The salts are the principal dissolved chemical components of seawater and they are conservative. Concentrations of non-conservative substances are altered by biological or physical processes in addition to simple mixing. An example of a non-conservative substance is lead. Some lead enters seawater from the atmosphere as particles, while other lead is dissolved in freshwater run-off and enters the oceans in rivers. Lead bound in this way is rapidly carried to the seafloor so that any dissolved lead remaining in seawater is present in vanishingly small amounts.
Conservative substances have the longest residence time of any substance in seawater.
Thus while seawater circulates through the ocean basins on a time scale of about a thousand years, the salts in the oceans have an average residence time of millions of years. Rates of removal and replenishment are roughly constant, so the concentration of salts stays constant in seawater. In sharp contrast, residence times of non-conservative substances are on the order of just days to thousands of years. Only when the rates of removal and replenishment change does the residence time of any substance change. A case in point is the residence time of total carbon dioxide in seawater.