Why seawater composition does not change?

Just as the salt content of seawater does not change, the concentration of its components also remains unchanged. To maintain the constant composition, it is necessary that dissolved ions are removed at the same rate as they are added. Processes by which ions are removed from seawater include the following:

1. Evaporative precipitation: In hot, dry climates where the sea is shallow and enclosed, solid deposits of salts such as rock salt (sodium chloride) and gypsum (calcium sulphate) are formed.
2. Chemical precipitation: When the concentration of a salt becomes too great, that salt forms a solid precipitate. For example, calcium ions and carbonate ions combine together to form insoluble limestone (calcium carbonate).
3. Biochemical removal: Organisms remove ions by scavenging them from seawater. Some animals like coral and bivalves make shells of calcium carbonate. Many organisms concentrate ions in their body tissues by a factor of 105 or more. For example, sea squirts concentrate vanadium, other tunicates concentrate niobium, oysters concentrate zinc, lobsters concentrate copper, and other shellfish concentrate mercury.