Based on Boyle's and Henry's gas laws, the amount of gas dissolved in water is determined by the partial pressure and solubility of the gas, and is independent of other gases. For practical purposes, atmospheric air is 80% nitrogen and 20% oxygen, or a ratio of 4:1. However, since O2 is twice as soluble as N2, the ratio of N2:O2 in air saturated water is 2:1, and remains at about at that ratio regardless of temperature or pressure.
The total amount of gas dissolved in water, however, increases as temperature decreases. At saturation under 1 atm pressure, O2 increases from 8.5 mg/L to 14.6 mg/L as temperatures fall from 25 degrees C to 0 degrees C. At saturation the total amount of gas dissolved in H2O increases proportional to pressure. For example, at 0 degrees C the amount of O2 in H2O at saturation increases from 14.6 mg/L to 29.2 mg/L as pressure increases from 1 to 2 atm.
When fish are exposed to deep H2O, they will seek neutral buoyancy by filling their swim bladder with gas captured at the surface or from bubbles produced from plants or substrate. When exposed to a sudden pressure reduction that exceeds their ability to evacuate their swim bladder, the bladder can rupture and cause mortality. Also, when fish are in deep water their serum will equilibrate with the dissolved gases in their environment. In lakes, water turover and gas recharging generally occur in winter months when temperatures are cool, and hence the ability of water to hold dissolved gas is greater. As the weather warms in the spring and summer, the dissolved gas remains in solution because of the hydrostatic head created with water depth. Fish brought from that depth faster than their ability to equilibrate with the decreasing hydrostatic pressure will suffer from the bends much the same as a deep water diver. In these cases when fish are depressurized, death occurs from gas expansion and/or from serum gas leaking out of the blood.
Ernest L. Brannon, Ph.D
Seattle, WA
I thought it would be good to revisit this topic since questions still arise from time to time. There are two previous posts concerning this topic that I will remind you about and they are:
CraigGipe..."How do you bleed an air bladder"
redneckshadrap..."Stripers and the F-n-F"
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