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. I., 

 PI. 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 



