PLANKTON PRODUCTION IN FISH PONDS 
153 
increased to 23.30 milligrams per liter again. This rise is associated with large 
increases in the algae Scenedesmus, Chroococcus, and Aphanizomenon. Still it is 
associated with a decrease in dissolved oxygen from 3.52 to 1.66 p. p. m., a slight 
increase in ammonia nitrogen, ancfa decrease in alkalinity from 85.94 to 23.42 p. p. m. 
The low oxygen and low alkalinity may be due to the fact that decomposition is going 
on more rapidly than photosynthesis. However, since there is a big increase in the 
algae, it seems more logical to assume that the low oxygen and low alkalinity were 
due to the fact that during the night respiratory changes in the algae had been using up 
the oxygen and at the same time produced the free C0 2 that reduced the alkalinity. 
Had the samples been taken at 3 p. m. instead of at 8 a. m., the dissolved oxygen and 
the alkalinity would probably both have been higher. After July 19 the organic 
matter is always relatively low. The minimum of 4.33 milligrams per liter was 
obtained on August 20. This minimum is accompanied by a low dissolved oxygen, 
2.30 p. p. m.; a minimum pH value, 7.6; a maximum for free C0 2 , 6.06 p. p. m.; and 
a maximum for soluble phosphorus of 0.90 p. p. m. The number of algae, however, 
is not at a minimum on August 20. The rise in organic matter on August 30 is 
accompanied by big increases in the numbers of algae Scenedesmus, Oocystis, and 
Chroococcus. 
The amount of organic matter in the control pond, C 4, was on the average much 
lower than in the ponds C 1, C 2, and C 3. (Table 4.) The maximum of 2.51 milli- 
grams per liter occurred on June 27. The minimum of 0.56 milligram per liter 
occurred on September 19, when the last determination was made. From July 19 to 
September 13 the amount of organic matter was practically stationary, ranging from 
1.76 milligrams per liter to 1.5 milligrams per liter. Figure 3 shows that the dissolved 
phosphorus was also uniformly low and suggests that in this case the soluble phos- 
phorus might have been a limiting factor. That inorganic nitrogen was not a limiting 
factor has been shown on pages 148 and 149. It is pointed out there that the average 
for ammonia nitrogen in C 1 was 0.047 and 0.044 p. p. m. for C 4, and that the average 
amount of nitrate nitrogen was 0.036 p. p. m. for C 1 and 0.035 p. p. m. for C 4. The 
low values for organic matter in C 4 are correlated with relatively small numbers of 
algae. (Table 8.) 
The average amount of organic matter present in 10 samples from C 1 was 7.22 
milligrams per liter, the averages for 9 determinations for C 2, C 3, and C 4 were 23.86, 
14.24, and 1.65 milligrams per liter, respectively. 
PLANKTON 
Net 'plankton . — The taking of net plankton samples was begun on June 7 and was 
continued until September 20. A total of 68 samples was taken from each pond. 
The dates on which samples were taken and the volumes of the samples are shown in 
Table 6. In Figure 4 the averages for 5-day intervals are plotted. These averages 
were obtained by dividing the sum of the volumes of plankton taken during the 5-day 
period by the number of samples taken during the period. 
Table 6 shows that the net plankton in C 1 remained low until July 23, when 
suddenly it increased to 3.0 cubic centimeters per 10 liters of water. On August 1 
the maximum of 9.0 cubic centimeters per 10 liters of water was reached. Then 
it dropped to 4.0 cubic centimeters per 10 liters of water on August 3 and 4, but on 
August 6 it was again up to 8.6 cubic centimeters per 10 liters of water. On August 
18 it is down to less than a cubic centimeter per 10 liters of water, but on the average 
