166 
BULLETIN OF THE BUREAU OF FISHERIES 
mum and the blanket alga, Hydrodictyon, is at its height of development. The 
variations in the different forms of nitrogen are shown in Figure 6. 
Table 10 shows that on May 9 the dissolved phosphorus amounted to 0.023 
p. p. m. On May 19 this had increased to 0.048 p. p. m. This increase might have 
taken place at the expense of the organic phosphorus, since the latter had decreased 
very considerably. On May 29, however, the dissolved phosphorus had disappeared 
completely. On June 9 a trace only was present. The rapid decline in the soluble 
phosphorus is accompanied with a rapid growth of plankton algae. After June 9 
dissolved phosphorus is present regularly. That is does not become exhausted in 
spite of the abundant growth of Hydrodictyon is due to the addition of fertilizer. The 
relatively large amount of dissolved phosphorus on September 9 and 29 is due, no 
doubt, to the decay of organic matter that had accumidated on the bottom during 
the summer. The organic phosphorus ranges from a minimum of 0.010 p. p. m. to 
a maximum of 0.205 p. p. m. If it had been possible to estimate quantitatively the 
amount of blanket algae each time a sample of water was taken the variation in the 
organic phosphorus would become more intelligible. The variations in the phosphorus 
are shown in Figure 6. 
Table 10 and Figure 6 show that the organic matter was relatively much more 
abundant during tbe early part of the season than later. (A comparison of Table 10 
with the notes on vegetation seems to indicate that the blanket algse are displacing 
the plankton algae in the surface waters. They accomplish this by depriving the 
latter of sunlight.) A maximum of 20.24 milligrams per liter of organic matter was 
present on June 9. A minimum of 1.09 milligrams per liter occurred on September 
19. It has already been pointed out that the maximum of 20.24 milligrams corre- 
sponds to very high counts for Aphanizomenon and Anabaena. It is also associated 
with a high degree of alkalinity as is evidenced bj 7 a pH of 9.0 and a phenolphthalein 
alkalinity of 30.34 p. p. m. The low value for organic matter in the centrifuge plank- 
ton during July, August, and September is due, perhaps, to the very abundant growth 
of Hydrodictyon and not to the exhaustion of either the soluble phosphorus or the 
inorganic nitrogen, for Table 10 shows that soluble phosphorus, ammonia nitrogen, 
and nitrate nitrogen were always present during these months. The variations in 
organic matter are shown in Figure 6. 
That phosphorus may, temporarily, become a limiting factor when a pond is not 
fertilized is suggested by the disappearance of the soluble phosphorus as shown by 
the determinations for May 29 and June 9. That the disappearance of the dissolved 
phosphorus is correlated with a maximum production of blue-green algse has already 
been referred to. 
PLANKTON 
Net 'plankton . — Table 10 shows that the volume of net plankton varied from 
0.05 cubic centimeter to 0.92 cubic centimeter per 10 liters of water. Only the 
animals of the net plankton have been counted and the results are as follows: Cy- 
clops was present throughout the entire season; the maximum of 52 per liter occurred 
in July. Diaptomus occurred for the first time on July 29 and after that date was 
present regularly with a maximum of 44 per liter occurring on August 8. Nauplii 
were always present except on September 29 and were most abundant in June with 
the maximum of 216 per liter occurring on June 9. The following Cladocera occurred 
in the net plankton: Bosmina, Ceriodaphnia, Chydorus, Daphnia, and Diaphana- 
soma. Of these Bosmina was tbe most abundant and was present in all the samples. 
