189 



including the plankton itself. The current of the river is doubt- 

 less responsible for the excess (61.4) whicli its waters carry 

 above that in the lakes (25.1 and 44.6). The greater amount 

 in Thompson's Lake (44.6) may be due to two sources, its 

 greater dependence on the river for its water supply and the 

 greater disturbances in its waters due to fish and to waves. 

 The fact that the total catches of the plankton net (3.91, 1.35, 

 2.24, and 7.68) do not on the average more nearly approximate 

 in their ratios to each other the ratios of the chemical resi- 

 dues (61.4, 274.3, 25.1, and 44.6) is due to the great leakage 

 of the finer suspended particles through the silk, especially in 

 Spoon and Illinois river waters. 



The residue in solntio)! contains the available supply of 

 mineral salts for the phytoplankton as well as some organic 

 materials which become sources of plant food, and its distribu- 

 tion in the four localities is correlated with the plankton pro- 

 duction in the direction of the differences, though not in their 

 quantity. Thus Spoon River with the least dissolved residue 

 (167.1) has the least plankton production (.384), and Quiver 

 Lake has likewise less residue (248.2) and less plankton (1.62) 

 than Thompson's Lake (282.9 and 6.68). The Illinois River ex- 

 ceeds all of the localities in its dissolved residue (304.1), which 

 may be attributed to the fact that the water is "older," afford- 

 ing greater time for solution, and that it is the recipient of 

 considerable sewage and industrial wastes which add to its 

 burden of substances in solution. The small amount in Spoon 

 River may be attributed to the fact that it is largely uncon- 

 taminated surface water of recent origin. The greater amounts 

 in the two lakes (248.2 and 282.9) are due in part at least to their 

 dependence upon the river, which in the case of Quiver Lake 

 is slight during the summer season. In so far as the total res- 

 idue held in solution is an index of fertility, the data indicate 

 that the river itself carries the greatest store of food (304.1); 

 Thompson's Lake, somewhat less (282.9) ; Quiver Lake, still less 

 (248.2): and Spoon River, least of all (167.1). On this basis and 

 in the light of the production of Thompson's Lake it would 



