457 



of shoaler waters, from 50 to 20 cm. Flag Lake also, in the shoal 

 waters of the autumn of 1895, yielded a large production ; and 

 Thompson's Lake, in the low water of the autumn of 1897, gave 

 the largest production on record for that season ^of the year in 

 that body of water. The vernal pulses, noted for their large 

 production, usually fall at the time of the run-off of large areas 

 of slight I >/ submerged bottom-lands. Shoal n ess, then, does not 

 prevent large production. Lideed, there are some important 

 reasons why shoal waters should, other things being equal, 

 produce more plankton. Light pervades the water more com- 

 pletely, aeration by wind and waves preserves the gaseous equi- 

 librium more perfectly than in deeper waters, and upon sedi- 

 mentation the suspended organic matters in the water are not 

 removed from the immediate proximinity of the growing phy- 

 toplankton. Their decay and solution renders them imme- 

 diately available, while in deep waters only the slow process of 

 diffusion, in the absence of vertical currents dependent upon 

 temperature or hydrographic changes, brings them within the 

 field of surface-dwelling plankton. 



It is well established that the plankton is relatively more 

 abundant in the surface than in the deeper waters of the ocean. 

 The researches of Reighard ('94), Birge ('95), and Ward ('95) 

 show conclusively that the surface waters of our larger lakes 

 contain the greater part of the plankton. Thus, Reighard ('94) 

 finds in Lake St. Clair from 1.2 to 37.2 times as much plankton 

 in the surface stratum of 1.5 meters in depth as in the remain- 

 ing bottom layer in depths of 2.2 to 8.4 meters. Birge ('95) 

 finds 50 per cent., or more, of the Crustacea of the plankton in 

 the upper 3 to 4 meters, and over 90 per cent, in the upper 9 

 meters in depths of 18 meters, and Ward ('95) reports 64 per 

 cent, of the plankton in the upper 2 meters. 



Of our total 640 collections, 389 were made in water over 2 

 meters in depth. In the Illinois there were 208 such out of 235; in 

 Spoon River, 34 out of 35; in Quiver Lake, 60 out of 115; in Dog- 

 fish Lake, 27 out of 48; in Flag Lake, 8 out of 44; in Thompson's 

 Lake, 48 out of 96; and in Phelps Lake, only 4 out of 67. Thus 



