458 



about 40 per cent, of our collections were made in water less 

 than 2 meters in depth and at all river stages, and at all sea- 

 sons of the year a considerable proportion of the Illinois and 

 its backwaters has a depth not exceeding this limit. Moreover, 

 less than 12 per cent, of our collections are in 5 or more meters 

 of water, and these are confined almost entirely to the two 

 streams. Our collections as a whole are therefore within the 

 limit of depth of the surface stratum, which in deeper wa- 

 ters contains by far the greater proportion of the plankton. 

 This feature of our environment has a tendency to increase 

 the plankton content per cubic meter and to lower it when 

 production is stated in volumes per square meter of surface. 

 Thus, if we compare production in the Illinois River in August 

 with that of Lake Michigan, as given by Ward ('95), we find 

 that in quantity per m.^ of water the river (3.88) exceeds that 

 of Lake Michigan. The average of Ward's August collections 

 is 3.69 cm.^ per m.^ by the gravity method, or 1.23 cm.^ if we 

 reduce it to centrifuge basis (see Ward '00). Production in 

 our environment in this month (see table following p. 342) 

 in all localities but Spoon River and Quiver Lake is from 1.5 

 to 47 times greater per cubic meter than in Lake Michigan ; 

 but if the average per sq. meter of surface be taken as a basis, 

 Lake Michigan exceeds the most fertile of our localities at this 

 season. The amount per sq, m. in Lake Michigan I find on 

 computation to be 37.98 cm.'^ estimated centrifuge measure- 

 ment, while in the Illinois at average August river-levels 

 (4 ft.) it is about 15 cm.^ in Thompson's Lake only 9.29, and 

 in Phelps Lake approximately 14 cm.'' 



The age of the ivater is, other things being equal, one of the 

 most vital of all the hydrographic factors environing the plank- 

 ton. Rain and spring water of recent origin entering the river 

 or its backwaters in tributary floods or by seepage from under- 

 ground storage beds, is practically barren of plankton. This 

 same water impounded in the reservoir backwaters, seeded by 

 the spores and resting eggs of the planktonts deposited on the 

 submerged territory on the recession of antecedent floods, and 



