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BULLETIN OF THE BUREAU OF FISHERIES 
one of the three fertilizers used was the most effective it is difficult to say. 
The amount of organic matter and the number of algse in the centrifuge plankton 
would rank the fertilizers in the following order: First, soybean meal; second, shrimp 
bran; and third, superphosphate. On the other hand, if the volume of net plankton 
is used the order would be: First, superphosphate; second, soybean meal; and third, 
shrimp bran. It has already been mentioned that the volume of net plankton is 
not an absolute index of productivity. This fact taken into consideration along with 
the data given in the tables, seems to warrant the conclusion that soybean meal was 
the most effective fertilizer. 
D POND EXPERIMENTS 
DESCRIPTION OF PONDS 
The D ponds studied in this investigation constitute Nos. 4, 5, and 9, of a series 
of 10 dirt ponds. The position and the shape of these ponds are shown in chart 1. 
D 4 has an area of approximately 42,920 square feet, or 0.99 acre. Its maximum 
depth is 4.35 feet and it average depth is 2.47 feet. That gives the pond a volume of 
approximately 106,012 cubic feet. D 5 has practically the same area as D 4, namely, 
42,860 square feet, or 0.98 acre. Its maximum depth is 4.5 feet and its average depth 
2.21 feet. The volume of this pond is, therefore, approximately 94,720 cubic feet. 
D 9 has an area considerably smaller than either D 4 or D 5. Its area equals 29,700 
square feet, or 0.66 acre. Its maximum depth is 4.8 feet and its average depth is 
3.61 feet. This gives the pond a volume of 107,217 cubic feet. 
PURPOSE OF THIS EXPERIMENT 
The object of the limnological observation on the D ponds was to determine the 
behavior of the plankton, the phosphorus, the carbon dioxide, dissolved oxygen, 
hydrogen-ion, chlorides, and the different forms of nitrogen in dirt ponds in which 
fish were being raised. The effect of fertilizer was also considered. (See below.) 
The observations on the D ponds cover the period from May 9, 1927, to Septem- 
ber 29 of the same year. The observations were made at approximately 10-day 
intervals. All samples were taken from the surface. All samples were taken 
between 6.30 and 7.30 a. m. 
POND D 4 
This pond had been wintered wet. It was drained during the last week in April. 
On April 30 it was refilled with water and stocked with fish. 
The pond was fertilized with a 3 : 1 mixture of sheep manure and superphosphate. 
On May 9 and June 18, 118 pounds of this mixture were added to the pond. Then 
from July 2 until August 23, 39 pounds of fertilizer were added every five days. 
NOTES ON VEGETATION 
May 15: Patches of cattails ( Ty-pha latifolia) along north bank. 
May 26: 
Patches of cattails ( Typha latifolia) along north bank. 
Blanket algse (mostly Hydrodictyon) common. 
June 16: 
Blanket algse (mostly Hydrodictyon) cover 10 to 15 per cent of area (estimate). 
Elodea, common, covers about 10 per cent of area. 
Typha, cover about 5 per cent of area. 
Sagittaria, cover about 2 per cent of area. 
Various grasses, pond lilies and narrow-leafed Potamogeton present. 
