174 



in temperature was somewhat delayed m 1896 and again in 

 1899, and the summer maximum was less pronounced in 1895 

 and 1897, though in compensation the summer heat was pro- 

 longed into September in these years. The spring rise in 1896 

 and the autumn declines in 1895 and 1898 are rather more 

 abrupt than usual. These annual differences extend and cur- 

 tail the plankton periods characteristic of the seasons, or 

 render their changes more abrupt. 



The difference between surface and bottom temperatures 

 is, as a rule, but slight. It is perforce usually lacking during 

 the period of decline in the autumn, and at other seasons 

 varies in amount with the air temperature, the wind, and other 

 attendant circumstances. So long as the temperature is above 

 the point of maximum density of water, 39.2°, the surface 

 waters are the warmer by an amount ranging from a fraction 

 of a degree to 5°, the latter occurring on still, hot days. With 

 air temperature falling below that of the water the surface and 

 bottom quickly come to have the same degree of heat. Below 

 39.2° the colder waters are at the surface, though at this season 

 of the year there is usually much less contrast at different 

 levels than in the warmer months. 



Temperature fluctuations, following those of the season and 

 the day, occur in the watersof this region to a degree not realized 

 in the typical lake, whose deeper waters respond but slowly to 

 the surface changes, and thus exercise an equalizing effect. 

 Examples of this quick response are found in the unusually 

 high temperature (82.3°) in both top and bottom waters of the 

 river on May 13, 1896, while temperatures of five days later 

 showed a drop to 71.2° in both regions. A decrease equal in 

 suddenness and extent occurred in September, 1898. The 

 surface layers of water, quickly affected by temperature changes, 

 form relatively a very large part of the volume of the river and 

 its backwaters, and thus instability of temperature becomes an 

 important feature of the environment of the plankton of the 

 river as contrasted with that of the lake. Changes of the ex- 

 tent above noted must affect considerably both the movements 

 and the multiplication of the plankton organisms. 



