Water, a universal solvent, invariably contains many soluble salts. In fresh water the total salt content remains under 15 percent. Different types of soluble salts released by weathering of rocks, soil erosion and decay of organic matter, readily dissolve in water.Dissolved salts are substances which have particular significance for floating aquatic vegetation and phytoplankton, since these organisms do not depend on the substratum for the supply of nutrients. Salts of nitrogen, phosphorus and silicon are most important substances found dissolved in fresh water. Nitrates, nitrites and ammonium salts are essential nutrients for plants. Dissolved silicates in the water are readily utilised by diatoms and sponges in the construction of their body structures e.g. shells in case of diatoms and spicules in case of sponges.
Many other elements like calcium, magnesium, manganese, iron, sodium, potassium, sulphur and zinc are found dissolved in water and influence the aquatic biota variously. Iron being an essential nutrient exists as ferrous oxide or ferrous sulphide in different fresh water bodies. Its availability is greatly modified by the pH of medium. Calcium is an essential element for plants as well as for animals. External coverings of arthopods and shells of molluscs and tubes of some worms need calcium carbonate.