188 



determined for each. Since the samples were collected at in- 

 tervals throughout the year, the averages may be regarded as 

 presenting in succinct form the chemical characteristics of the 

 stations examined, and they may therefore serve as a basis for 

 a comparison of the relative fertility of the localities. 



The residue upon errtporafion, which comprises the solid 

 matters left upon evaporating the water and drying the residue, 

 includes both organic and inorganic substances. The inorganic 

 constituents are salts, and comprise mainly compounds of lime, 

 magnesia, soda, potash, iron and alumina with chlorine and with 

 carbonic, sulphuric, nitric, and silicic acids. In this residue lie 

 both the mineral constituents of the food of the phytoplank- 

 ton and the undecayed organic matter found in the water. Not 

 all of the constituents of the residue are equally utilized as 

 food by the phytoplankton, so that the quantity of the residue 

 gives a basis only for a very rough estimate of the fertility of 

 the different waters. Some significance, however, attaches to 

 the marked differences shown in the table. 



The differences in total residue in Illinois and Spoon rivers 

 (367.5 and 522.3 ) and Quiver and Thompson's lakes ( 268.9 and 

 326.4) show no particular correlation with those of the average 

 plankton production of these waters for corresponding periods 

 (1.91, 0.384, 1.62, and 6.68 cm.^ per m.^ as shown in Tables X.- 

 XIII.). The amounts and relative proportions of the dissolved 

 and suspended residue in these localities show some relation to 

 the plankton production. The residue in suspension is not, in 

 its present form at least, available for plant food. Its occurrence 

 in the four localities is almost directly correlated with the rela- 

 tive turbidity of the water Spoon River has from four to eleven 

 times as much suspended matter (274.3) as the other localities, 

 and this consists largely of clayey material with considerable 

 fine quartz, neither of which contributes any considerable 

 source of nutrition to the phytoplankton. The suspended ma- 

 terial in the other locations at times of flood partakes of the 

 character of that in Spoon River. At other times it contains 

 a considerable proportion of debris of plant or animal origin 



