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 shght 
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, 1597, when surface waters were 
at $8.2°, the temperature was but 80° at 15 ecm. 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 Ceratophyllwm also interfere with 
the movements of the water,and thus tend to establish and 
