384 Wisconsin Academy of Sciences, Arts, and Letters. 
(b) a thermocline, or mesolimnion, a zone of rapid decline in 
temperature and in oxygen content;'and (c) the Jiypolimnion, 
a bottom layer, which is poor in oxygen, cool, and constitutes 
a zone of stagnation and decomposition. 
As an essential of life the distribution of oxygen exerts a de¬ 
cisive influence on the distribution of plants and animals. De¬ 
spite the fact that bottom areas of the lake are without oxygen 
and exceedingly rich in carbon dioxide for several months of 
the year, a fair number of species lives in the decomposition 
area thruout the year; all of the species are present in large 
numbers. 
Light and Mineral Matter. —The relation of these tAvo is not 
a direct one, but may become so. The depth to Avhich light may 
penetrate water depends on the clearness of the Avater. Par¬ 
ticles of solid matter in suspension control its clearness and 
hence its penetrability. This is Avell illustrated by a compari¬ 
son of Lake Mendota and some of the alpine lakes. Lake 
Mendota contains a large amount of matter in suspension; a 
Avhite Secchi’b disk of 15 cm. diameter can be seen at a depth 
of about 2.5 m. in the open lake, and about 3 meters near the 
shore. In some of the alpine lakes a similar disk can be seen 
at 10 and more meters. Ekman (1915) reports 18 meters as 
the limit of vision in Lake Vaettern. 
The degree of penetrability affects the photosynthesis of 
plants. Aquatic plants receive a more ideal light for photo¬ 
synthesis than those on land. That is, the best light for photo¬ 
synthesis is a diffuse or dispersed light of about the intensity 
of bright moonlight, or one twelfth of sunlight. Such a diffuse 
condition prevails in the Avater. The light is dispersed by the 
particles; much of the light is reflected by the surface so that 
the intensity is somcAvhat lessened. This intensity decreases 
with the depth, and we actually And an increase in groAvth as 
depth increases, up to an optimum point, until Ave finally come 
to the minimum where light is too insufficient for photosyn¬ 
thetic activity. That is to say, plant growth is more luxuri¬ 
ant as the depth increases, and the optimum is not found at 
the surface, but some distance below; in Lake Mendota this 
optimum lies between 3-5 meters. 
