140 POND COMMUNITIES 



A decrease in depth, due to the accumulation of humus and the lowering 

 of the ground-water level, is to be noted in the older ponds. The series 

 is, then, an ecological age-series, and throughout our discussion we refer 

 to earlier and later phases of the various associations concerned. 



III. Communities of Ponds 



I. THE PELAGIC FORMATION 



We have in the ponds a pelagic formation. Though it is limited 

 in number of species, many of which breed on the bottom, it is similar 

 to that of larger lakes. We have found little difference in the pelagic 

 species inhabiting younger and older permanent ponds. Diaptomus 

 reighardi has not been taken from ponds filled with the vegetation 

 which reaches the surface. Other species are about the same in 

 the different permanent ponds. The pelagic formation is poorly 

 developed. 



2. PIONEER FORMATION (TERRIGENOUS BOTTOM) 



(Ponds, i, 5, 7) (113) (Stations 9 and 32; Tables XXVII and XXXIV) 



The youngest ponds of the Chicago area are near Waukegan. The 

 outer end of the Dead River receives the force of the winter waves from 

 the lake and the bottom is bare, with a few scattered aquatic plants. 

 Here animals are few. We have taken only a few invertebrates. The 

 fish present probably get their food from the older parts farther back 

 from the lake. The fish are: the pike (Esox lucius) which prefers clear, 

 clean, cool water (79); the red-horse {Moxostoma aureolum) which dies 

 in the aquarium if the water is the least bit impure, and which also suc- 

 cumbs to any impurities in its natural environment (79); Notropis 

 cayuga, which prefers clear waters; the common shiner (Notropis 

 cornutus) which breeds on bare bottom (105), and the white crappie 

 {Pomoxis annularis) which lives in streams. On the bottom at such a 

 period one is likely to find the larvae of caddis-flies {Goer a sp.), snails, 

 mussels, etc., but we have found none in the Dead River. 



Vegetation quickly captures parts of such a pond. Chara is the 

 first plant to cover parts of the bottom. After this has happened, the 

 pioneer formation may still continue. In Pond 1 of the series of special 

 study (Fig. 85) we have a considerable area of bare sand, and the forms 

 present are the caddis- worm (Goer a sp.) and the mussels (Anodonta 

 marginata and grandis, and Lampsilis luteola). These are preyed upon 

 by muskrats (Fig. 86). There are a number of fish that belong to this 



