380 Biology in America 



oxygen content of the water may be so reduced as to smother 

 many of its inliabitants. Tlic sliores of shallow lakes in the 

 north, after an unusually lonji', hard winter, may be lined 

 in spring with the deca3'ing bodies of hosts of fish, which 

 have thus perished. For aquatic animals require fresh air 

 as well as those who inhabit terra firma, or rather the oxygen 

 which is dissolved in the water from the air. At least most 

 of them do. Professor Juday at Wisconsin has however re- 

 cently made the very interesting claim that many species of 

 animals, Pi-otozoa, worms, insect larvfe and even molluscs 

 may inhabit the oxygen-free ooze at the bottom of lakes. 

 While Professor Juday 's observation needs confirmation, 

 there is no question that many aquatic animals can live in 

 water with a very low oxygen content. This does not mean 

 however that they are living without oxygen, which is a sine 

 qua nan for all living things, but merely that they obtain 

 it in some other way, possibly through breaking down oxy- 

 gen-containing substances in the water, or it may be directly 

 from their food. 



Another effect of winter upon shallow lakes is the con- 

 centration of dissolved substances in the lower levels. When 

 ice forms on the surface of water, any substances dissolved 

 in the latter are in some mysterious way filtered out of the 

 freezing water and as a result become more concentrated in 

 the unfrozen water, and very much less so in the ice, sea ice 

 containing only about one-fifth as much salt as sea w^ater. 

 This increase in concentration of the salt content of brackish 

 lakes in winter may materially affect the life which they 

 contain, and may even be a crucial factor in determining the 

 presence of various species of animals in the water. In 

 Devils Lake, North Dakota, we have a fine example of one 

 of these shalloAv, brackish lakes, which are characteristic of 

 much of our western territory. It has a maximum depth of 

 not more than eighteen feet, while much of the lake is so 

 shallow that it freezes solidly in winter. In earlier days 

 when the lake was deeper it abounded in pickerel, and in 

 recent years many efforts have been made to restock it. In 

 many of these experiments fish were kept alive for weeks 

 during the summer, but with one or two possible exceptions 

 no results of these experiments were evident the following 

 spring. The probable explanation of these failures is that 

 the lake had about reached that degree of concentration (14,- 

 000 parts of solids in 1,000,000 parts of water) which the ex- 

 perimental fish can stand, and that with a considerable increase 

 in this concentration in winter, due to a three-foot layer of 

 ice, they were unable to survive. 



Not alone are dissolved substances separated from freezing 



