. Fresh-water biology. Freshwater biology. CONDITIONS OF EXISTENCE 59 IV. Lake-Pond Communities (see Figs. 7 and 8) (Shelford). 1. Pelagic sub-formations 2. Terrigenous bottom formations 3. Vegetation formations a. Submerged vegetation associations b. Emerging vegetation associations 4. Temporary pond formations (Shelford) Conditions of existence in fresh water at any given point are changing in a definite direction. This change involves every item of the environment which has been enu- merated on the preceding pages. Streams wear down their beds, wear their valleys wider, reduce the speed of
. Fresh-water biology. Freshwater biology. CONDITIONS OF EXISTENCE 59 IV. Lake-Pond Communities (see Figs. 7 and 8) (Shelford). 1. Pelagic sub-formations 2. Terrigenous bottom formations 3. Vegetation formations a. Submerged vegetation associations b. Emerging vegetation associations 4. Temporary pond formations (Shelford) Conditions of existence in fresh water at any given point are changing in a definite direction. This change involves every item of the environment which has been enu- merated on the preceding pages. Streams wear down their beds, wear their valleys wider, reduce the speed of their current, grind their coarse bottom materials into the finest silt. The waves of lakes cut away the shores, grind up the rocks they break off in this process, and deposit the silt thus produced in the bottom. Streams lower the outlets of lakes and carry detri- tus into them. Ponds and small lakes support vegeta- tion which decays, filhng their bottoms with putrescible material which is gradu- ally transformed to humus with a lowering of oxygen and the development of poison- Diagrammatic representation of a OUS decomposition products. The ponds d^sLsmaStli^re^onofero/b^and ITT ji nil IT 1" 1 sandy bottom. Vertical dashes indi- and lakes are thus filled as well as dramed cate the region ot emerging vegeta- T .. , ^ r Ti t 1 1 *'°"' Crosses indicate the region of and all become swamp and finally dry land, submerged vegetation. stippUngin- ^ "^ "^ dicates the region of deep water or the Streams gradually erode their way down hypoiimnion. The region of piank- *^ â ' .' ton occupies the entire lake except to sea level and become meandering base S4 ^fdSd^X^^ttetuom'! level streams with fine silt bottom, sluggish ^o^'s""^'-^ current and an abundance of vegetation. The base level streams and dry land are the ultimate fates of all bodies of fresh water. With the changes enumerated, there is always almost complete change of animal and plant life. The
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectfreshwa, bookyear1918
