REACTION IN WATER 95 



(c) emergent. These stages of the hydrosere may be assigned with 

 ahiiost equal propriety to land or water, though developmentally they 

 are an intrinsic part of land climaxes. Since this is largely an out- 

 come of the shallowing effect of plant and animal matter, such accumu- 

 lation has already been considered under land reactions. Reaction 

 upon the medium belongs properly to the consideration of the w^ater. 

 The same is true of influences on the bottom in sluggish rivers and 

 the larger lakes with silt (terrigenous) bottom. The small floating 

 plants and animals are properly considered in relation to both bottom 

 and the medium itself. 



Small Lakes and Ponds 



Accumulation and Decomposition. In its general features, accumu- 

 lation in shallow still water resembles that on land, marsh and swamp 



Pig. 19.— View in Little Barren (Mile 474, 760 kilometers from The Pas, Hudson 



Bay Railway), showing tundra vegetation over several feet of frozen sphagnum. 



(Photo by V. E. Shelford.) 



forming the connection between the two. However, there are several 

 important differences, as is well known. One of these is that detritus 

 is sometimes transported and deposited far beyond its place of origin, 

 another that excrement plays a larger part in consolidation, and a 

 third is the retarding of decomposition and the emphasis upon anae- 

 robic processes. Furthermore, calcareous and chitinous skeletons may 

 accumulate to the point of constituting a definite layer. 



The nature of the filling process is related to temperature and 

 length of season and is different in different climates. Tundra may 

 overlie deep sphagnum deposits resulting in part from failure of the 

 dead plant bodies to decompose. An area of wet tundra is traversed 

 by the Hudson Bay Railway, and the railway itself rests on frozen 

 sphagnum extending to a depth of as much as 20 feet (6.4 meters) 

 near the southern tundra edge and to 7 or 8 feet (2.3-2.6 meters) near 

 Churchill. The difference in depth is related to topography; Fig. 19 



