0. 

 UJ 



o 



Figure 31.--Seasonal and vertical distribution of phyto- 

 plankton and protozoan groups at three stations in 

 Brooks Lake from June 27 to September 3, 1957, 

 Width of each spherical curve corresponds to the 

 cube root of individuals per liter at the indicated 

 depths. Dot indicates absence of representatives of 

 a group in that sample. 



Green algae showed two pulses in abundance, 

 the larger in late June and early July and the 

 smaller in early August. Maximum density of 

 the June pulse of green algae was 147,000 

 individuals per liter at station I and was 

 coincident with a dense bloom of green algae 

 on the littoral zone. 



Blue-green algae (two genera. Microcystis 

 and Lyngbya) also showed two pulses, one on 

 July 4 and the other in mid- August. This abund- 

 ance of blue-green algae is interesting because 

 oligotrophic lakes such as Brooks are usually 

 deficient in blue-green algae (Rawson, 1956). 

 Maximum density of blue-green algae was 

 57,000 individuals per liter at the 50-meter 

 depth at station III on August 23. 



On the basis of phytoplankton population. 

 Brooks Lake does not entirely fit the tradi- 

 tional oligotrophic classification. Throughout 

 the summer, diatoms were the dominant form, 

 and Melosira was usually the dominant genus. 

 Melosira is usually regarded as an indicator of 

 eutrophy rather than oligotrophy, but at Brooks 

 and Great Slave Lakes Melosira was the domi- 

 nant diatom. Other eutrophic indicators were 

 also found, namely, Synedra, Fragillaria, and 

 Asterionella. Maximum density of diatoms 

 was 181,000 per liter at station 1 on June 27. 

 Possibly an even greater density occurred 

 in the spring before sampling began. 



TABLE 16. 



-Genera of plankton fovind in water samples from 

 Brooks Lake, 1957 



Green algae 



Blue-green algae 



Diatoms 



Protozoa 



AnkistTodesmus 



Botryococcus 



Closterium 



PediastTum 



Phacotus 



PleuTococcus 



Scenedesmus 



Spirogyra 



StaurastTum 



Lyngbya 

 Microcystis 



Asterionella 



Cyclotella 



Cymhella 



Diatoma 



Fragillaria 



Gomyhonema 



Melosira 



Navicula 



Stephanodiscus 



Synedra 



Tabellaria 



Ceratium 

 Dinobryon 

 V orticella 



48 



