The more saline waters are chemically similar to seawater. Major constituents include chlorides, sodium, magnesium, calcium and potassium. Dissolved salts are important to the life cycles of many organisms. Some fish spawn in fresh or slightly brackish water and must move to more saline waters as they mature. These species have internal mechanisms that enable them to cope with the changes in salinity.
Seawater also contains hundreds of trace elements that are important in many biological reactions.
For example, living organisms require minute quantities of cobalt to make vitamin B-12. Metals such as mercury, lead, chromium and cadmium also occur in low concentrations. The composition of seawater is relatively constant from place to place. Freshwater, however, varies depending upon the soil and rocks with which the water has come in contact. Both fresh and saltwater contain a myriad of natural dissolved materials.