PLANKTON PRODUCTION IN FISH PONDS 
169 
used up as much as 92 per cent of the half-bound C0 2 . The results show that the 
half-bound C0 2 (unpublished data) was never completely exhausted, although in 
the 3 p. m. sample from C 1 August 9 only 5.06 p. p. m. were left. This suggests 
the possibility that on unusually bright days the available C0 2 may temporarily 
become exhausted. The data on C0 2 show that this occurred very rarely, if ever. 
In D 5 free C0 2 was always present when samples were taken in the morning, yet 
the production in this pond was poor. 
(5) Dissolved oxygen was probably not a limiting factor. In the ponds that 
had fish in them there was always enough oxygen to meet the requirements of these 
fish. In the C ponds the lowest value for dissolved oxygen was 1.66 p. p. m. This 
should have been sufficient to supply the respiratory needs of the plankton organ- 
isms. Moreover, this minimum value for oxygen occurred when the algie were 
very abundant in that pond (C 3), and the dissolved oxygen was undoubtedly much 
higher during the day than it was in the morning when the samples were taken. 
(That the amount of dissolved oxygen varies during the day was shown by some 
observations made in 1928 on 3 of the D ponds. Samples were taken at 6 a. m. 
and again at 3 p. m. During this interval the amount of dissolved oxygen increased 
in D 3 from 6.0 to 8.6 p. p. m., in D 9 from 4.68 to 15.90 p. p. m., and in D 10 from 
6.57 to 7.83 p. p. m. The increase in the amount of dissolved oxygen in these ponds 
was roughly proportional to the amount of algse present. D 9 had a water bloom 
of Anabaena.) 
(6) There is, on the whole, very little correlation in the variations in the dis- 
solved chloride and the amount of organic matter in the centrifuge plankton. This, 
taken together with the fact that chloride was always present in relatively large 
quantities — that is, as compared with the quantities of inorganic nitrogen or dis- 
solved phosphorus — suggest very strongly that there was at all times an adequate 
supply of chloride available. Whether the unusually high concentrations of dis- 
solved chloride in C 3 had any detrimental effect has not been determined. The 
high counts for algse (Table 8) would tend to show that as much as 21.0 p. p. m. of 
chloride did not have an inhibitory effect on the algse. 
(7) That inorganic nitrogen was not a limiting factor is shown by the fact that 
nitrogen as free ammonia and as nitrates was always present. Moreover, the amounts 
of nitrogen as free ammonia and as nitrates in the unfertilized ponds compared 
favorably, in most instances, with the amounts of these substances present in the 
fertilized ponds. In D 5, which was not fertilized, the average amount of free am- 
monia present equaled the average amount present in D 4. The average amount of 
nitrate nitrogen in D 5 exceeds that in D 4 and D 9, both of which were fertilized. 
The data for the C series show that the averages for free ammonia nitrogen and 
nitrate nitrogen in C 1 and C 4 differ but slightly. Yet C 1 produced 4.37 times 
as much centrifuge plankton as C 4 did. 
(8) That the soluble phosphorus may be a limiting factor is shown by the data 
presented in this paper. In all of the D pond studies, as well as in F 1, C 2, 
and C 4, the soluble phosphorus became completely exhausted for short periods of 
time. That the soluble phosphorus is not the only limiting factor is suggested by 
the behavior of the C ponds. In C 1 the dissolved phosphorus never fell below 
0.045 p. p. m. and in C 3 not below 0.005 p. p. m. Still plankton production dropped 
markedly at that point. Again, in C 3, a subsequent rise in the dissolved phos- 
phorus was not followed by a proportionately large increase in the amount of organic 
