329 



conditions prevalent during this pulse (PI. XLV.) does not re- 

 veal anything peculiar to this season. The nitrogenous sub- 

 stances are neither greater nor less than at other times when 

 production was at a minimum. It is only evident that there 

 is an overplus of these substances in which all correlation be- 

 tween chemical and plankton contents of the stream is lost. 

 The slight quantitative rise in nitrates (.75 to 1,1 parts per mil- 

 lion) a week prior to the culmination of the plankton pulse 

 (PI. XLV.) may contribute to the increase, but similar rises in 

 nitrates elsewhere are not followed by like results. We may 

 therefore dismiss chemical factors — in so far as our data reveal 

 them. — as affording only the basis of nutrition, but neither re- 

 vealing nor explaining their meteoric utilization in this remark- 

 able pulse of growth and reproduction of the plankton 

 organisms. 



The thermal factor peculiar to this season of the vernal 

 pulse is the vernal rise in temperature. This pulse in produc- 

 tion follows immediately upon it, rising with it and culminat- 

 ing shortly before summer heat is reached. The vigor and ra- 

 pidity with which growth and reproduction ensue in the aquatic 

 world is comparable with that which we see in field and wood 

 at this same season of the year. On the very days in which this 

 plankton pulse culminates, the bursting buds are releasing leaf 

 and flower in growth unsurpassed for rapidity during the whole 

 year. The animal world, notably the insects, also begin their 

 rapid multiplication at this period. The same fundamental 

 causes, whatever these may be, underlie these responses of or- 

 ganisms to the vernal rise in temperature both on land and 

 in the water. The prolongation and gradual ascent of the vernal 

 pulse in 1898 may have made its cumulative effect much greater, 

 and thus increased the amplitude of the vernal pulse beyond 

 that of years of more sudden approach of spring. 



The hydrographic conditions in 1898 thus conduce to the 

 presence in channel waters of an unusually abundant plankton at 

 this season. Reference to Plates, XXIX., XXXIX., and XLIl. 

 will show that the backwaters have a greater plankton con- 



