152 
BULLETIN OF THE BUREAU OF FISHERIES 
September 13. The low values early in the season are associated with small numbers 
of algse in the centrifuge plankton. (Table 8.) The increase in organic matter from 
July 19 to August 2 is due to a very rapid rise in the phytoplankton, due almost 
entirely to the growth of green algae, especially the flagellates Pleodorina and Pan- 
dorina. Table 8 shows that on July 28 each liter of water contained 960,000 colonies 
of Pleodorina and on July 30 each liter of water contained 115,200 colonies of Pleo- 
dorina and 756,000 colonies of Pandorina. The net loss on ignition on July 30, 
August 2, and August 9 is not as great as might be expected from the volume of net 
plankton (Table 6), but this is probably due to the fact that the major portion of the 
net plankton was made up of hollow spherical colonies. From August 20 on the 
organic matter increases again, even though the volume of net plankton is far below 
the figure for the interval from July 31 to August 17. This is due to a change in the 
make up of the plankton. As Table 8 shows, the plankton is now dominated by the 
smaller algse: Scenedesmus, Oocystis, Chroococcus, etc. The changes in the amount 
of organic matter are shown in Figure 2. The relationship of organic matter to 
phosphorus has already been discussed. 
In C 2 the maximum amount of organic matter was found when the first deter- 
mination was made on June 27. This maximum of 44.64 milligrams per liter of 
water is correlated with the maximum numbers for the algae Scenedesmus and 
Aphanizomenon. (Table 8.) From June 27 to August 9 the tendency is down- 
ward except for the rise on July 30. This rise on July 30 occurs in spite of a decrease 
in the number of algae. On August 9 the minimum of 5.19 milligrams per liter of 
water was reached. This minimum is correlated with the minimum for algae counts. 
(Table 8.) The temporary rise on July 30 is accompanied by an increase in the dis- 
solved oxygen from 6.76 to 11.97 p. p. m.,an increase in pH from 8.7 to 9.0, and an 
increase in the alkalinity from 20.22 to 57.62 p. p. m. The large decrease in organic 
matter from 27.99 milligrams per liter on July 30 to 5.19 milligrams per liter on 
August 9 is accompanied by an increase in ammonia nitrogen (Table 4) from 0.144 
to 0.60 p. p. m., a decrease in the dissolved oxygen from 11.97 to 2.59 p. p. m., and 
an increase in free C0 2 from —57.62 to 7.58 p. p. m., also a decrease in pH from 9.0 
to 7.55. (Table 2.) There is also a slight increase in the dissolved phosphorus. (Fig. 
2.) (The dissolved phosphorous was rather low until after the minimum for organic 
matter had been reached.) On August 20 the organic matter had increased to 24.74 
milligrams per liter. The increase is accompanied by a large increase in the algse 
Scenedesmus, Oocystis, and Chroococcus. (Table 8.) After August 20 the organic 
matter remained fairly constant until the end of the experiment. The variations in 
the organic matter are shown in Figure 2. 
In C 3, as in C 2, the maximum for organic matter, namely, 55.80 milligrams 
per liter, occurred on June 27, when the first determination was made. This maxi- 
mum is associated with the maximum number of algse in the plankton. (Table 8.) 
After that the organic matter decreases very rapidly, so that on July 7 it is down to 
1 1 .41 milligrams per liter. Table 8 shows that the number of algae in the plankton has 
also decreased enormously. This rapid decline in the organic matter is, however, not 
accompanied by an increase in the ammonia nitrogen, but there has been some increase 
in the nitrate nitrogen. (Fig. 3.) The dissolved oxygen has remained practically the 
same, and the alkalinity and the pH have increased. (Fig. 3.) All of which suggests 
that decomposition was not taking place at a very rapid rate and that the dead 
plankton had merely settled on the bottom. On July 19 the organic matter had 
