428 Birge — The Crustacea of the Plankton. 



algae which on calm days accumulate near the surface. When 

 the lake is rough the algae are distributed to a greater depth r 

 and the Crustacea follow them to some extent; although, even 

 when the wind blows with considerable force, the young Crus- 

 tacea still form the chief population of the upper meter of the- 

 water. I have not been able to discover any descent of the 

 Crustacea during windy weather, but, on the contrary, have 

 always found the upper meter fully occupied by them even when 

 the lake was so rough as to make it very difficult to go- 

 out with a row-boat. 



The wind may affect the vertical distribution, also, by creating 

 currents in the water. These are either lateral or vertical; we 

 are concerned only with the latter. During the summer the 

 vertical currents can penetrate no deeper into the water than 

 the thermocline; that is, from six to fifteen meters, according 

 to the time of year. These currents, however, seem to produce 

 very little effect on the distribution of the Crustacea — at any 

 rate, at a distance of 850 m. from the shore, where my observa- 

 tions have been made. In the next section it will be shown 

 that Crustacea must be able to move through a distance of at 

 least 100 meters vertically per day, and that the larger individ- 

 uals move through four or five times that distance. There is, 

 therefore, no difficulty in their maintaining any position in the 

 water they may choose to occupy, against the somewhat slow 

 vertical currents produced by the wind. Indeed, the wind 

 affects the vertical distribution of the limnetic algae much less 

 than would be expected. I have frequently collected after severe 

 gales, and, in summer, have never failed to find the algae of 

 the upper three meters far more numerous than those from 

 lower levels. I have never been able to detect vertical currents, 

 produced either by wind or sun, which were capable of dis- 

 tributing the algae uniformly through the mass of water in 

 summer, and of course the active Crustacea are far more inde- 

 pendent of these currents than are the algae. 



In the autumn the entire mass of water in the lakes is put 

 into somewhat active circulation by the autumnal gales. The 

 algae are at a maximum and are pretty uniformly distributed 

 through the water. Neither the quantity nor the quality of the- 



