396 



age in the unstable bottom, and refuge was had under the lee 

 shore, but still in usual depths and open water. In several in- 

 stances in overflow stages, when the ice was too heavy to break 

 and too light to carry our load, it was necessary to make the 

 collection near the margin of the lake in effluent waters. These 

 variations in the location of the point of collection introduce 

 no error of consequence into the series, judging by the results 

 of an examination of the local distribution of the plankton in 

 this lake, the details of which cannot be given in the present 

 paper. 



With the exception of the single pump collection on Feb- 

 ruary 28, 1896, all collections prior to May 20 of that year were 

 made by the oblique-haul method, and thereafter by the plank- 

 ton pump. 



This lake is a type of the larger reservoir backwaters, such 

 as Meredosia Lake, Clear Lake, and others found in the bottom- 

 lands of the Illinois and maintaining a constant connection 

 with that stream. An examination of its plankton content 

 will therefore serve to throw light on the relation which lakes 

 of this type bear to plankton production in channel waters. 



PLANKTON PRODUCTION. 



1894. 

 (Table VIII., PI. XXXV.) 



There are but 5 collections in this year, from June to De- 

 cember, at an interval of a month or more, with an average 

 production of 8.89 cm. 3 per m. 3 and a maximum of 24.92 on 

 June 7. 



An inspection of the hydrograph (PI. XXXV.) of this year 

 reveals the fact that only the first two collections were taken 

 under conditions which permitted any run-off from the lake to 

 the river, and both of them at times — that is, in falling levels 

 below 6 ft.— when the run-off was largely, if not wholly, through 

 the tortuous slough at the up-stream end of the lake. The pro- 

 duction in the lake (24.92 and 10.74) at these times was 33- to 



