Dissolved Oxygen
If you look around on the net for important water quality parameters affecting your fish, oxygen is a parameter that is often overlooked. However, like terrestrial animals, fish and other aquatic organisms do need oxygen to live. Sensitive fish and invertebrates will quickly die when the amount of dissolved oxygen in the water falls below the limit for the species concerned.
If the oxygen content of the water is too low, the fish will demonstrate symptoms as frequently gasping at the water surface, or trying to jump out of the tank. Although this might seem a bit suicidal, the fish simply has a desire to swim into more favourable waters, which would be possible in nature. These symptoms are not exclusive to oxygen problems however. Gasping at the surface and escape responses will also occur in case of high CO2 or NO2 concentrations.
Physical processes influencing oxygen saturation
A number of physical variables influence the actual amount of dissolved oxygen. An increasing barometric pressure will increase the amount of dissolved oxygen. Increasing the salinity will decrease the amount of dissolved oxygen. However, these factors only influence dissolved oxygen concentrations to a small extent in aquaria. A more important physical factor is temperature.
There is a direct correlation between temperature and the level of oxygen saturation in water. At increasing temperature the level of oxygen saturation decreases. This implies that tropical waters are saturated with dissolved oxygen at much lower levels than chilly temperate waters. Fishes are evolutionarily adapted to certain oxygen levels and cannot be acclimated to lower ones. Hillstream loaches, living in cool, clear and well-oxygenated streams, will show respiratory distress long before gouramis will.
INTERMEZZO
Why does temperature affect the solubility of oxygen? Oxygen is more soluble in cold water than in hot water. The decrease in oxygen solubility with increased temperature has serious consequences for aquatic life. Power plants that discharge hot water into rivers can kill fish by decreasing the dissolved oxygen concentration. But why does oxygen solubility change with temperature? Well, consider a cup of tea in which you add a lump of sugar. The bottom of the cup will be covered with sugar crystals. When a tiny amount of sugar dissolves, heat is absorbed. When a tiny amount of sugar crystallizes out of solution, heat is released. We can write:
heat + solid sugar + water = dissolved sugar
The equation represents two processes: dissolution going left to right, and crystallization going right to left. When the sugar crystals are dissolving at exactly the same rate that sugar is crystallizing out of solution, the system is at equilibrium. Altering the balance between dissolution and crystallization can be achieved by changing the temperature of the solution. Adding heat will favor dissolution while cooling the solution will favor crystallization. The temperature dependence of solubility can be explained using Le Chatelier's principle. It states that when a system at equilibrium is placed under stress, the equilibrium will shift so to relieves that stress. In this case, the "stress" is the addition of heat. Le Chatelier's principle predicts that heating the solution mixture will shift the equilibrium in favor of dissolution, to remove the added heat. This explains why sugar is more soluble in hot water than in cold. The same sort of analysis can be applied to solutions of gases. Dissolving oxygen in water releases a small amount of heat:
gaseous O2 + nearly saturated O2 solution = saturated O2 solution + heat
Heating the solution will shift the equilibrium to the left, again to remove the added heat. This results in less oxygen dissolving at higher temperature (Senese, 2007).
Biological processes influencing oxygen saturation
Besides physical factors that influence the oxygen saturation of your tank water, there are also a number of biological processes influencing oxygen saturation. First of all there are the fish. Fish need oxygen to generate the energy necessary for activities like locomotion and growth (Van Dam, 1995). During feeding, the increased activity to find and eat food leads to an increased respiration rate. After feeding, the oxygen consumption rate increases due to demands for ingestion. The consequence of this is that the oxygen consumption rates of well-fed fish are higher than those of starving fish, and increase with feeding level. When the amount of dissolved oxygen in the water is low, food intake is suppressed, because at low oxygen concentration fish cannot take up enough oxygen to support the energy demands linked to feed intake.
Algae and macrophytes (your aquarium plants) produce dissolved oxygen and consume CO2 during the day by photosynthesis. Because it requires light, photosynthesis stops during the night. Through respiration, algae and macrophytes consume O2 and produce CO2 throughout the day and night. Because photosynthesis and respiration do not always occur concurrently and at different rates, diurnal variations in O2 and CO2 occur. In natural systems (e.g. in case of dense algal blooms) oxygen concentrations might become depleted at night, affecting fish respiration (Moriarty, 1982). O2 concentrations are lowest early in the morning, when photosynthesis resumes. In an aquarium such drops in oxygen saturation are unlikely, especially when a sufficient water circulation is applied. This will encourage the interchange of oxygen in the atmosphere to enter the aquarium at the water surface.
A third type of oxygen consumers in the aquarium is often overlooked. Bacteria consume 30 times as much oxygen per body weight as fish (Horton 2007). They use this oxygen for the aerobic decomposition of organic material. More decomposition means more oxygen consumption, that’s why water high in decaying organic material is also low in oxygen. Next to that, nitrifying bacteria breaking down ammonia also require oxygen, i.e. your filter is breathing!
Take home messages
To conclude this article some important points regarding dissolved oxygen in the aquarium.
1. As the temperature rises the oxygen content of the water decreases.
2. Fish have varying abilities to tolerate reduced oxygen levels, depending on amongst others the habitat they originate from.
3. Consumption of oxygen by bacteria is an important factor, using up 30 times as much oxygen per body weight as fish.
4. Oxygen enters the aquarium at the surface of the water, from the surrounding air. Provide a sufficient water circulation to encourage oxygen uptake from the atmosphere.
References
Horton, A., 2007. Environmental health in marine aquaria. British Marine Life Study Society. http://www.glaucus.org.uk/Oxygen-1.htm, 15-09-2007.
Jobling, M., 1994. Fish Bioenergetics. Chapman & Hall London. pp. 155-166. Moriarty, D.J.W., 1982. Physiology. In: R.S.V. Pullin and R.H. Lowe McConnel (Eds.). The biology and culture of tilapias. ICLARM Conference Proceedings 7, 432 pp. International Centre for Living Resource Management, Manila, Philippines, pp 115-117.
Senese, F., 2007. General Chemistry Online. http://antoine.frostburg.edu/chem/senese/101/index.shtml, 15-09-2007.
Van Dam, A.A. and Pauly, D., 1995. Simulation of the effects of oxygen on food consumption and growth of Nile tilapia, Oreochromis niloticus (L.). Aquaculture Research 26, 427-440.