96 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



shows the late stages in the filling of a depression, the Little Barren 

 at mile 474 (Hudson Bay Railway). The filling of small lakes far- 

 ther south, for example in the latitude of the Great Lakes of North 

 America, is often accompanied by floating bogs, the deposition of marl, 

 etc. (Transeau, 1905, p. 364, Fig. 4). 



Still farther south where decomposition is more rapid, the plant 

 contribution to bottom deposits is most important in the zones of 

 floating and submerged angiosperms. Some of this material is carried 

 out into deeper waters as coarse fragments and detritus. Where the 

 bottom is suitable, much of this material is consumed by crayfish, 

 mussels, worms, insect larvae, fishes, etc., and further modified for 

 incorporation in the deposit. Dead animal bodies, and excrement 

 especially, often accumulate at a relatively rapid rate, A consider- 

 able amount of material arises from the plankton. The phytoplankton 

 and plant fragments quite regularly form a thin brown layer over 

 depositing bottoms in summer, which afford residence for certain types 

 of small organisms. 



Such deposits have long been studied by Swedish investigators, 

 who have distinguished them especially on the basis of the amount of 

 coprogenous matter in them (von Post, 1862; Naumann, 1929; cf. 

 Gams, 1921). Finally, the bottom is the chief site of decomposition, 

 which is due to the coactions of micro-organisms, but in its turn exerts 

 reactions upon the medium. 



Reactions upon the Medium. These may operate upon light and 

 heat, the color and transparency of the water, or much more signifi- 

 cantly upon the materials dissolved in it. The penetration of solar 

 energy may be modified by floating or submerged plants, such as 

 Nymphaea, Lemna, and Potamogeton, by the plankton, or usually to 

 a much less degree by the nekton. The first is a frequent result in 

 shallow waters of temperate climates especially, but its importance 

 relative to the aggregate surface of lakes is slight. The influence of 

 plankton depends upon its abundance, being considerable at times of 

 its maximum and of little effect at other periods. Both the quantity 

 and quality of light are affected, the latter especially by the phyto- 

 plankton. 



The paramount reactions upon the medium are the consequences 

 of metabolism, in the course of which gases are exchanged and nutri- 

 ents absorbed and returned, and of decomposition. The three proc- 

 esses concerned are photosynthesis, respiration, and decomposition, the 

 last involving also the respiration of bacteria. The role of plants in 

 absorbing carbon dioxide and releasing oxygen, and of all living organ- 

 isms in taking up oxygen and emitting carbon dioxide, is too well 



