THE LIFE OF DEVIDS LAKE 33 



The biological problems may be grouped under three main 

 heads ; first, the life zones of the lake and their characteristic inhabi- 

 tants; second, the seasonal distribution of the plancton, and third, 

 an annotated list of species. 



LIFE ZONES 



The life zones of the lake are not well defined regions because 

 of its shallowness, the gradual slope of the bottom, and especially 

 because of the influence of the wind in mixing its waters. They may 

 in general, however, be defined as four, namely (1) littoral, the 

 shore zone to a depth of 0.6 ra., (2) Ruppia, the region between 

 0.6 and 2.0 ra. in depth, where Ruppia grows abundantly, (3) 

 pelagic, the area over 2.0 m. deep, and (4) bottom, the ooze cover- 

 ing the floor of the lake. Zone 1 is characterized in places by Enter- 

 omorpha which grows abundantlj' on the rocks at some points along 

 shore. It is not common, however, occurring only where the shores 

 are covered with rocks, chiefly on the islands or "rock piles" in the 

 lake. Its presence indeed is so exceptional that it should perhaps not 

 be considered a "characteristic" of this zone. 



Apart from Enteromorpha, zone 1 has no specific character- 

 istics. Cladophora al.so occurs here to a considerable extent, 

 attached to the rocks or to submerged logs, but this is more char- 

 acteristic of the following zone. It is a region where, at times, 

 certain species occur in large numbers, only to disappear again as 

 quickly. This erratic occurrence of organisms in zone 1 is due 

 chiefly to two factors ; first, temperature, and second, wind. The 

 former is one of the most important factors controlling the multi- 

 plication, movement and consequent distribution of organisms in 

 the lake. Consequently it is not surprising that wide variations in 

 the abundance of organisms in the littoral zone should accompany 

 the temperature differences in this zone, to which reference has 

 already been made. 



Wind is also an important factor in determining the distribu- 

 tion of organisms in this zone, especially the massing of filamentous 

 algae which occasional]}' occurs here, as already noted. 



Further reference to these features will be made in the discus- 

 sion of plancton distribution. In general it may be said that 

 zone 1 is marked rather bj' the absence than the presence of char- 

 acteristic forms. 



The Ruppia zone is the most interesting, as well as important 

 one in the lake. It is interesting because of the variety of its 

 inhabitants, and important because of the influence of the great 

 mass of Ruppia, which develops here in summer, upon the chemistry 

 of the water, as an important source of the organic matter in the 

 ooze accumulating on the lake bed and as a breeding place for 

 numerous organisms, both animal and plant. 



