419 



Phelps Lake at weekly intervals for any length of time. Such 

 intervals extend from the end of March to the last of August, 

 and since similar series were made elsewhere we have an excep- 

 tional basis for comparison of the course of production in the 

 several localities. 



A comparison of the planktographs of the river and Phelps 

 Lake (PL X. and XL.) reveals certain general similarities. 

 These are expressed in the three major fluctuations of the year, 

 the vernal, midsummer, and autumnal rises in production, which, 

 in the main, have coincident limits, but very divergent ampli- 

 tudes, in the two bodies of water. The absence of the sudden 

 diluent action of flood waters is noticeable in Phelps Lake rec- 

 ords, though declines in plankton content coincide with the 

 flood invasions of both June and August. The absence of col- 

 lections in September in Phelps Lake at the time of the decline 

 in plankton content between the midsummer and autumnal 

 rises is due to the very low stage of water in the lake, per- 

 mitting no collection. Replenishment by autumn rains is 

 followed by the large development in October (51.6 cm. 3 ). 



Not only is this general similarity between the movement 

 in plankton production in Phelps Lake and the Illinois River 

 traceable in 1896, but there is a more detailed agreement in the 

 changes in the direction of movement in production in coinci- 

 dent or approximate collections. This is most clearly seen in 

 the months of April to August, when collections are of sufficient 

 frequency to trace with some certainty the course of produc- 

 tion. During these five months there are 16 agreements in the 

 direction of the change out of a possible 20 — a total of 80 per 

 cent. — between Phelps Lake and the Illinois River. In the year 

 as a whole the agreements number 19, or 68 per cent., out of a 

 possible 28. Two of the exceptions in April-May are due to the 

 dislocation of the vernal pulses in consequence of the higher 

 temperatures in the lake above noted. When we take into con- 

 sideration the marked differences in the local environment of 

 the plankton in these two localities and the considerable inde- 

 pendence of this lake as contrasted with other reservoir lakes, 



