80 COMMUNITIES OF LARGE LAKES 



recorded below 9 meters and not on the open shores. All are found in 

 small lakes and sluggish streams. 



d) Lower shore formation (8-25 meters) (Station 3; Tables XI, XIII, 

 XV). — The belt immediately below the shore belt is characterized by 

 wave-action sufficient to move only the finest material. Its lower limit 

 is the limit of wave-action; the beginning of light diminution; the lower 

 limit of daily fluctuation in temperature; and the lower limit for most 

 of the species of Mollusca (75, appendix). Practically all the forms that 

 have been recorded here are inhabitants of still, shallow water also. 

 Notable among these are the common still-water amphipod Eucrangonyx 

 gracilis, the little bivalve Sphaerium striatinum, and several species of 

 Amnicola and Valvata which, together with Lymnaea woodruffi, are more 

 characteristic of Lake Michigan than of shallow waters. While a large 

 number of Mollusca are recorded from the lake above 25 meters only the 

 Sphaeridae are found below this limit. Small annelids, midge larvae, 

 and leeches are very abundant north of Gary, Ind., in 1 1 meters of water. 



This belt is the principal breeding-ground of the whitefish. The 

 eggs are deposited on the bottom and left unguarded. It appears that 

 the young fish stay in the shallow waters for a considerable time. Wher- 

 ever the bottom is firm the lake trout breeds also. Nearly all the fish 

 traps are set in the upper edge of this belt and in the lower boundary of 

 the one above. 



e) Belt of overlapping: upper deep-water belt (25-54 meters) (Tables 

 XIV, XV). — This belt is characterized as below wave-action, below 

 daily fluctuations of temperature, with seasonal fluctuations not exceed- 

 ing 3 C. It is intermediate between the belt above and the deep belt, 

 and is the characteristic feeding-ground of the whitefish and the regular 

 home of the long-jaw (Argyrosomus prognathus, Fig. 21). On the other 

 hand, it is the upper limit for some of the deeper-water forms, such as the 

 well-known My sis relicta and Pontoporeia hoyi (Figs. 22, 23), the deep- 

 water crustaceans which are the chief food of the whitefish. 



/) Deep-water formation (54 meters to bottom) (Table XV). — This 

 belt is characterized by weak or no light and by seasonal changes in 

 temperature less than 1 degree. Below 115 meters there are no light 

 and no seasonal changes, and the temperature is 4 C. throughout the 

 year. Off Racine in 82 meters (265 ft.) the bottom is of reddish-brown 

 sandy mud (82); in 95-125 meters (311-410 ft.) dark-colored impalpable 

 mud, depressions with decaying leaves (82 a). In the Grand Traverse 

 Bay region, Milner found decaying sawdust in 183 meters (600 ft.) (81). 

 Except for unimportant variation in bottom, conditions are practically 

 uniform throughout. Milner (81) states that the invertebrates are 



