5 months from May to October 1970. As a result of these additions, surface primary produc- 

 tion was increased approximately tenfold while the primary production of the euphotic zone was doubled. 

 The standing stock of primary producers and water clarity were substantially the same as in the pre- 

 vious year when no fertilizer was added. The productive index (mg C/mg Chi a/hr) was increased, 

 especially in the immediate area of nutrient enrichment. However, the principal phytoplankton species 

 were very similar at locations near and distant from the area of fertilization. In conclusion, it appears 

 that as a result of adding nutrients at a low but sustained level, primary productivity was increased 

 without substantially changing the nature of the food chain at the primary level of production. 



In the Pacific northwest, an earlier study (Nel- 

 son and Edmondson, 1955) on the fertilization of 

 a small salmon-producing lake in Alaska showed 

 that the addition of phosphate and nitrate fer- 

 tilizer increased the production of sockeye salm- 

 on (Oncorhynchus nerka); in more recent 

 studies by Donaldson et al. (1968), an increase 

 in the production of steelhead trout (Salmo 

 gairdneri) was demonstrated in a small lake in 

 the state of Washington. The natural fertiliza- 

 tion of lakes from decomposing salmon carcasses 

 has been discussed by Krokhin (1967), who has 

 suggested that the potential deficit from salmon 

 removed by the fishery should be replaced by ar- 

 tificial fertilization. In the report presented 

 here we have carried out a fertilization experi- 

 ment which differs from the two previous reports 

 (Nelson and Edmondson, 1955; Donaldson et al., 

 1968) in several respects. These include the 

 size scale of the experiment which was very 

 much larger than any previous experiments, the 

 application of fertilizer as a solution, control of 

 the N:P ratio, and, finally, sustained weekly nu- 

 trient additions over a period of 5 months. 



^ Fisheries Research Board of Canada, Biological Sta- 

 tion, Nanaimo, B.C.; present address: Institute of 

 Oceanography, University of British Columbia, Van- 

 couver, B.C., Canada. 



^ Fisheries Research Board of Canada, Biological Sta- 

 tion, Nanaimo, B.C., Canada. 



Manuscript accepted September 1971. 



FISHERY BULLETIN: VOL. 70, NO. 1, 1972. 



Preliminary results of our experiment have 

 been reported (Parsons et al., in press) together 

 with our conclusion that lake production was 

 increased by the addition of fertilizer and that 

 this was achieved without causing a condition 

 of eutrophication. The following account deals 

 specifically with the effect of nutrient enrich- 

 ment on the primary level of production. Inten- 

 sive studies on the effect of nutrient additions 

 were carried out during the period May to 

 August 1970 while a more general monitor pro- 

 gram has been maintained from 1969 to the 

 present (March 1971). The first sustained nu- 

 trient additions were made during the period 

 June to October 1970 and further additions are 

 planned for the next 5 years. 



The primary purpose in this study is to in- 

 crease levels of production in an oligotrophic 

 lake, but not to change the trophic relationships 

 which lead to the production of young sockeye 

 salmon. In this respect the ultimate desideratum 

 of the experiment is to produce larger sockeye 

 smolts at their time of seaward migration; 

 earlier reports have demonstrated that there is 

 a close positive relationship between smolt size 

 and survival (Ruggles, 1965; Johnson, 1965). 

 Since previous studies (Parsons et al., in press) 

 have shown that the migrant smolts from Great 

 Central Lake are small (63 ± 1 mm) and that 

 the primary productivity is very low (ca. 5 g 



13 



