445 



The average production in the Illinois for the year is 2.0S 

 cm.'' per m.^ or 2.13 cm.'^ if the average of all collections is taken 

 instead of the mean of monthly averages. This is 25 per cent- 

 below the mean of monthly averages, or 3 per cent, below 

 that of all collections. This depression in production is due to 

 the disturbed and irregular hydrographic conditions which 

 throughout most of the year left insufficient time for the plank- 

 ton to breed. 



As shown by the + and — signs in the table, production in 

 channel waters is below the average in 7 of the 12 months, and 

 4 of the 7 deficiencies fall continuously in the disturbed period 

 of August-November. The decline below the average produc- 

 tion in this disturbed period ranges from 72 to 86 per cent. The 

 other 3 months of deficient production are April, June, and Ju- 

 ly^ — 4, 45, and 86 per cent, below their averages. The April defi- 

 cit is due to the delay in the vernal pulse, while those of June 

 and July are due, possibly, to the after effects of the high ver- 

 nal pulse of May. Production in excess of the average is found 

 in January-March, during the unusually slow rise of the spring 

 flood, in which the catastrophic effect alike of the sudden 

 and higher floods and of stagnation under ice in low water is 

 eliminated. Production is also high, by 86 per cent, in May, as 

 a result of the delayed culmination of the vernal pulse — the 

 largest one, moreover, found in our records in channel waters. 

 It is also high by 40 per cent, in December, when declining levels 

 (PI. XII.) afford the stability necessary for the breeding of the 

 plankton. 



Production in Spoon River, as might be expected in a year 

 of much flood water, falls to a barren level exceeding .1 cm.' in 

 but a single instance, in March (.124), when waters of general 

 overflow mingle with those of the tributary to a considerable 

 extent. 



Production in the backwaters in 1898 again bears a striking 

 resemblance, throughout, to the course it presents in the chan- 

 nel. The higher levels conduce to greater unity in the envi- 

 ronment, and to greater interchange between many localities, 



