Dissolved Gases and Supersaturation in Aquarium Water

How temperature, salinity and pressure set gas solubility, what total gas pressure and percent saturation mean, and how warming water or pump air leaks push gases past saturation.

Why gases dissolve in water

Henry's law states that, at equilibrium, the amount of a gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. Each gas has its own proportionality factor, the Henry's law constant. At 25 °C the dimensionless solubility constant listed for carbon dioxide (about 0.83) is roughly 26 times that of oxygen (0.032), and oxygen's is about twice that of nitrogen (0.015), so CO2 is far more soluble than the two main atmospheric gases. Water exposed to air therefore holds a mixture of nitrogen, oxygen, argon and CO2, each in proportion to its share of air pressure and its own solubility.

Temperature, salinity and pressure

Near room temperature, gas solubility in water falls as temperature rises; the van 't Hoff equation describes this dependence. Dissolved salts lower solubility further, an effect called salting out and described by the Sechenov equation. Pressure acts the other way: higher atmospheric or hydrostatic pressure lets more gas dissolve, and lower barometric pressure at altitude reduces it. The USGS DOTABLES program, built on the Benson and Krause equations, gives the following oxygen saturation values at sea-level pressure (760 mm Hg):

At 25 °C, full-strength seawater holds about 18% less oxygen than fresh water at saturation (6.8 versus 8.3 mg/L, calculated from the USGS table). This is one reason the same temperature leaves less oxygen headroom in a marine tank than in a freshwater one. Oxygen requirements and low-oxygen problems are covered separately in the dissolved oxygen guide.

Total gas pressure and percent saturation

Water is supersaturated when it contains more gas than it would hold at equilibrium with the atmosphere at the existing temperature and barometric pressure. Total gas pressure (TGP) is the sum of the pressures of all dissolved gases, including water vapour. Percent total dissolved gas saturation compares that sum with barometric pressure, and ΔP is the difference between total gas pressure and barometric pressure in mm Hg. A worked example from Auburn University aquaculture material: water at 20 °C carrying 30.17 mg/L of dissolved air, where saturation is 25.06 mg/L, is at about 120% saturation.

Measuring a single gas is not enough. The EPA notes that water can be strongly supersaturated with oxygen while total gas pressure stays at or below 100% if nitrogen is undersaturated. Nitrogen, about 80% of the atmosphere and nearly inert biologically, is the most significant cause of gas bubble disease, because oxygen is consumed by tissues while nitrogen is not. Total gas pressure is measured with a saturometer: a thin-walled silicone tube permeable to gases but not to water, connected to a pressure gauge.

How supersaturation happens

• Warming of cold, gas-saturated water. As temperature rises, the same gas content represents a higher percent saturation. Using the USGS fresh-water values, water saturated with oxygen at 10 °C (11.3 mg/L) and warmed to 25 °C without degassing would be at about 136% oxygen saturation (11.3 ÷ 8.3, calculated). Minnesota hatcheries recorded supersaturation both from spring and well water and from heating saturated water to speed egg hatching.
• Air leaks on the suction side of a pump, or a pump intake that is not submerged deeply enough, draw air into the water stream, where pressure forces it into solution.
• Groundwater and well water, which can be supersaturated when it reaches the surface.
• Strong photosynthesis by algae or plants, which supersaturates oxygen specifically; the EPA notes that algal blooms often coincide with warmer water, which adds to supersaturation.
• Air entrained under pressure, such as water plunging into deep pools below dam spillways, or very efficient submerged aerators.

Effects on fish and invertebrates

Gases in the water equilibrate with the fish's blood across the gills and other surfaces. When body fluids become supersaturated, gas comes out of solution as bubbles (emboli) that can block blood vessels. Signs include bubbles in the fins, skin, opercula and eyes, eye haemorrhage and exophthalmia, buoyancy problems from an overinflated swim bladder, and death from emboli in the heart and large vessels. Clinical diagnosis and treatment of gas bubble disease are described in the dedicated gas bubble disease guide.

Sensitivity differs widely. EPA criteria documents report that, for salmonids, levels of 115% total gas saturation and above are acutely lethal to most species, 120% and above is rapidly lethal to all salmonids tested, and 110% can already cause external emphysema. In a Minnesota experiment with 7-hour exposures, salmonid mortality generally began at 120–125% total gas saturation, bluegills (a fish without a duct between swim bladder and gut) showed mortality at 113%, and white sucker fry showed no effects at 127%. Daphnia magna proved about as sensitive as salmonids, while crayfish tolerated close to 140%. Chronic sublethal exposure reduced growth and food conversion and was linked to eye damage and secondary fin infections.

Depth, agitation and degassing

Hydrostatic pressure offsets supersaturation by about 10 percentage points per metre of depth, so water at 130% saturation at the surface is at about 100% at 3 m. Aquariums are too shallow for fish to use this compensation in a meaningful way. Gases leave water when it is exposed to air in thin films or droplets: aquaculture practice uses degassing towers or packed columns, in which water runs over screens or media, and aerators that splash water into the air. In hatchery trials, letting water spill between two raceways removed 3–4% of gas saturation, and lake trout in the degassed water grew about 25% faster.