Further comparisons could be made, but it 

 is clear that the zooplankton of Brooks and 

 Karluk Lakes are generally similar. It is 

 not known whether the abundant rotifers in 

 Karluk Lake were utilized by sockeye salmon. 

 In conclusion, there is no evidence of a de- 

 ficiency in numbers of genera or actual abund- 

 ance of zooplankters utilized by Brooks Lake 

 sockeye salmon in 1957. 



Zooplankton probably do not limit sockeye 

 salmon production at Brooks Lake where 

 fish densities are low. Even the lowest plankton 

 densities measured were relatively high and 

 were similar to densities in Karluk Lake 

 during its period of high sockeye salmon 

 production (Juday, Rich Kemmerer, and Mann, 

 1932). 



Phytoplankton 



To measure the relative abundance of phyto- 

 plankton and protozoa, samples were taken at 

 the same sampling times and locations as for 

 zooplankton, as described on page 43. Only 

 2 liters of each 3-liter sample were used to 

 measure phytoplankton abundance, the third 

 liter being utilized for chemical analyses. 

 Samples were transported to the lakeside 

 laboratory as quickly as possible, and the 

 entire 2 liters was immediately centrifuged 

 in a high-speed plankton centrifuge. The vol- 

 ume of plankton for each sample was deter- 

 mined by transferring the concentrate from the 

 metal centrifuge cup into a small graduated 

 glass centrifuge tube and recentrifuging in a 

 geared hand-operated centrifuge. After the 

 volume was determined, a measured quantity 

 of 3 percent formalin was added to preserve 

 the sample for later counting and identification. 



Counts were made with a large-aperture 

 pipette by placing 1 ml. of the concentrate into 

 a Sedgwick-Rafter cell and counting random 

 fields until a minimum of 100 individual plank- 

 ters (or colonies In multicellular species) 

 were counted. Many green algae and protozoa 

 were fractured during the centrlfuglng. 

 Hiitryi)r.ori:us colonles in particular were evi- 

 dent in uncentrifuged samples but were rarely 

 seen afterward. 



The underside of the Sedgwick-Rafter cell 

 was marked with a diamond-tipped pencil to 

 partition the sample into sections as an aid in 

 preventing counting fields more than once. 



Counts were converted to plankters per 

 liter with this formula: 



n = 



KJCv 

 1 



in which n^ = number of plankters per liter of 

 original water, K = a constant, x = average 

 count per field, v = volume of original pre- 

 served concentrate, and 1 - volume of original 

 water sample (in liters). 



The constant (K) was the number of fields 

 of view in the entire cell. It was calculated 

 by first measuring in millimeters the diameter 

 of a field of view under the microscope and 

 then calculating the area of the circular field 

 of view. The area of the entire cell (1,000 

 square mm.) was then divided by the area of 

 a single field; the resulting quotient was K. 

 The average count per field (x) was the 

 quotient resulting from dividing the total count 

 (about 100) of each plankter by the number of 

 fields of view examined. 



This procedure produced a relative measure 

 of seasonal and depth distributions of phyto- 

 plankton and protozoa but was only an ap- 

 proximation of actual numbers per original 

 liter of lake water. Only recognizable frag- 

 ments or cells were counted, which tended 

 to minimize counts; on the other hand, counts 

 were increased by regarding as individuals 

 each fragment of colonial phytoplankton broken 

 apart during centrlfuglng. The numbers per 

 liter of each of the three most abundant 

 phytoplankton groups (green algae, blue-green 

 algae, and diatoms) and protozoa were plotted 

 In the same way as were zooplankton (fig. 31). 

 A checklist wascompiledof all genera of phyto- 

 plankton and protozoa encountered (table 16). 



Phytoplankton and protozoan distributions, 

 both vertically and in relative seasonal abund- 

 ance, were much more variable than zoo- 

 plankton (figs. 30 and 31). Since diatoms were 

 abundant at all depths throughout the season, 

 zooplankters had sufficient diatoms for food. 



47 



