A second problem posed by ocean water is the lack of oxygen.
Oxygen enters water in two ways, from mixing on the surface and from aquatic
plants. It is removed through respiration by animals, plants, and decomposition.
The amount of oxygen in water depends on temperature and on the amount
of solutes dissolved in the water. As temperature and solutes increase,
the amount of oxygen decreases. The ocean contains less oxygen than most
freshwater because, it has a poor surface-to-volume ratio, it has plants
in a small proportion (the lighted zone), and because it has a salt concentration
of 30 parts per thousand.
