344 ANIMALS IN INLAND WATERS 



lakes may be divided into two types: the oligotrophic type, whose 

 water is poor in nutritive plant material and shows nearly equal 

 distribution of oxygen during summer and winter, and whose mud 

 bottom contains little organic material; and the eutrophic type, 

 which, in contrast with the former, is rich in nutritive plant material, 

 and which at sufficient depth below the thermocline in summer shows 

 a considerable reduction in oxygen content, and whose mud bottom 

 is composed of typical muck. All gradations between these two may 

 be found, and some even recognize an intermediate type, the meso- 

 trophic; the pronounced representatives of the two types differ con- 

 siderably from each other. 



The lakes of the Alps and of the Alpine foothills are examples of 

 oligotrophic lakes; in North America the Finger Lakes of New York 

 are of this type. Such lakes have narrow shore lines and blue to light 

 green water which is very transparent. The oxygen content of the 

 deep water never goes below 70% of saturation. The consequence of 

 the narrow width of the littoral region is a sparse plant growth, and 

 the small amount of nutritive plant material makes possible only a 

 small mass of plankton. The grass-green algae prevail in the phyto- 

 plankton, the blue-green algae being of less importance. As a result 

 of the plentiful oxygen supply, the deep waters are inhabited by many 

 genera and abound in individuals in spite of sparse plankton produc- 

 tion; larvae of biting gnats of the genus Tany tarsus are characteristic 

 of the deep water. 



Eutrophic lakes include lakes of the Baltic lake plateau and many 

 other parts of Europe. They are plentiful in North America; the 

 well-studied Lake Mendota in Wisconsin is an example. They are 

 generally more shallow and therefore relatively warm in summer, 

 they have a wide littoral zone, and their water is rich in nutritive 

 plant material. The color of the water is green to yellow or blue- 

 green. Their transparency is often much reduced. In consequence of 

 the flat shores, plant growth is plentiful and the water contains a 

 large amount of plankton in which the blue-green algae outnumber 

 the grass-green; such algal coloration is of frequent occurrence. The 

 rich bottom mud is the foundation for the processes of decay, which 

 result in a rapid decrease of the oxygen content of the deeper waters 

 (Fig. 93). 



The deeper pedonic fauna is limited to animals which do not de- 

 mand a large supply of oxygen. Chironomus larvae and slimeworms 

 (Tubificidae) are characteristic of the bottom, and the larvae of the 

 gnat, Corethra, of the open water. The Chironomus larvae are the 

 only free-living insects with haemoglobin in their blood, and it is this 



