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in temperature was somewhat delayed in 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,asarule, 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 waters of this region toa 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 18, 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. 
