324 



Birge — TJie Crustacea of the Plankton. 



differ no more widely than catches might differ though made on 

 the same day and close together. 



In each of the two years where the conditions of the preced- 

 ing winter were known, the summer maximum was close to ten 

 times the winter average. In all three years there was a 

 marked decline of numbers to a late summer minimum in Au- 

 gust; at which time the average number is ^ to £ of the max- 

 imum. In 1895 there was a very marked drop in numbers 

 about the first of July; while in 1896 the maximum number was 

 maintained throughout June and early July and then there was 

 a steady decline for a month or more. In 1894 observations began 

 on the first of July. Diaptomus was practically stationary during 

 the month and rapidly declined after the early part of August. 



These variations in number in different years are at present 

 without complete.explanation. Yet the most singular fact — the 

 notable drop in numbers about July first, 1895 — certainly extended 

 to the species all over the lake. Observations were made between 

 the first and tenth of July in that year even in the remoter 

 parts of the lake, and with substantially uniform results. What- 

 ever the cause it was probably the same as produced a similar 

 fall in the numbers of Daphnia hyalina at the same time. 



The autumnal condition of Diaptomus varies with the temper- 

 ature of the early fall. In 1894 and 1896 there was substan- 

 tially no recovery from the August minimum. 1896, indeed, 

 showed minor variations of number but on the whole the num- 

 ber did not increase. In 1895 on the other hand there was a 

 very marked rise of numbers in September, culminating in the 

 third week of that month. We shall hardly be wrong in at- 

 tributing this additional brood of Diaptomus in 1895 to the 

 higher temperature of the water in that year. There was very 

 little decline of temperature until the very last days of the 

 month as the following observations will show: 



1895. 



meters 

 10 meters 

 18 meters 



Sept. 2, 

 6 a. m. 



Sept. 26, 

 6 a. m. 



21.9° 



20.0° 



20.9 



20.0 



13.9 



17.7 



Sept. 30, 

 6 a. m. 



16.3° 



16.5 



16.5 



