168 
BULLETIN OF THE BUREAU OF FISHERIES 
SUMMARY OF D PONDS 
The data for the D ponds recorded in the preceding pages show that in each of 
these 3 ponds there occurred at least one instance when the soluble phosphorus was 
completely exhausted. This fact would seem to point toward the soluble phosphorus 
as a limiting factor. However, since there was an abundance of soluble phosphorus 
in D 4 and D 9 after June 9, and still the organic matter and the plankton remained 
low, it becomes very apparent that some other factor besides the soluble phosphorus 
plays a part in limiting productivity. The data also show that inorganic nitrogen 
(free ammonia and nitrates) was never completely exhausted and that the amounts 
of inorganic nitrogen present in the water of the unfertilized pond D 5 compared 
favorably with the amounts of inorganic nitrogen in D 4 and D 9, which were fertilized. 
The data on organic matter and on the volume of net plankton show that D 4 and D 9, 
which were fertilized, were more productive than D 5, which was not fertilized. The 
differences in the productivity of D 4 and D 5 becomes much more apparent when 
the fish production is taken into consideration. D 4 produced much more fish than 
did D 5. The fish production will be considered in a separate report. 
GENERAL SUMMARY AND CONCLUSIONS 
The discussion in the preceding pages has been limited to the data obtained from 
the C ponds and the D ponds. A series of observations similar to those made on the 
C ponds in 1928 was made on the A series of cement ponds in 1927. Observations 
similar to those made on the D ponds in 1927 were made on F 1 and on the E ponds 
in 1927. The observations on the E ponds were repeated in 1928. The data obtained 
from the A and the E ponds in the main confirm the data obtained on the C ponds 
and the D ponds but are too voluminous to include in the present paper. The follow- 
ing conclusions apply to all series alike: 
(1) Although temperature is a very important factor and may govern the seasonal 
distribution of plants and animals, it can not be considered as a limiting factor in the 
course of this experiment. The differences in the temperatures from the different 
ponds as shown in Tables 3 and 10 are too small to account for the differences in the 
amount of plankton. 
(2) Transparency also did not act as a limiting factor in the surface water of 
these ponds. 
(3) The data on pH determinations do not point toward the hydrogen-ion as a 
limiting factor. In fact, they rather suggest that the hydrogen-ion concentration 
is controlled by productivity; that is, in any one pond when photosynthesis is going 
on at a high rate the pH is high also. When the rate of photosynthesis decreases 
and the amount of C0 2 resulting from respiration or from the decomposition of organic 
matter exceeds the C0 2 used in photosynthesis, the pH is low. The data suggest 
also that the variations in the concentration of the hydrogen-ion are due largely, if 
not altogether, to the variations in the amount of free C0 2 . 
(4) If the algae were dependent on the free C0 2 in the water only, then C0 2 
might be considered a limiting factor, for free C0 2 was often absent from the surface 
waters of these ponds. The algae, however, can make use of a large proportion of 
the C0 2 present as the half-bound C0 2 . Juday (1911) reports that certain algae 
used as much as 83 per cent of the half-bound C0 2 . In the course of this investi- 
gation it was found that plankton algae, principally Pandorina and Pleodorina, 
