IX 



PAGE 



Phelps Lake i8i 



Color of the water i8i 



Causes of turbidity 182 



Silt in catches and method of estimation 183 



Solids in suspension as determined by Berkefeld filter 184 



Silt and turbidity in fiuviatile and lacustrine environments 185 



Chetnical Conditions 185 



Source of data 1S6 



Acknowledgments 187 



Comparison of chemical conditions and plankton at stations, from 



averages of all analyses 187 



Residue upon evaporation 188 



Residue in solution i8g 



Chlorine igo 



Oxygen consumed. 191 



Nitrogen ig2 



Total organic nitrogen 193 



Albuminoid ammonia 194 



Free ammonia 194 



Nitrites 195 



Nitrates 196 



Sewage in the Illinois River 198 



Cities in drainage basin of river with sewage systems 199 



Cities without sewage systems 200 



Sewage of Chicago 201 



Sewage of Peoria 202 



Chemical Conditions and Plankton Production 203 



Comparison of nitrogenous matters and plankton production on the 



basis of averages of all analyses 203 



Illinois River 204 



Spoon River 204 



Quiver Lake. 205 



Thompson's Lake 206 



Seasonal changes in chemical conditions and plankton production 206 



General character of seasonal changes in chemical conditions 206 



Comparison of the years 1896-1899 207 



Relation of chemical changes to plankton production 208 



Illinois River 208 



Spoon River 209 



Quiver Lake 210 



Thompson's Lake^ 212 



Comparison of plankton production with seasonal changes in nitrog- 

 enous substances 213 



With seasonal changes in nitrates 213 



The maximum-minimum cycle 213 



Relation to plankton maxima in the several localities 214 



With seasonal changes in nitrites 215 



With seasonal changes in albuminoid ammonia and total organic 



nitrogen 216 



