499 



The submerged vegetation — such as that found in Quiver 

 Lake— affects the conditions of nutrition in other ways than 

 those above indicated. The absence of roots and the slight 

 hold which its lowermost stems can obtain upon the soft bot- 

 tom facilitate its removal by floods and seines, and the nutri- 

 ment stored in its tissues is thus taken from the lake, and its 

 waters are impoverished to that extent. Again, both Cerato- 

 phyllum and Elodea are perennial, continuing beneath the ice 

 from year to year and never wholly yielding to decay. The lit- 

 toral vegetation of Flag Lake, with its large annual growth and 

 well-marked periods of decay in autumn and spring, contrib- 

 utes more generously to the enrichment of the water. Thus, 

 while robbing the water of its food material, the submerged 

 vegetation often fails to make equivalent returns. 



The submerged vegetation also interferes with the free 

 operation of certain other factors which affect the plankton of 

 open water. It shuts out the sunlight, and effectually modi- 

 fies the temperature thereby. Thus, on a midsummer day the 

 water in Thompson's Lake rarely shows a difference of more 

 than three degrees (Fahr.) between surface and bottom in 

 two meters of water. In the vegetation, on the other hand, 

 the temperature contrast is much greater and within much 

 narrower limits. On July 15, 1897, when surface waters were 

 at 88.2°, the temperature was but 80° at 15 cm. below. The 

 diurnal range of temperature is thus much less in vegetation 

 than in open waters. The growing portion of the submerged 

 vegetation is usually at or near the surface, while the deeper 

 portions are older and often moribund. This vegetation thereby 

 enjoys the full benefit of the sunlight, so essential to the growth 

 of chlorophyll-bearing plants, while its occupation of the water 

 — especially at the surface — shuts out the light to a consid- 

 erable degree from the more open deeper waters, and in this 

 way adds another effective barrier to the growth of the phyto- 

 plankton in surrounding water. 



The dense growths of the Ceratophyllum also interfere with 

 the movements of the water, and thus tend to establish and 



