LIMNOLOGICAL STUDIES OP KARLUK LAKE 
417 
phorus was larger in the lower water than in the surface water in all cases. This is 
due to the decomposition of organic matter in the lower stratum. The plankton and 
other organic material which settles into the lower water and decomposes there con- 
tains organic phosphorus which is changed to the soluble form during the decomposi- 
tion process. As a result of this change the soluble phosphorus shows a more or less 
marked increase in quantity in the lower water during the summer; the extent of this 
increase is dependent upon the amount of organic matter which decomposes in this 
stratum. 
The organic phosphorus consists of that part of this element which is combined 
with the organic matter that is present in the water in the form of living organisms, 
or in organic material derived from such organisms. The quantity of organic phos- 
phorus is dependent upon the amount of organic matter, but the former is not directly 
proportional to the latter. The various organic compounds possess different per- 
centages of phosphorus, so that there is no direct quantitative relation between the 
two. The quantity of organic phosphorus is usually larger than that of the soluble 
phosphorus; frequently the former is several times as large as the latter, as shown in 
the surface water of Karluk Lake on August 13, 1927. In the bottom water, however, 
the quantity of soluble phosphorus may be several times as large as that of the organic 
phosphorus; the 10-meter sample of Thumb Lake on August 12, 1927, contained more 
than three times as much soluble as organic phosphorus. In Karluk Lake there was 
only half as much organic phosphorus in the 125-meter sample as in the surface sample 
on August 13, 1927. A similar result was obtained in Thumb Lake on August 12, 
1927; on July 21, however, there was a larger amount of organic phosphorus in the 
lower than in the upper water of this lake. 
The quantity of titratable silica in the waters of the three lakes varied from none 
to 2 milligrams per liter. This substance is used by the diatoms in making their 
siliceous shells, so that the quantity of silica in the water is correlated more or less 
closely with the growth of these organisms. A large crop of diatoms may completely 
exhaust the supply of available silica as shown in some of the surface samples of these 
three lakes. The largest amounts of silica were found in the lower water where light 
conditions were unfavorable for the growth of diatoms. The plankton contained a 
rather large number of diatoms, so there was an active demand for silica in the upper 
water. 
Only small amounts of ammonia nitrogen were found in the upper waters of the 
three lakes, but larger amounts were obtained from bottom samples. The activities of 
the phy toplankton in the upper strata create a demand for nitrogen, and free ammonia 
can be used by these organisms as well as nitrite and nitrate nitrogen. This accounts 
for the small amount of ammonia nitrogen in the upper water. The free ammonia 
of the lower water is derived from the organic matter which decomposes in that 
region; since light conditions are unfavorable for photosynthesis there is no demand 
for the ammonia nitrogen in the lower strata. 
Only a very small amount of nitrite nitrogen was found at any depth. In Karluk 
Lake the nitrate nitrogen varied from 0.02 to 0.052 milligram per liter, the largest 
amount being found in the bottom water. The waters of Thumb and O’Malley 
Lakes yielded from 0.012 to 0.06 milligram of nitrate nitrogen per liter. 
Two liter samples of surface and bottom water from Karluk Lake were evapo- 
rated in July and September, 1930, and the residues obtained from them were 
analyzed for various constituents. Residues were obtained from surface samples of 
Thumb and O’Malley Lakes also. The results of these anatyses are given in Table 9. 
