distribution through the summer (table 18). 

 Nitrite was detected at each station on every 

 sampling date, although it was not found at 

 certain depths on July 4 and August 23. The 

 zero measurements may not be absolute be- 

 cause 0.0008 p.p.m. was the smallest quantity 

 our equipment could detect. Quantities be- 

 tween absolute and 0.0008 were recorded 

 as zero. 



The absence of an increase of nitrite in the 

 autumn indicates that decaying salmon car- 

 casses are not important in increasing de- 

 tectable nitrogen supplies in the lake, at 

 least when relatively small numbers of sal- 

 mon are present, as they were in 1957. If 

 appreciable nitrogen had been added, its ef- 

 fects should have been apparent on the August 

 23 and September 3 sampling dates. Further- 

 more, the prime source of nitrogen in lakes 

 is probably from the atmosphere rather than 

 through organisms (Ruttner, 1953). 



Silica 



Silica determinations (table 19) were made 

 with a Klett- Summer son photoelectric colori- 



meter. Silica is the major constituent of the 

 skeletal structure of diatoms, which are usu- 

 ally the principal phytoplankton food for zoo- 

 plankton in oligotrophic lakes. A major 

 decrease in silica often occurs after a 

 bloom of diatoms (Ruttner, 19 53), so a 

 shortage of silica could limit diatom produc- 

 tion. 



Silica ranged from a low of 9.4 to a high 

 of 12.0 p.p.m. Si02 and was relatively uniform 

 throughout the season and at all water depths. 

 This is consistent with the uniform distribu- 

 tion and abundance of diatoms, which are the 

 main cause of variations in silicon content. 

 Brooks Lake has a high silicon content com- 

 pared with many other lakes, probably because 

 of the presence of volcanic ash on the water- 

 shed from the eruption of nearby Katmai 

 Volcano in 1912 and subsequent continuing 

 minor eruptions of several nearby volcanoes. 

 This explanation is suggested by the fact that 

 tropical lakes with volcanic watersheds have 

 high silicon contents (Hutchinson, 1957), and 

 the material from the 1912 eruption was 60 to 

 75 percent silica (Griggs, 1922). The Brooks 



TABLE 19. --Silica determinations (SiO^). July 4 to October 3, 1957, Brooks Lake 



[In parts per million] 



54 



