309 



Its mean falls on the 23d. This is the vernal pulse, often the lar- 

 gest of the year, this distinction being attained in 1896. It rises 

 in 25 days from a minimum of .13 cm.^ on March 30 to a max- 

 imum of 9.39 on the 24th — an average daily increase of .37 

 cm.^ This pulse, as elsewhere, follows immediately upon the 

 vernal rise in temperature, which in this spring reaches 72° on 

 the day of the maximum of the pulse and passes from 46° to 66° a 

 week prior to it. The maximum thus lies a fortnight after the 

 most rapid vernal rise in temperature begins. It attends a sharp 

 decline in nitrates and free ammonia, and its maximum coin- 

 cides (PI. XLIII.) with that in the organic nitrogen. It also 

 occurs in a period of apparently stable hydrographic condi- 

 tions, the total movement in April in this year being only 1.4 

 ft. less than in any other year. This stability is more appar- 

 ent than real. The decline of the March flood (PI. X.) was 

 checked, and slight rises resulted from spring rains which 

 brought large quantities of silt into the stream, so that move- 

 ment in river levels is not in this instance a sufficient index of 

 hydrographic stability. The result was apparently the sup- 

 pression to some extent of the vernal pulse (cf. on this point 

 1896 and 1898, PI. X. and XII.). The amplitude of this vernal 

 pulse (9.39) is less than that of 1898 (35.68)— the only other 

 year in which our collections are frequent enough to lo- 

 cate and delineate this pulse with sufficient accuracy. 

 This may be due to the operation of one or more of the 

 following factors. First, to the spring rains above referred 

 to, at the time of the apparent maximum of the pulse, which 

 flush it out and dilute it, and to some extent destroy the 

 plankton. In the second place, there was no general overflow 

 at this season, and plankton bred in the less current-swept, 

 impounded backwaters is not entering the channel to the 

 usual extent at this period of the year (cf. PI. X. and XII.). 

 Again, the periods of standstills in levels and those of rise 

 check the outflow from impounding areas or turn channel 

 water into the bottom-lands, conditions which obtained in 19 

 of the 30 days in April. Lastly, there is some possibility that 



