36 
in the environment common to them all to which these pulses may be 
attributed. On the following page the seasonal distribution of the 
total diatoms has been plotted for 1898, along with that of the ni- 
trates and of the total plankton (volumetric), the thermograph, and 
the hydrograph. _ An examination of the changes in nitrates yields 
no marked evidences of correlation. The vernal pulse of diatoms 
follows the high nitrates of winter and spring, and the hiemal pulse 
in December appears after their autumnal rise, and in this particular 
year develops at the time of an unusual drop in nitrates (Pt. L., 
Pl. XLV.). The diatom pulses do not show any constant relation 
to the movement in nitrates either in amount or direction. Whipple 
(94) has noted the importance of nitrates in the development of 
diatoms in reservoir waters. The fact that little correlation 
appears in our waters between the fluctuations of the nitrates and 
the growth of diatoms may be due to the presence here of nitrates 
—owing to sewage contamination—far in excess of the demands 
which the diatoms make, and the limitations placed by other elements 
in the environment are reached before that of the nitrate food- 
supply becomes operative. The distribution of these diatom pulses 
throughout the whole year, even in seasonal extremes, seems to pre- 
clude the factor of temperature as the immediate cause of the 
pulses except as it may affect the growth of individual species, 
which is sometimes apparently the case, as is shown in subsequent 
pages. 
The vernal pulse is attained each year about May 1, at which 
time the water passes the temperature of 60°. The average of the 
recorded surface temperatures of 1898 in the river is about 58°. 
Surface temperatures, except in winter months, are usually several 
degrees higher than bottom temperatures (Pt. I., Table III.). Our 
records are always of diurnal temperatures. The true average tem- 
perature, owing to colder water at lower levels and to the nocturnal 
decline, will lie several degrees below 58°—probably about 55°. 
The greatest development of diatoms thus takes place at a temper- 
ature a few degrees higher than the average temperature for the 
year. Owing to the somewhat greater abundance of diatoms dur- 
ing the warmer months, the average thermal exposure of the plank- 
ton diatoms will be somewhat higher than the average temperature 
of the year. There may be some significance in this phenomenon 
of the occurrence of the optimum temperature for development at 
