144 
BULLETIN OF THE BUREAU OF FISHERIES 
No fish were kept in these ponds during the course of the experiment. The effects 
of the fertilizers are determined by the amount of plankton and the weight of the 
organic matter per unit volume of water. 
LIMNOLOGICAL DATA 
The original plan for these experiments had been to take net plankton samples 
only, but it was later decided to make a complete series of limnological observations. 
The first plankton samples were taken on June 7. Then from June 13 to September 6 
they were taken regularly several times a week — sometimes daily. From September 
15 to 20 they were again taken daily. The other limnological observations made on 
this series of ponds cover the period from June 27 to September 19, 1928. The 
results discussed below are tabulated in Tables 2 to 5, and the variations are shown 
graphically in Figures 2 and 3. 
Temperature . — Tests made at the beginning of the experiment and at intervals 
during the experiment showed that the maximum variation in the temperatures for 
this series of ponds did not exceed %° C. at any one time. Therefore, the temperature 
of one pond was assumed to hold good for the series. I might repeat here that the 
C ponds are all of the same size and depth, have the same exposure, and are all free 
from any kind of rooted vegetation. 
Table 3 shows that the minimum temperature of 18.3° C. occurred on June 27. 
The maximum of 25° C. occurred on July 7 and on August 9. Temperature does not 
seem to be a limiting factor in this experiment. 
Transparency . — No measurements of transparency were made on these ponds. 
They were practically water-tight, and very little water had to be added during the 
season. This made it possible for the suspended silt to settle. Any differences in the 
transparency were due, therefore, to differences in the amount of plankton and the 
dust-fine detritus resulting from the decomposition of the dead plankton. In the 
control pond, C 4, the bottom was plainly visible throughout the season. In C 1 the 
bottom was visible until the plankton became very abundant. C 3 was very turbid 
early in the season, but as the plankton, especially the phyto-plankton, decreased, the 
water became more transparent, so that at the end of the season the bottom was 
visible. C 2 was always very turbid. This is correlated with the large amount of 
organic matter present throughout the season. It appears that at least as far as the 
C ponds are concerned productivity is not governed by transparency, but rather 
that the reverse is the case. 
Hydrogen-ion concentration— The results of pH determinations are shown in 
Table 2. In C 1 the pH value in samples taken at 8 a. m. ranged from 8.5 on June 27 
to 9.0 on July 30. In samples taken later in the day the pH varied from 8.8 to 9.1. 
These figures show that the water in this pond was at all times distinctly alkaline in 
reaction to phenolphthalein. The variations in pH and temperature are shown in 
Figure 2. In C 2 the pH of 8 a. m. samples ranged from 7.55 on August 9 to 9.0 on 
July 30. In the afternoon samples it ranged from a maximum of 8.75 on September 
13 to a minimum of 8.5 on September 19. The minimum of 7.55 occurred at the 
same time as the maxima for free C0 2 and ammonia nitrogen. One of the maxima 
for temperature occurs on the same date. The variations of temperature and pH 
are shown in Figure 2. In C 3 the pH in the morning samples varies from 7.6 on 
August 20 to 9.3 on July 7. In the afternoon samples it varies from 8.7 on August 
9 to 9.1 on September 13 and 19. The maximum pH value occurs here after an enor- 
