as follows: The volume of preserved concen- 

 trate was measured with a graduated cylinder, 

 and the number of each organism determined 

 by the formula 



cv 



n = 



in which n = number of plankters per liter of 

 original water; c = number of plankters in 

 1-ml. cell; v = volume of concentrate in milli- 

 liters; and 1 = volume of original water sam- 

 pled in liters. For example, if the sampler 

 recorded 75 revolutions (1 revolution = 4 

 liters), the sample measured 100 ml., and the 

 count was 60 plankters in a 1-ml. Sedgwick- 

 Rafter cell, the calculated number of organisms 

 per liter of lake water would be 



n = 



60 X 100 

 300 



n= 20 organisms per liter. 



Counts were made by filling a 1-ml. Sedg- 

 wick-Rafter cell with thoroughly mixed con- 

 centrate. Guidelines were scored on the under- 

 side of the cell with a diamond- tipped pencil 

 to help prevent duplication of counts. Total 

 numbers of each species in the cell were 

 counted. Initially, duplicate samples of con- 

 centrate were counted to check the technique 

 and develop skill in counting. Since counts of 

 organisms in duplicate samples were found 

 to be consistent, counts were later made only 

 on single samples of concentrate. 



The following six genera and species of zoo- 

 plankters were collected: Daphnia longispina, 

 Bosmina longirostris, Bosmina coregoni, Holopedium 

 gihherum, Cyclops sp., and Diaptomus sp. Spe- 

 cies identification of Cyclops and Diaptomus, 

 could not be made with the microscopes avail- 

 able at the field station. Eurycercus was found 

 in stomachs of round whitefish but never in 

 plankton collections, and the rotifer Conochilus 

 was found once in a sockeye salmon stomach; 

 therefore, the two zooplankters were present 

 in the lake in 1957 but were not abundant. 



An unidentified cladoceran was found in 

 stomachs and plankton collected between water 



depths of 5 and 25 meters at stations II and III. 

 Samples of live and preserved individuals of 

 this species were sent to D. K. Milliard of the 

 Arctic Health Research Center at Anchorage, 

 Alaska, but he was unable to identify them. 

 At Hilliard's suggestion, samples were sent 

 for identification to Kaj Berg, Freshwater 

 Biology Laboratory, University of Copenhagen, 

 Copenhagen, Denmark. Berg and a colleague, 

 Mr. Ren, identified it as an arctic form of 

 Holopedium gibberum. An excerpt from Berg's 

 letter follows: "The Cladoceran in question 

 belongs to the species Holopedium gibberum 

 Zaddach. However, they do deviate from the 

 typical form of the species as the gelatinous 

 case is considerably lower and the abdomen 

 somewhat slimmer than on this species. It is 

 a hunger form or what the Germans call 

 'Kummer form.' But these differences are 

 not enough to establish a variety or subspecies 

 on the existing material, as there is an even 

 transition from the typical form to the one 

 present, which is of Arctic origin. It turns on 

 a form, which — owing to the short period of 

 activity in the Arctic fresh waters — must 

 carry out its development and reproduction 

 in the shortest time possible and with a mini- 

 mum of use of material on account of the low 

 content of food in the Arctic lakes. From a 

 biological point of view the animal can fur- 

 thermore do without the big gelatinous case 

 in the fairly cold water, as the viscosity here 

 is great. We know analogous forms of the spe- 

 cies from Greenland." 



The average numbers of zooplankters per 

 liter of water sampled for each of the three 

 stations were significantly different (fig. 28). 



Oi— 

 27 



JUNE 



14 

 JULY 



24 



13 

 AUG. 



23 3 



SEPT. 



Figure 28.— Numbers of zooplankters per liter ot lake 

 water, averages of all depths for each sampling date, 

 June 27 to Sept. 3, 1957, Brooks Lake. 



44 



