Factors Determining Vertical Distribution. 423 
several species of Crustacea. So long as the Crustacea are mul¬ 
tiplying, the higher strata may contain as high a percentage 
as they do in summer. (Cf. p. 398.) 
One indirect effect of temperature should be noticed. A 
higher temperature increases the sensitiveness of the limnetic 
Crustacea to light, and thus aids in driving from the upper 
strata those species which are negatively affected by light, es 
pecially Daphnia hyalina. 
Chemical relations of the water. 
The abrupt limitation of the downward extension of the 
Crustacea in lake Mendota by the thermocline is not due to the 
change in temperature. This is shown by the fact that in 
lakes which are poor in plankton the Crustacea extend far below 
the thermocline and in many cases the colder water is the more 
densely populated part of the lake. The Crustacea are excluded 
from the lower water by the accumulation in it of products of 
the decomposition of the plankton plants and animals. Thes 
accumulate in the stagnant water below the thermocline and 
their decomposition finally, and in lake Mendota rapidly, fills 
the water with decomposition products and exhausts the oxygen. 
The State Board of Health of Massachusetts in 1889 and 1890 
made elaborate examinations of the condition of the deeper water 
of numerous ponds in that state. It was found (Drown, ’90, 
p. 554) that in the deep water there was “ an accumulation of in¬ 
termediate products of decomposition of nitrogenous organic 
matter, the hydrogen compounds of carbon, sulphur, phosphorus, 
and nitrogen, which, owing to the exhaustion of the supply of 
free oxygen, cannot be further oxidized. ” It was found also 
that “in foul water of this character the varieties of animal and 
vegetable life which we find in water nearer the surface are 
almost, if not altogether, absent. ” In 1891 investigations were 
made of the amount of oxygen in the bottom water, showing 
(Drown, ’91, p. 373) a rapid decline in the dissolved oxygen 
below the thermocline and its total disappearance from the bottom 
water of the ponds. It is not possible to state positively 
whether it is the absence of the oxygen or the presence of the 
decomposition products which excludes the Crustacea from the 
