SUCCESSION OF COMMUNITIES 133 



formations are quite similar in many respects to the formations of the 

 vegetation in sluggish rivers but resist lack of oxygen and stagnant 

 water much better. 



d) The anaerobic formation. This is the bottom and deep-water 

 formation. We have already stated that the circulation of water (see 

 Fig. 10, p. 61) is not known for any of the lakes discussed. Old lakes like 

 those about Chicago are usually covered with humus on the bottom. In 

 this humus and probably just above it there is little or no oxygen. 

 Analyses of the bottom water from ponds with humus-covered bottoms 

 showed that it contained no oxygen. The open water of the lakes with 

 the incomplete circulation in summer is without sufficient oxygen to 

 support life, below the level of circulation (Fig. n, p. 61). There 

 are, however, numbers of animals that pass the summer under these 

 conditions (no, in). These are protozoa belonging to eleven genera, 

 worms belonging to two genera, one rotifer, one ostracod, and the small 

 bivalve (Pisidium idahoense). Dr. Juday kept these animals in jars 

 without oxygen and observed their activities. The rotifer was always 

 active. The ostracod showed little activity, and the bivalve kept its 

 valve closed, showing no activity whatever. 



There are occasional midge larvae in the mud of such bottoms, but 

 they are rare. Some of these have haemaglobin in their blood and are 

 supposed to be able to use oxygen when it is present in the minutest 

 quantities. In the open oxygenless water there are phantom larvae 

 (Corethra) which are able to carry a supply of oxygen with them from 

 the surface. 



III. SUCCESSION IN LAKES 



The general tendency of succession in lakes has been indicated. The 

 first formation is the bare-bottom type, which is locally transformed to 

 the vegetation of open-water type. This usually begins in the protected 

 situations first; the bays are ecologically oldest. These bays pass 

 rapidly from the third open-lake type to the bay conditions. When such 

 a stage has been reached the situations that have a less degree of protec- 

 tion from waves have reached the second stage and we have lakes as we 

 find most of the larger ones about Chicago. They contain, at various 

 points, the three formations which we have discussed. The lake is 

 reduced in size by filling near its shores and the lowering of its outlet. 

 The older stages are continuously encroaching on the younger. The 

 area of barren shoal is constantly becoming less as the lake fills and the 

 outlet, if it has one, is' lowered. Around the shores the development of 

 prairie or forest is usually well begun and one or the other of these types 

 of land vegetation finally displaces the lake. 



