Other chemical properties of water

The effect of dissolved substances on the quality of the water environment cannot be individually assessed just from the viewpoint of the content of available nutrients because at the same time all biological effects of present elements and compounds play a combined role. Some substances have 

Soluble substances influence the course of physiological phenomena also by changing the osmotic value of water and its capability to absorb gases. Substances which are unimportant from the viewpoint of biochemistry or toxicology also participate in these effects since they do not metabolize at all or only slightly. 

All aquatic organisms tolerate only a certain range of osmotic values of neighbouring water, which means that they tolerate only certain limited concentrations of dissolved substances, particularly salts [24]. At a higher concentration (hypertonic) the osmotic value of the environment increases and the organisms are damaged or die because of the loss of water drawn from their cells. At too low concentration (hypotonic) the net osmotic flow of water is reversed and water endeavours to overfill and swell the cells, even to bursting point. 

In fresh waters the content of salts does not exceed 0.5%o and the osmotic pressure is considerably lower than the osmotic pressure in the cells. There- 

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