426 
BULLETIN OE THE BUREAU OF FISHERIES 
series of catches which were taken at station 1 in 1927. The average number from 
surface to bottom in the series of catches taken on July 19 was 244 per liter; it was 
257 on July 31, then declined to 213 on August 13, fell to 106 on August 24, and 
reached a minimum of only 30 per liter on September 13. In 1928 the average was 
107 per liter on July 10 and 55 on September 3, and similar decreases were noted in 
the September series taken in 1929 and in 1930. 
The protozoan population of Karluk Lake consisted principally of Epistylis and 
Vorticella. These organisms were attached to the Copepoda; they were found in 
considerable numbers on both Cyclops and Diaptomus, but they were not present on 
the Cladocera. A third protozoan, namely, Peridinium, was found in some of the 
catches taken at station 1. The largest number of protozoa noted at station 1 was 
obtained in the series taken on July 31, 1927, namely, an average of 1,279 per liter 
from surface to bottom; at station 2 the largest average number was 1,990 per liter 
down to a depth of 45 meters, the maximum depth at the station, on August 2, 1927. 
In the other catches taken at the three stations in Karluk Lake, the average number 
of protozoa varied from 11 to 729 per liter. 
Phytoplankton . — The results obtained in the enumeration of the phytoplankton 
forms of the net plankton are given in Table 12. This table shows that the blue- 
green algae played only a minor role in the catches secured between 1927 and 1930, 
leaving the green algae and the diatoms as the dominant elements of this group of 
organisms. As might be expected these forms were most abundant in the upper 20 
meters where light conditions were favorable for their photosynthetic activities. 
The phytoplankton material obtained in the lower strata represented senile or dead 
individuals that were settling to the bottom, or else organisms that were living sapro- 
phytically in this region. A maximum of 102,000 green algse per liter was found in the 
0-10-meter stratum of station 1 on July 31, 1927, and the next largest number was 
noted in the 15-20 meter catch of this same series. The largest average number of 
green algse from surface to bottom was obtained in this series also, namely 28,500 
per liter. The diatom maximum at station 1 yielded 25,700 cells and colonies per 
liter in the 10-15 meter stratum on July 31, 1927 ; the average number of diatoms from 
surface to bottom on this date was 4,800 per liter. An average of 7,000 was obtained 
on July 12, 1930. 
The September catches in general showed a smaller number of green algae and 
of diatoms than the July catches. The average number of green algae declined from 
28,500 per liter on July 31 to 3,680 on September 13, 1927. Decreases were noted 
also in 1929 and 1930, but in 1928 the September average was larger than that ob- 
tained in July. The diatoms, on the other hand, showed a smaller average number in 
September than in July from 1927 to 1930, inclusive. 
The vertical distribution of various net plankton organisms at station 1 is shown 
graphically in Figures 3 to 6. The number of individuals belonging to the different 
forms varies so widely that it is necessary to use the spherical type of curve in order 
to get all of the groups on the same diagram. The construction of this type of curve 
has been discussed by Birge and Juday (1922), so that it need not be repeated here. 
For the purpose of bringing out more clearly the vertical distribution of the Cladocera 
and Copepoda, the diagrams have been platted on the basis of the number of organ- 
isms per cubic meter of water instead of on the liter basis as given in the tables. The 
blue-green algae were omitted in some of the diagrams because they were present in 
such small numbers; for the same reason the Cladocera were omitted in Figure 5. 
