PLANKTON PRODUCTION IN PISH PONDS 
165 
Table 10 shows that the dissolved oxygen ranged from a minimum of 2.17 
p. p. m. to a maximum of 11.34 p. p. m. The low value of 3.63 p. p. m. on June 30 
and of 2.17 p. p. m. on July 10 follow the disappearance of the water bloom caused by 
the blue-greens and the temporary disappearance of the blanket algae from the 
surface. The minimum for dissolved oxygen occurs on the same date as the maxima 
for free C0 2 and the hydrogen-ion concentration. The high values for dissolved 
oxygen of 9.47 p. p. m. (91.0 per cent saturation) on May 29, 11.34 p. p. m. (129.0 
per cent saturation) on August 29, and 9.75 p. p. m. (88.1 per cent saturation) on Sep- 
tember 29, correspond to pH values of 8.7, 9.0, and 8.9, and to the phenolphthalein 
alkalinities of 10.12 p. p. m., 38.42 p. p. m., and 28.32 p. p. m., respectively. The 
high oxygen values for May 29 and September 29 are due to the lower temperatures 
of the water. The degree of saturation shows that the water was capable of absorb- 
ing still more oxygen from the atmosphere. The difference between the minimum 
and the maximum amounts of dissolved oxygen can not be explained on a temperature 
basis, for the temperature on July 10, when the minimum 2.17 p. p. m. (24.2 per cent 
saturation) occurred, was 23.9° C. ; the temperature on August 29, when the maximum 
11.34 p. p. m. (129.0 per cent saturation) occurred, was 22.2° C. — a difference in 
temperature of 1.7° C. but a difference of 104.8 percent in the degree of saturation. 
It has already been mentioned that the minimum was associated with the disap- 
pearance of the water bloom and is, therefore, in all probability caused by a decay of 
organic matter. The maximum, since the water is 129 per cent saturated with oxygen 
may be the result of photosynthetic activity. The variations in dissolved oxygen 
are shown in Figure 6. 
The amount of chloride in solution ranges from 1.5 p. p. m. to 4.0 p. p. m. (Table 
10.) The generally lower values for chlorides during the latter part of the season, 
when the plankton was low is due, probably, to the abundant growth of blanket algae 
already referred to. There is no evidence, however, that chloride is a limiting 
factor. 
The results of the nitrogen determinations are given in Table 10. This table 
shows that the ammonia nitrogen varied from a minimum of 0.016 p. p. m. on June 9 
and September 8 to a maximum of 0.224 p. p. m. on July 10. The minimum of 0.016 
p. p. m. corresponds to the maximum for organic matter. The maximum of 0.224 
p. p. m. is associated with a decrease in organic matter. The nitrate nitrogen ranged 
from 0.120 p. p. m. on May 9 to 0.015 p. p. m. on August 18 and 29. Nitrite nitrogen 
was never present even in traces. The organic nitrogen varied from a minimum of 
0.169 p. p. m. on August 29 to a maximum of 3.92 p. p. m. on May 29. As Table 10 
shows the maximum for organic nitrogen does not correspond to the maximum for 
organic matter. This discrepancy is probably due to the fact that the sample on May 
29 was centrifuged only once, while that on June 9 was centrifuged twice. Plankton 
counts made on the May 29 sample show that every liter of water contained 3,025,000 
filaments of Aphanizomenon and 4,680,000 filaments of Anabaena. Now Juday 
(1926) has pointed out the fact that by centrifuging the water once only a small 
proportion of Aphanizomenon is removed. Hence, it seems permissible to assume 
that the value for organic matter on May 29 as given in Table 10 is far below the true 
value. The total nitrogen follows the organic nitrogen very closely. The minimum 
for the total nitrogen is 0.235 p. p. m. and the maximum is 4.04 p. p. m. The maxima 
for the organic and the total nitrogen occur when the blue-green plankton algae are at 
the height of production. The minima occur when the plankton algae are at a mini- 
