The proportion of dissolved substances in seawater is usually expressed in

The proportion of dissolved substances in seawater is usually expressed in ppm, ppb or ppt

Explanation:

The concentration of very diluted solutions should be expressed in parts per million, billion or trillion.

ppm = mass from the solute. 10⁶ / mass or volume of the solution

ppb = mass from the solute. 10⁹ / mass or volume of the solution

ppt = mass from the solute. 10¹² / mass or volume of the solution

ppm = mg/kg, μg/g, μg/mL → These are the units

ppb = ng/g

ppt = pg/g