254 



from the plankton in September-October. In 1898 (Table I.) the 

 last record was made October 11 the latest in any year with the 

 exception of an isolated record October 26, 1897. Moina micrura 

 is thus distinctly a summer planktont. 



It appears in the plankton only after maximum summer tem- 

 peratures of approximately 80 have been reached, and decreases 

 rapidly as soon as the autumnal decline passes this point, and soon 

 thereafter vanishes from the plankton. Its optimum temperature 

 in channel waters is thus near 80. 



The relation which hydrographic conditions bear to the ap- 

 pearance of Moina in channel plankton appears upon a comparison 

 of the Moina population and the movement in river levels in differ- 

 ent years, as shown in the following table. 



MOINA AND HYDROGRAPHIC CHANGES. 



While the correlation is not proportionate between the extent of 

 movement in levels and the Moina per m. 3 , it is still very evident 

 that in years of continued and more stable low water Moina is 

 found in much greater numbers, as appears on a comparison of 1897 

 and 1898. It is also confined largely to the more stable part of the 

 year, appearing in 1895 in June- July in large numbers, but falling 

 off when the minor floods of August-September occur, while in 1897 

 the large numbers are found in the stable levels of August. 



