LIMNOLOGICAL STUDIES OF KARLUK LAKE 
431 
that of Karluk Lake, but the average number of the other five lakes was smaller than 
that of Karluk. 
The blue-green and the green algse were more abundant in Karluk Lake than in the 
other six lakes. With the exception of the series of net catches taken on Cayuga 
Lake on August 12, 1910, the average number of diatoms was considerably larger in 
Karluk Lake than in the other six lakes included in the table. The general results 
given in this table indicate that the net plankton is more abundant in Karluk than in 
the other lakes; the most striking difference is the great abundance of green algse in 
the former as compared with that group of phytoplankton organisms in the latter. 
This greater production of plankton in Karluk Lake is due, in part at least, to the 
nitrogen and phosphorus compounds that are contributed to the water of this lake 
and to that of the streams flowing into it by the decomposing carcasses of the red 
salmon which spawn in its watershed. Both of these substances are essential food 
materials for the phytoplankton and they serve the function of fertilizers; thus the 
addition of these compounds derived from the dead salmon to those already present 
in the water makes it possible for the lake to support a larger crop of phytoplankton. 
A more abundant crop of phytoplankton will, in turn, support a larger crop of zoo- 
plankton forms which depend upon the algse for food; as a result this abundant supply 
of phytoplankton and zooplankton provides a good supply of food for the young red 
salmon, since they feed almost exclusively on plankton material while they inhabit the 
lake. Thus the fertilizers derived from the adult salmon are very important factors 
in producing an abundant food supply for the young red salmon. Gilbert and Rich 
(1927) state that the Karluk fingerling red salmon are unusually large and form a 
sturdy stock; these authors attribute the large size to the very favorable conditions for 
growth which they find in this watershed. It seems probable from the results 
obtained in the present investigation that the chief factor in these favorable growth 
conditions is the abundant crop of plankton which is available for them in Karluk Lake. 
CENTRIFUGE PLANKTON 
A series of centrifuge catches was taken at station 2 in Karluk Lake on August 8, 
1927, and two sets of these samples were obtained from Thumb Lake, one on August 3 
and another on August 12, 1927. These centrifuge catches were taken for the purpose 
of comparing the number organisms obtained by this method with that obtained with 
the net; many plankton organisms are so small that they readily pass through the 
meshes of the net and are thus lost in such a catch. A hand centrifuge operated at a 
speed of about 1,200 revolutions per minute was used for the centrifuge catches; from 
100 cubic centimeters to 120 cubic centimeters of water were used for each catch. 
Numerical results for the various catches are given in Table 14. The protozoa and 
blue-green algse have been omitted from the table because they were found in only 2 
of the 11 catches. 
The six centrifuge samples taken at station 2 in Karluk Lake on August 8, 1927, 
yielded an average of 65,600 green algse and 88,200 diatoms per liter of water. No net 
catches were taken at station 2 on this date, but a series obtained there on August 12 
gave an average of 2,500 green algse and 3,600 diatoms per liter. The centrifuge 
material yielded a little more than 25 times as many green algse and almost 25 times 
as many diatoms as the net samples obtained on the latter date. ' The series of net 
catches taken at station 2 on August 2, 1927, yielded an average of 16,700 green 
algse and 10,300 diatoms per liter, so that the centrifuge catches contained only about 
four times as many green algse as these net catches and only a little more than eight 
times as many diatoms. 
