Habitat classification: The classification of fresh-water habitats

[First paragraph(s)...]
Habitats are places where animals live; their essential characteristics are therefore those which enable the animals to survive, grow and breed there. These characteristics may be either positive or negative. The positive ones are those which provide some essential requirement; the negative ones are those which denote the absence of some condition which would make it impossible for the animal to survive.The outstanding characteristic of fresh water habitats, when compared with terrestrial ones, is the far greater basic significance of the physico-chemical conditions. While these are of great importance to a land animal, they are largely the result of the biotic conditions, particularly the vegetation, among which it is living. In the water, the relation is very different; the physico-chemical factors are the result of the topographic and geological structure of the locality and are virtually uninfluenced by vegetation or other biotic features. The physico-chemical factors therefore provide a much better basis for habitat classification in water than they do on land.