Atmospheric pressure results from the
weight of atmospheric gases pressing
down on the earths surface. The atmospheric
gases and their proportions are:
nitrogen, 78.084%; oxygen, 20.946%;
argon, 0.934%; carbon dioxide, 0.038%;
water vapor and other gases, 0.036%.
According to Daltons Law of Partial Pressures, each gas in the atmosphere exerts a pressure (partial pressure) in direct proportion to its percentage composition. For example, standard atmospheric pressure at sea level is 760 mm of mercury (Hg), and the partial pressure of nitrogen is 593.4 mm (760 mm x 0.78084).
Henrys Law states that each gas has a characteristic solubility, and its molecules will diffuse from the atmosphere into water until the pressure in water equals the partial pressure of the gas in the atmosphere. When this state is reached, the pressure of the gas in the atmosphere is at equilibrium with its pressure in water, and no further net exchange of its molecules occurs between the atmosphere and water.
At this stage, the water is said to be saturated with the gas. When the pressures of gases in the atmosphere and in the water are equal, the water is saturated with air.
Under certain conditions, water can contain either a lower or higher concentration of one or more gases than it should at equilibrium. When water is undersaturated with a gas, that gas enters the water from the atmosphere, and an equilibrium state is attained. Likewise, if water has more of a gas than it should a state called supersaturation the gas will diffuse from water to the atmosphere until equilibrium is reached.
Such diffusion does not occur rapidly in still water. Still water can remain undersaturated or supersaturated for several hours or days under certain conditions.
The saturation concentrations of
gases and air vary with water temperature
(Table 1). A sudden rise in temperature
would result in temporary gas supersaturation,
while a drop in temperature would
cause temporary gas undersaturation.
Concentrations of air and individual gases in water sometimes are given as percentage saturation. For example, saturation with dissolved air at 20 C is 25.06 mg/L (Table 1), but if water at this temperature contains 30.17 mg/L air, its percentage saturation with air is 120.4% [(30.17/25.06) x 100]. Obviously, at equilibrium, the percentage saturation of a gas (or air) in water is 100%.
There are several reasons why water
becomes supersaturated with air. Natural
warming of water in a pond or heating of
water in hatcheries are common causes. Air
leaks on the suction side of pumps or an
improper submergence depth of pump
intakes can cause gas supersaturation. Highly
efficient submerged aerators have also been
reported to cause gas supersaturation.
One of the best-known causes of gas supersaturation is entrainment of air bubbles when water falls over spillways of high dams. In cooler climates, water that infiltrates downward into aquifers in winter can be quite cold and contain a high concentration of air. In warm weather, water from wells in such aquifers tend to be supersaturated with air in respect to ambient air temperature.
Individual gases can be below or above saturation, with the most common example being dissolved oxygen. It is not unusual for surface waters in ponds to have dissolved-oxygen supersaturation of 200 to 300% during the afternoon because of photosynthesis. At night, photosynthesis stops, and respiration can cause dissolved-oxygen concentrations to fall to 50% or less of saturation.
Concentrations of Individual Gases and Atmospheric Air in Freshwater at Different Temperatures and 760 mm Hg.
Gas Bubble Trauma
Gases dissolve in the blood of fish,
shrimp and other aquatic animals. Suppose
animals are held in water at a certain
temperature, and their blood equilibrates
with gases in the water. Then suppose
the water is suddenly warmed, resulting
in gas supersaturation. The fish blood
also will be supersaturated with gases,
and gas bubbles can form in the blood. In
fact, anytime the blood of animals becomes supersaturated with gas, bubbles
This condition is known as gas bubble trauma, and it can lead to stress or mortality. Eggs may float to the surface, and larvae and fry may exhibit hyperinflation of the swim bladder, cranial swelling, swollen gill lamellae and other abnormalities.
A common symptom of acute gas bubble trauma in juvenile and adult fish is gas bubbles in the blood that can be seen in the surface tissues on the head, in the mouth and in fin rays. The eyes of affected fish also tend to protrude.
A variable known as DeltaP is used to
assess gas supersaturation in water relative
to gas bubble trauma. The DeltaP is
defined as the difference between the
total gas pressure in water and the barometric
pressure at a given location.
The DeltaP can be calculated by measuring the partial pressure of each gas in the water [(percentage saturation/100 x partial pressure in the atmosphere], summing the partial pressures and subtracting from the sum the barometric pressure. Fortunately, a relatively inexpensive instrument called a saturometer can be used to measure DeltaP directly.
Aquatic animals exposed to DeltaP values of 25 to 75 mm Hg on a continuous basis may exhibit some symptoms of gas bubble trauma, and low-level mortality may occur over an extended period of time. Acute gas bubble trauma occurs at greater levels of DeltaP. Symptoms will be more pronounced, and mortalities typically are 50 to 100%.
Supersaturation of pond waters with dissolved oxygen during afternoons is a common occurrence. This condition usually does not harm culture animals, for supersaturation does not persist for long and is often limited to surface waters. Animals can move to greater depths, where the combination of lower dissolved- oxygen concentration and greater hydrostatic pressure result in a lower DeltaP.
Nevertheless, carp reportedly had a greater frequency of disease when percentage saturation with dissolved oxygen exceeded 150% (DeltaP above 225 mm Hg). Mortality of fish and shrimp has been reported in culture systems where dissolved- oxygen supersaturation exceeded 300% (DeltaP above 450 mm Hg).
Supersaturating gases can be removed
from water in degassing towers in which
water is passed through screens or other
media to increase exposure to the atmosphere.
In ponds, aerators that splash
water into the air can lessen afternoon
supersaturation with dissolved oxygen
caused by a high rate of photosynthesis.
Of course, managers always should guard
against gas supersaturation caused by
heating water, pumps, submerged aerators
and air entrainment.