170 
BULLETIN OF THE BUREAU OF FISHERIES 
matter in the centrifuge plankton. The facts presented in this paper do show that 
Harvey’s (1927) statement to the effect that the productivity of the sea depends 
only on the amount of available nitrates and phosphates does not apply to these 
fish ponds. The data on phosphorus are only partially in agreement with the con- 
clusions arrived at by Atkins and which have been reviewed in the introduction. 
However, they are more nearly in agreement with Atkins’s theory than with the 
results obtained by Juday et al. on the lakes of Wisconsin. 
(9) That the addition of various fertilizers increases the production of plankton 
in a pond is shown by the data for the C ponds. (Tables 4, 6, and 8.) It is also 
shown by the fish production in the D and the E ponds (data to be published in a 
separate paper). The large number of algae in C 1, C 2, and C 3, as compared with 
the small numbers for C 4, are in agreement with the results obtained by Jaernefelt 
(1926) and those of Von Alten (1919). Jaernefelt used various combinations of 
inorganic salts and cellulose in half barrels and in glass aquaria. Gaerder and Gran 
(1927) and Gran (1927) in cultural experiments with raw sea water at times obtained 
an increase in the production of algae when either nitrates or phosphates or a mixture 
of the two were added to the culture flasks. At other times the increase in the 
number of algae in the treated flasks was no greater than in the untreated flasks. 
Gran concludes that when no additional increase was obtained in the treated flasks 
“That the occurrence of nutritive salts at that time was not yet the limiting factor 
for the nourishment of algae.” If in the experiments reported here the dates on 
which the ponds were fertilized are placed on the curves for the organic matter, the 
result is that often there is no immediate response to the addition of fertilizer. At 
times the plankton will keep on decreasing through several applications of fertilizer. 
Finally, however, when the necessary nutrient materials have accumulated and the 
conditions are physiologically right, the plankton goes up. That the naturally 
occurring plankton maxima are augmented through the addition of fertilizer would 
seem to follow from the fact that the average amount of plankton produced is 
increased. 
(10) The results on the C ponds show that the organic fertilizers, soybean meal, 
and shrimp bran, which in addition to containing phosphorus also contain large 
amounts of nitrogen, give better results than superphosphate which contains phos- 
phorus mainly. This is contrary to the conclusion reached by Fisher (1924) and to 
which reference has been made in the introduction. It seems also to contradict the 
conclusion arrived at in (7) that inorganic nitrogen was not a limiting factor. This 
latter contradiction may, however, be more apparent than real. It may be that the 
soybean meal and the shrimp bran contain along with the nitrogen some other substance 
that makes a greater utilization of nitrogen possible. Allen and Nelson (1908-9), 
who tried to rear marine algae in artificial sea water made up of highly purified salts, 
report that the algae would not grow in this culture medium. When, however, 
extracts of ulva or of fish tissue were added to the artificial sea water good growths 
were obtained. 
Whatever the explanation may be, the fact that the addition of fertilizers 
increases the productivity of fish ponds seems to be fairly well established. 
