THE COMPOSITION, ABUNDANCE, AND DEPTH DISTRIBUTION 

 OF THE 1957 SUMMER NET ZOOPLANKTON OF BARE LAKE, 

 ALASKA, AFTER FERTILIZATION 



by 



Robert F. Raleigh 



Fishery Research Biologist 



Bureau of Commercial Fisheries 



U.S. Fish and Wildlife Service 



Juneau, Alaska 



ABSTRACT 



Bare Lake was artificially fertilized with various phosphates and sodium nitrate 

 each summer over a 7-year period (1950-56). During the third season of fertilization 

 (1952), it was found that although many factors in the lake's ecology had changed, the 

 zooplankton population had remained remarkably stable. In 1957, the zooplankton was 

 again examined and was found to have increased at least threefold in abundance over 

 the 1952 population. In addition, although the lake has a maximum depth of only 7.5 

 meters, the distribution of zooplankters by species showed a definite change with 

 depth. 



INTRODUCTION 



Bare Lake is a small, unstratified lake 

 located on southwest Kodiak Island (fig. 1). 

 It occupies an oval-shaped basin with a maxi- 

 mum length of 1,222 meters and a maximum 

 width of 495 meters, a total area of approxi- 

 mately 49 hectares. It has a maximum depth 

 of 7.5 meters and a mean depth of 4 meters 

 (fig. 2). 



The lake was artificially fertilized with di- 

 ammonium phosphate and sodium nitrate each 

 summer from 1950 through 1956 by the U.S 

 Fish and Wildlife Service, Bureau of Com- 

 mercial Fisheries. The amount of fertilizer 

 added was calculated to increase the nitrate 

 nitrogen concentration of the water 0. 25 mg./l. 

 and the phosphate phosphorus concentration 

 0.05 mg./l. Primarily, the fertilization study 

 was designed to determine if the addition of 



inorganic nutrients to the lake waters would 

 increase the food supply and, subsequently, 

 the growth and survival of juvenile sockeye 

 salmon, Oncorhynchus nerka (Walbaum), before 

 their seaward migration. 



According to Nelson and Edmondson (1955), 

 preliminary results, 1950 through 1953, in- 

 dicated that fertilization had (1) increased the 

 rate of photosynthesis during the 10-day period 

 after fertilization by a factor of 2.5 to 7 as 

 compared with the 10-day period before fertili- 

 zation, (2) increased the phytoplankton popula- 

 tion manyfold, (3)increasedthepHof the water, 

 and (4) decreased transparency, but (5) had 

 little or no detectable effect on the zooplankton 

 population. 



The same authors state (1955, p. 434): 

 "Some rotifers seemed to show an effect of 

 increased food supply, in that egg production 



