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, 
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 
