TABLE 20.-- Turbidity determinations in terms of silica (fuller's earth), July 4 to 



October 3, 1957, Brooks Lake 



[In parts per million] 



Station and 



depth (m. ) July 4 



Turbidity 



July 14 July 24 Aug. 3 Aug. 13 Aug. 23 Sept. 3 Oct. 3 



I 



2 



5 



II 







5 



10 



25 



35 



m 







5 



10 



50 



65 



0.4 



0.5 



Lake silica content from year to year fluc- 

 tuates but is always high. In determinations 

 made at Brooks Lake between 1947 and 1955, 

 the range was 7.0 to 16.2 p.p.m, Si02.« 

 Even this lowest figure is far greater than in 

 most North American and European lakes and 

 contrasts with relatively small amounts found 

 in Karluk Lake (Barnaby, 1944). 



Turbidity 



The amount of turbidity provides a measure 

 of total particulate matter in the water. In 

 Brooks Lake this material is mostly plankton 

 and so provides a check on quantitative plank- 

 ton measurements. 



Turbidity was measured July 4 and 14 with 

 a Klett-Summerson photoelectric colorimeter, 

 and for the remainder of the summer, starting 



6 Theodore R, Merrell. 1956. Evaluation of Brooks 

 Lake Research Station. [Typewritten report.] On file 

 at BCF Biological Laboratory, Auke Bay, Alaska. 



July 24, with a turbidimeter, as described by 

 Welch (1948). The method was changed be- 

 cause the colorimeter was not sensitive enough 

 to measure accurately the extremely low 

 turbidities. The results of July 4 and 14 were 

 corrected by using the turbidimeter as a stand- 

 ard and calibrating the colorimeter. Turbidi- 

 ties were extremely low, ranging from 0.4 to 

 10.3 (p.p.m. silica (fuller's earth)) with only a 

 single sample exceeding 4.0 (table 20). Most 

 of the particles were plankton, although spruce 

 pollen was also present early in the season. 



Transparency 



Transparency, as measured by Secchi disk 

 visibility, is a widely used and simple deter- 

 mination for comparing relative water trans- 

 parencies of different lakes. Usually, low 

 visibility indicates high productivity and vice 

 versa. 



Secchi disk visibility measurements were 

 made only at stations 11 and III. The readings 



55 



