Factors determining the distribution of animals and plants in freshwaters

Conditions in freshwater differ to saline water in many ways; for example, lower concentrations of salts (especially Na, SO₄ and Cl) and smaller volumes of water. Smaller volumes result in a greater range and more rapid fluctuations in temperature, oxygen and other important parameters, making freshwater ecosystems more vulnerable to pollution. The smaller volume and area of freshwater bodies also act as a physical restriction on certain species due to size and feeding behaviour, restricting free dispersion of organisms. Rivers and lakes are isolated entities that make colonization difficult, resulting in comparatively low diversities. In terms of pollution, both lakes and oceans act as sinks with the former having only limited dilution and assimilative capacity.