LIMNETIC COMMUNITY 125 
similar to that of the large lakes and rivers. Oxygen is usually abundant 
in the surface waters, but is often wanting in the bottoms of lakes (74) 
with incomplete summer circulation. Muck bottoms in deep water 
or in bays have little or no dissolved oxygen. Dissolved nitrogen is 
important, but has been little studied. In the open water light and 
pressure are governed by the same factors as in the large lakes (see 
pp- 62-64). The bottom in small lakes varies with exposure to waves. 
Where the waves are eroding, the bottom is stony or sandy; where deposit- 
ing, it contains siltand humus. There are often deposits of marl, which is 
a calcium carbonate deposit, frequently reaching a depth of 18 feet in the 
Indiana lakes. It frequently reaches to the surface of the water, but 
when it does so is often covered by muck. Muck bottom is common in 
the deeper water andin bays. The vegetation in such lakes is very much 
like that in base-level streams. The vegetation of the shores of rivers 
like Fox River is duplicated in these lakes, and in fact, small lakes are 
strictly comparable to sluggish rivers in many respects. We have 
patches of vegetation, patches of sand and gravel bottom, but also much 
bottom which has more organic matter than river silt. The principal 
difference is that currents in the lakes vary with the wind, and in sluggish 
streams are mainly in one direction. 
II. COMMUNITIES OF SMALL LAKES 
(Stations 30, 30a, 31; Table XXVI) 
These are divided into the limnetic formation, the formations of 
sandy and stony shores, the formations of muck bottom in shallow 
water, the formations of the vegetation, and the formations of deep 
water (anaerobic). 
I. THE LIMNETIC FORMATION (104) 
(List IT) 
The limnetic formation of the smaller lakes is very similar to that of 
the larger lakes. It is made up of the same groups, but with the addition 
of a few pelagic insects such as the phantom larva (Corethra sp.). The 
species of crustaceans, rotifers, and protozoa are different. The char- 
acters of the formation are similar to those of Lake Michigan (p. 75). 
2. SHALLOW WATER FORMATIONS 
a) Terrigenous bottom formation (105).—Vegetation sparse or absent 
—water o-3 meters. Crawling over the sandy bottom are usually found 
caddis-worms (Goera sp. or Molanna sp.) (Figs. 70, 71). These forms 
