Distribution in Depth 



309 



uppermost part, the thickness of this productive 

 stratum varying directly with the transparency of the 

 water. 



It is not at the surface, however, but usually a little 

 below it — a depth of a meter, more or less — at which 

 the greatest mass of the plancton is found. Full sun- 

 light is perhaps too strong; for average planctonts a 

 dilution of it is preferred. Free-swimming planctonts 

 such as rotifers and entomostraca move 

 freely upward or downward with changes 

 of intensity of light. Anyone who has seen 

 Daphnes in a sunlit pool congregating in 

 the shadow of a water-lily pad will under- 

 stand this. These animals rise nearer to 

 the surface when the sun goes under a 

 cloud, and sink again when the cloud 

 passes. The extent of their regular diur- 

 nal migrations appears to be directly relat- 

 ed to the transparency of the water. 



Temperature also is an important factor 

 determining vertical distribution. Forms 

 requiring the higher temperatures are 

 summer planctonts that live at or near 

 Others that are attuned to lower tem- 

 peratures may find a congenial summer home at a 

 greater depth. Thus the flagellate Mallomonas (fig. 

 185) in Cayuga Lake is rarely encountered in summer 

 in the uppermost fifty feet of water, though it is com- 

 mon enough at depths between 50 and 100 feet, where 

 the temperature remains low and constant. The 

 average range of Daphne pulicaria is said to be deeper 

 than that of other Daphnias. 



The gases of the water have much to do with the 

 distribution of animal planctonts, especially below the 

 thermocline, where the absence of oxygen from some 

 lakes during the summer stagnation period excludes 



Fig. 185. Mal- 

 1 o m o n a s 

 ploessi. 



(After Kent.) 



the surface. 



