COMMUNITIES IN STANDING WATERS 343 



near the beginning of winter; Leucichthys reighardi and L. hoyi spawn 

 in deeper water, the latter between 36 and 55 m. (in lakes Michigan 

 and Huron) in early spring. Another group of three or more species 

 spawn in still deeper water down to at least 110 m. 44 The siscowet, 

 a variety of lake trout, appears to be limited to the depths in Lake 

 Superior. 45 



Some species of fish living permanently in the deeper waters show 

 variations which may be correlated with the lack of light in their 

 habitat. Thus the pedonic charr (Salvelinus salvelinus profundus) of 

 Lake Constance is pale yellowish brown in color and its eyes are en- 

 larged. The peculiar fish Comephorus baikalensis, which lives in the 

 abyssal region of Lake Baikal, has lost all its pigment and is a pale 

 rose color. 



Lake Superior. — Lake Superior is the largest body of fresh water 

 in the world in its own right, and in addition it is in close connection 

 with two others of the five Great Lakes of North America. Although 

 mid-continental in position, it is only 602 ft. above sea level. As has 

 been stated already, Lake Superior has a surface area of 82,360 sq. 

 km.; its mean depth is 475 ft., and its maximum depth is 1008 ft. 

 The biota of the lake has been inadequately studied; there exists a 

 short report of work done before 1871 46 on which the following notes 

 are based. Of 28 dredgings, 7 revealed sand and gravel, 5 showed mud 

 and sand, 14 showed mud and soft clay. Deeper portions of the lake 

 have a clay and mud bottom, or mud alone. In shallow water the mud 

 is mixed with sand, and the bottom of the most shallow parts is mainly 

 pure sand. Water samples from the bottom are perfectly fresh. August 

 and September temperatures below 60-80 m. were uniform at 4°, while 

 the surface waters varied between 15 and 18°. 



In shallow water near the shores the animal life varies with the 

 character of the bottom. Deep-water communities begin at 60-80 m., 

 and in greater depths the same pedonic species are distributed through- 

 out. Substrata of soft clay-like mud are unfavorable to animal life 

 except for certain worms. No invertebrate species are found peculiar 

 to the depths, which are characterized rather by the absence of shal- 

 low-water species. 



Types of lakes. — Nauman 47 and Thienemann 48 have distinguished 

 between different types of lakes in the temperate zones according to 

 their various environmental factors and their biotic inhabitants which 

 depend upon these. Thienemann distinguishes between (a) the clear- 

 water lakes and (b) the brown-water lakes, whose waters are colored 

 brown from the high humus content. The brown-water lakes will be 

 considered below together with the moorland waters. The clear-water 



