15] VEGETATIONAL TYPES OF FRESH WATERS 497 



on numerous factors such as local physiography and the composition 

 of the rocks whence inflowing waters came; seasonally, pollen grains 

 may form a particularly impressive form of dust. Autochthonous 

 materials are the precipitations (such as ' lime ' and ' iron ') that 

 take place in water, usually as a result of life-processes, and the 

 sedimentation of plant and animal remains. The so-called ' lime ' 

 is mainly calcium carbonate, which is precipitated primarily through 

 the photosynthetic activity of plants that withdraw carbon dioxide 

 or the HC03-ion from dissolved bicarbonates. Much of it is apt 

 to float as particles in the water and be deposited in shallows as 

 greyish-white marl ; at deeper levels, however, the more abundant 

 carbon dioxide commonly redissolves any settling particles which 

 then remain in solution as calcium bicarbonate. 



Unlike the situation with lime, the secretion of silicates takes place 

 directly on living organisms — particularly on Diatoms, whose siliceous 

 ' shells ', on sinking to the bottom, greatly enrich the ooze with 

 silica. Such sedimentation takes place chiefly in the pelagial region 

 of free and deep water and is the origin not only of currently accumu- 

 lating diatomaceous deposits but also of the ' diatomaceous earth ' 

 in the sediments of long-extinct bodies of fresh and salt waters. 

 Consequently, and in contradistinction to lime, silica tends to be 

 far more plentiful in the sediments of the deep central plains than 

 of the shallow shore-terraces, etc., of lakes — at least when these 

 latter do not contain a large amount of allochthonous quartz 

 material. 



The organic components of sediments enable lake-bottoms to be 

 transformed into spheres of often intense vital activity, while even in 

 shallow waters the carbon dioxide produced by respiring organisms 

 can lead to extensive re-dissolving of lime. Humic matter in solution 

 may result in a browning of waters coming in from leached soils 

 and may be flocculated on encountering calcium or other dissolved 

 salts when entering lakes — hence the brownish gelatinous sediment 

 that is commonly found in lakes in boggy regions. Otherwise the 

 organic component results largely from plankton and dust sedimenta- 

 tion, from the sinking of pleuston, and from washing in from the 

 watershed and the littoral zones. Even in temperate regions the 

 total sediment deposited may amount to several thousands of kilo- 

 grams of dry weight per hectare annually. Herein thrive ' decom- 

 position ' Bacteria, particularly, forming such ' end ' substances as 

 carbon dioxide, water, ammonia, hydrogen sulphide, and methane. 

 These are thereby returned to circulation, or, in the cases of ammonia 



