175 



The temperature conditions here described are those as- 

 signed by Whipple ('98) to lakes of the temperate type and 

 third order, those whose bottom temperatures are seldom very 

 far from their surface temperatures, and in which there is con- 

 siderable vertical circulation at all seasons when the surface is 

 not frozen. At no place in the region examined by us has a 

 depth been found sufficient to permit the occurrence of a 

 stratum of cold water at the bottom unaffected by the vertical 

 circulation and warming process in the surface regions, such, 

 for example, as has been found by Birge ('97) in Wisconsin 

 lakes. This absence in the river environment of the "thermo- 

 cline" and of summer and winter periods of stagnation in 

 lower levels, marks another point of contrast between the river 

 and some lakes as units of environment. 



The temperature conditions in the bodies of water adja- 

 cent to the river do not differ to any considerable degree from 

 those here discussed. The limited extent, greater amount of 

 vegetation, shallower waters, or greater access of spring water 

 in some of these will cause slight variations from the condi- 

 tions found in the river. 



The ice conditions attending the winter minimum are of 

 profound biological significance, since they produce important 

 alterations in the winter routine. As a result of the presence 

 of an ice sheet on a body of water, the temperatures become 

 more constant, the mingling of waters due to winds ceases, the 

 usual processes of aeration are interrupted, and the propor- 

 tions and amounts of the gases dissolved in the water may be 

 very much altered, the degree of the change depending upon 

 conditions such as the completeness with which the surface is 

 sealed by the ice, the amount of sewage, the relative abundance 

 of plant and animal life, the duration of the ice, and the exist- 

 ence of currents. So far as our observations go at Havana, the 

 stage of stagnation attended by the destruction of the animal 

 life which is sometimes found in small lakes is rarely realized 

 in this environment. Several reasons may be assigned, the 

 principal one being being the instability of river levels in the 



