372 Birge, Crustaceen des Mendota-Sees. 



July 1894. I there showed that there were about 50/ of the crusta- 

 eea in the upper three metres of the lake; nearly 30/ between 3 and 

 6 metres; 15/ between 6 and 9 metres; 5/ between 9 and 12 metres; 

 and less than 1/ between 12 metres and the bottom at 18 metres 

 or in deeper parts of the lake at 22 metres. A complete account of 

 the work was published in the 10 th volume of the Transactions of 

 the Wisconsin Academy of Sciences, Arts and Letters, p. 421484. 



This study has been carried on since July 1894, with the design 

 of determining the annual and vertical distribution of the limnetic 

 Crustacea, and the series of more than 400 observations was closed 

 with the end of 1896. The results of the study of the annual distri- 

 bution of the Crustacea will be published in the forthcoming 11 th 

 volume of the Transactions of the Wis. Academy, but the full report 

 of the vertical distribution must be further postponed in order to com- 

 plete the investigation of some details. I therefore present a brief 

 synopsis of the results thus far obtained, regarding vertical distribution. 



1. The vertical distribution of the limnetic Crustacea, in summer, 

 is like that of Juli 1894, in all essential particulars. 



2. Soon after the formation of the transition stratum of tempera- 

 ture (,,Sprungschicht") about July 1, the Crustacea in the lower water 

 either die or migrate into the water above the transition stratum. 

 This forms thereafter the lower limit of the Crustacea, 95I or more 

 of the whole number present being found above it. This condition 

 lasts as long as the transition stratum is found until after the 

 middle of September - - and the Crustacea follow the transition stratum 

 as it gradually moves downward through the lake. It lies at a depth 

 of about 9 meters in July when it becomes the lower limit of the 

 Crustacea, and lies at about 15 metres in September, when the autumnal 

 gales cause it to disappear. 



3. This limitation of the downward extension of the Crustacea is 

 not due, for most species, to the change in temperature occurring at 

 the transition stratum. The Crustacea are excluded from the deeper 

 water by the accumulation there of the products of the decomposition 

 of the plankton. The exact nature of these substances is the chief 

 point requiring further study. 



4. In plankton - poor lakes the Crustacea are found in and far 

 below the transition stratum, though not always to the bottom of the 

 lake. Certain species are found in the deeper water in greater abun- 

 dance than near the surface. Diaphanosoma has not been found in 

 numbers below the transition stratum and very probably never enters 

 the cooler water. 



5. The limitation of the Crustacea is very abrupt. Ten times as 

 many Crustacea may be found in a single metre in and above the 

 transition stratum as are contained in the whole 8 or 10 metres below 





