What is the difference in O2 availability for the salmon? Please help?

The oxygen concentration (dissolved oxygen) is normally higher in freshwater than it is for saltwater. The differences in the amount of dissolved oxygen are due to five (5) primary factors:

- The salinity (amount of salt) in the water

- The depth of the water

- The amount of salt and other minerals in the water

- Naturally occurring aquatic plants in the water

- The temperature of the water

Salinity:

Higher salinity reduces the amount of oxygen that can dissolve in the water.

The depth of the water:

Normally 20.9% of the gas in the atmosphere is oxygen. In water however; oxygen levels are much lower. The higher concentration of oxygen in the atmosphere will try to equalize itself by diffusing into the water - which is at a lower oxygen concentration and pressure. However, the diffusion of oxygen is limited by the pressure of both the atmosphere and the water. Because there is more hydrostatic pressure in deep water, less oxygen will diffuse. Thus, the oxygen concentration is less in deep water than it is in shallow water.

Minerals in the water:

Generally, oxygen concentrations are reduced due to the presence and oxidation of decaying organic matter (suspended, benthic, or sediment). In addition nitrogenous materials may exert an oxygen demand through bacterial oxidation of ammonia to nitrate.

Aquatic Plants:

Aalgae acts as both a source and sink of dissolved oxygen. During daylight hours algal photosynthesis produces oxygen in excess of algal demands (respiration). During nighttime periods when photosynthesis is absent, algal respiration may reduce dissolved oxygen levels significantly.

Temperature of the water:

The concentration of dissolved oxygen drops with increasing temperature because cold water can hold more dissolved oxygen than warmer water. Of all the variables, temperature has the greatest effect on dissolved oxygen concentrations in fresh water.

Provided below are two charts that compare the solubility (availability) of oxygen in fresh water versus saltwater for various temperatures.

A chart showing the drastic differences in dissolved oxygen concentration based on water temperature is available at: http://www.engineeringtoolbox.com/oxygen-solubilit...