PLANKTON PRODUCTION IN FISH PONDS 
141 
the length of time taken by the phosphate and nitrate so formed to come again within 
the range of algal growth.” 
Juday et al (1928), who studied 88 lakes situated in northeastern Wisconsin, 
state, “No definite evidence was found to indicate that soluble phosphorus is a limiting 
factor in the production of phytoplankton in those lakes.” Again, “In some lakes 
which support a relatively large crop of plankton there is no decrease in the amount of 
soluble phosphorus, or only a very slight one, in the upper water from May to July 
or August.” They therefore failed to confirm Atkins’s theory at least in as far as the 
88 lakes studied are concerned. 
Data will be presented in this paper that tend to show that soluble phosphorus 
may be a limiting factor in fish ponds, but it will also be shown that phosphorus is 
not the only limiting factor. Evidence will also be presented to show that inorganic 
nitrogen was not a limiting factor. 
Czensny (1919) calls attention to the fact that the free C0 2 may directly limit 
the production of algae and indirectly the production of those organisms that feed on 
algae. Birge and Juday (1927) have shown that the soft-water lakes in northeastern 
Wisconsin that are extremely low in fixed C0 2 do as a rule contain considerable 
quantities of free C0 2 . In the hard-water lakes studied by Birge and Juday there is 
generally enough of what has been called the half-bound C0 2 to make up for any 
deficiency in free C0 2 . The algae can make use of the half-bound as well as of the free. 
The data that will be presented in this paper confirm the conclusions of Birge and 
Juday as far as the hard waters are concerned. 
That the addition of fertilizer to the pond water has an effect on plankton pro- 
duction has been shown by Von Alten (1919). In Von Alten’s experiments, the effect 
of fertilizer was specially noticeable in the case of diatoms. He observed an increase 
in the number of species, the number of individuals, and an increase in size. Pauly 
(1919) noticed that inorganic fertilizers exerted a beneficial effect upon Volvox, 
rotifers, Cladocera, and copepods, but the number of diatoms was decreased. 
METHODS OF ANALYSIS AND EXPRESSION OF RESULTS 
The different forms of nitrogen and the chlorides were determined according to the 
procedures outlined by the American Public Health Association in Standard Methods 
of Water Anatysis (1926). The free C0 2 , the phenolphthalein alkalinity, and the 
dissolved oxygen were determined as outlined by Juday (1911). The soluble phos- 
phorus was determined by Denige’s method (1921). The total phosphorus was 
determined by a method outlined by Juday et al. (1928). The difference between the 
total and the soluble phosphorus has been designated as the organic phosphorus. All 
phosphorous determinations were made on centrifuged water. For the determination 
of hydrogen-ion a La Motte colorimetric outfit was employed. Transparencies were 
determined by means of Secchi disk. The organic matter in the plankton was 
determined as described by Juday (1926). The net plankton was determined 
volumetrically by straining a definite volume of water through a'jWisconsin plankton 
net. The concentrated sample obtained in this way was then transferred (from) the 
plankton bucket to a graduated tube of an electric centrifuge and ‘was centrifuged 
at a moderately high speed for two minutes. The volume in cubic (centimeters 
was then read off directly from the tube. It might be stated here that volumetric 
determinations of net plankton are not always a very good index of productivity. 
