Gas pressure can be expressed in several ways. The pressure of the atmosphere on the ocean's surface (or your lungs if you are standing on the beach) is about 15 lbs/in2, which is termed one atmosphere (A). If you fill a glass tube that has one end closed with a fluid and immerse the open end in an open container of the fluid, the fluid will fall in the tube and pull a vacuum until the atmospheric pressure pushing it back up equals the weight of the column of fluid. This is a barometer, and the level of fluid will vary slightly as the total air pressure varies. Mercury is commonly used as the fluid. Standard air pressure at sea level will support a 760 mm (29.9 inch) column of mercury (in physiology, mmHg is the common unit of gas pressure, meteorologists use inches). Sometimes a mmHg is referred to as a torr, honoring Torricelli the inventor of the barometer. What is commonly termed suction is really the pressure of the atmosphere acting in the opposite direction [ha!].
Air is a mixture of gases (about 80% nitrogen, 20% oxygen and 0.035% carbon dioxide). These have a combined total pressure of 760 mmHg. Each gas has a partial pressure proportional to its fraction of the total. [An Example]
The oxygen of concern in physiology is not the covalently bound O2 in the H2O molecule, but the O2 in simple solution in the water that is available to the fish for respiration. O2 concentration of the air is a virtually constant 260 mg/L, while the O2 dissolved in water readily varies from 0 - 14 mg/L. O2 concentration in water depends on O2 solubility and the partial pressure of oxygen (PO2 ) as it goes into solution.
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The difference in partial pressure will tell you which way oxygen (or any gas) will diffuse, oxygen will flow from a region of high pressure to a region of lower pressure until the PO2 is the same in both regions. When air and water have the same partial pressure (in equilibrium) the water is said to be saturated. Aeration increases the rate at which oxygen will flow into water, but it does not change the partial pressure, so aeration speeds the movement of O2 into the water if the water is below saturation (PO2 < 150 mm Hg), but cannot raise the concentration above air saturation. If the atmospheric pressure is doubled from the normal 760 mm Hg to 1520 mm Hg, you also double the PO2 . [An Example]
Partial pressures increase and decrease in nature. For example, water at high elevations will have slightly less PO2 than water at sea level. More substantial changes occur as the result of biological activity. Algae blooms during a sunny day can increase the PO2 by 75 mmHg or more. On the other hand, if the water has a lot of biological oxygen demand from decomposition, the PO2 can fall. When the PO2 falls substantially below 150 mmHg the water is hypoxic. When this happens, it means that oxygen isn't being "forced" into the fish as hard and the fish must increase respiration or decrease oxygen consumption or O2 depletion in the tissue of the fish occurs, eventually killing the fish. If the PO2 of the water falls to 0, then it is termed anoxic and all aerobic aquatic life will cease.
For every 10 m down into the water column that a gas bubble is driven, there is an increase of 1 A pressure (the weight of 10 m of water is about the same as the entire atmosphere). Consequently, as gas dissolves into the water from that gas bubble it enters at high pressure and this increases the quantity and pressure of the dissolved gases in the water. [An Example] This is termed supersaturation. If a fish is swimming at the same depth (pressure) as that at which the dissolution occurred the gas pressure in the fish's blood is high, as well. This is not a problem as long as the fish stays at that depth. However, if the fish swims upward the hydrostatic pressure decreases and, since the solubility remains constant, the amount of gas the water can hold decreases and the excess gas is released as bubbles. Bubbles of gas in a fish's blood can quickly lead to fatal embolisms. This condition is called "gas bubble disease". Human divers can suffer from the same disease, but it is called "the bends". The deadly bubbles are usually not oxygen, but nitrogen. Gas supersaturation can occur below dams where the bubbles are driven deep into a plunge pool or are entrained under pressure in the turbines or passageways. It can also occur in the plumbing of hatcheries in ground water.
Note: supersaturation can only occur when gas goes into solution at pressure greater than 1 A, if gas goes into solution at the surface (1 A) then it can flow down to great depths without a change in gas pressure.
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