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 



