Lake program in 1957, and was accomplished 

 by arranging to support a project by a Univer- 

 sity of Michigan graduate student, Charles R. 

 Goldman. The results have been published 

 separately (Goldman, 1960), but because this 

 work is interrelated with the rest of the pro- 

 gram reported here, a summary of the tech- 

 nique and results follows. The primary pro- 

 ductivity program was planned and executed 

 by Goldman and was the subject of his Ph. D. 

 thesis. 



Determining primary productivity by means 

 of radioactive carbon tracers is a rapid and 

 sensitive technique for measuring rate of or- 

 ganic increase at the lowest trophic level 

 (phytoplankton). Because production at this 

 base of the food chain may limit production 

 of successive trophic levels (e.g., sockeye 

 salmon) a measure of its production may be 

 a valuable key to assessing the productive 

 potential of an entire lake's biomass. How- 

 ever, this technique has limitations similar 

 to those characteristic of other food pyramid 

 analyses. Even when a good measure of pri- 

 mary productivity is obtained, to be of prac- 

 tical use in production of sockeye salmon the 

 intermediate steps through each trophic level 

 must be related to each other. Thus, it is 

 possible that a highly productive lake might 

 have its productivity concentrated in some 

 presently economically useless end product. 

 The 1957 program was designed to develop 

 suitable field techniques and compare primary 

 productivity seasonally and in different loca- 

 tions in Brooks Lake. Collection of limnologi- 

 cal data and fish samples was made concur- 

 rently with the primary productivity work. 



Determining primary productivity consists 

 essentially of measuring the rate of uptake of 

 radioactive tagged carbon atoms by phyto- 

 plankton during the process of photosynthesis — 

 the greater the rate of uptake, the more pro- 

 ductive the water. 



Primary productivity measurements with 

 sodium carbonate C'^ began on June 14. A 

 second measurement was made on the 18th, 

 and thereafter determinations were made every 

 fifth day throughout the summer under all 

 weather conditions. Duplicate determinations 

 were made at each Brooks station once during 

 the season to assess experimental error. 



The three limnological and gill net sampling 

 stations (fig, 21) were also used for primary 

 productivity samples. Two additional stations 

 were established exclusively for primary pro- 

 duction measurements, one in Naknek Lake 

 which was sampled every 15 days and the 

 other near the south shore of Brooks Lake 

 which was sampled once a month. 



Primary productivity was measured at 1-m, 

 intervals from surface to 5 m. at all stations, 

 and below this depth, at 10, 15, 20, 25, and 

 3 5 m, At station III measurements were also 

 made at 50 and 65 m. 



The following successive steps were taken 

 in the primary productivity determination; 



1. Clear glass 150-ml, bottles, numbered 

 and with ground glass stoppers and ties, were 

 washed in detergent and rinsed with distilled 

 water. 



2. A metal harness snap was tied to the 

 neck of each bottle so that it could be readily 

 attached to hog-ring loops at intervals on a 

 line that was run from the surface to bottom 

 of the location where a measurement was 

 desired. 



3. The bottles were placed in numerical 

 order in a heavy wooden box, which acted as 

 an insulated carry-case, and transported by 

 boat to the sampling station, 



4. A light reading was taken with an 

 ordinary photographic photoelectric light me- 

 ter, always in the same direction (on Brooks 

 Lake samples the reading was always on Mt. 

 Katolinat (fig. 1).) 



5. A Secchi disk reading was taken. 



6. At the station where a measurement 

 was desired a sample was taken with a 

 Kemmerer bottle from each depth and trans- 

 ferred to a numbered bottle. 



7. To each sample bottle was added 2 ml, 

 of sodium carbonate C^"* (5 microcuries). 



8. The bottle was snapped to the line at 

 the depth corresponding to the depth at which 



59 



