Dissolved Oxygen

Dissolved oxygen is oxygen gas that is dissolved in water. Like terrestrial animals, oxygen is essential to the survival of fish and other aquatic organisms to live. As water moves past their gills (or other breathing apparatus), oxygen gas is transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen is also required by virtually all aquatic plant life, and is important for many chemical and biological reactions that occur in water.

Light, temperature, pH, the number of photosynthetic organisms, depth, turbulence, altitude, and salinity are all factors that affect the dissolved oxygen level in water. By manipulating these factors, the dissolved oxygen in water can be increase or decreased. Oxygen is produced during photosynthesis and consumed during respiration and decomposition. Because it requires light, photosynthesis occurs only during daylight hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in dissolved oxygen concentrations.

During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, dissolved oxygen concentration may steadily decline. It is lowest just before dawn, when photosynthesis resumes. Low dissolved oxygen concentrations in a water body can also lead to associated toxic effects, such as the release of ammonia and sulphides from bottom sediments to the water column. The consequences of these processes are sometimes seen in tropical rivers in Australia where the combined effects may be the death of fish and other aquatic animals.

In Australian rivers, dissolved oxygen levels are naturally affected by wet-season floods, and run-off from nutrient rich soil. This increase in nutrients and available light contribute to the growth of aquatic plants (including algae and cyanobacteria). For this reason, waterbodies with high plant biomass can show a daily pattern of dissolved oxygen cycling, where oxygen levels are high in the evening but drop during the night to a minimum at dawn. Dissolved oxygen levels can fluctuate between 2% and 80% daily for several weeks (Pearson et al., 2003). These changes in dissolved oxygen that occur every 24 hours are called the

Diurnal Oxygen Cycle’. Because the solubility of oxygen is lower at higher temperatures and because of increased metabolic demand by aquatic organisms at high temperatures, aquatic ecosystems in the tropical climate of northern Australia are under particular threat of experiencing oxygen deficiency, predominantly at times when water levels are low and lack of water movement reduces oxygen diffusion at the air-water interface.

Photosynthesis is a fundamental biological process that uses light energy to produce sugars from carbon dioxide and water. Submerged aquatic plants (including planktonic algae), increase dissolved oxygen levels in water by directly releasing oxygen into the surrounding water. Oxygen may also be released into the water by some floating plants from their rootlets of which oxygen actively diffuses into water. Submersed macrophytes oxygenate the water more effectively than floating-leaved macrophytes, as solid cover of the latter prevents effective reoxygenation from the air. Decaying macrophytes consume large amounts of oxygen and macrophyte leachate stimulates oxygen consumption by suspended bacteria. Large mats of floating aquatic plants, such as Salvinia molesta and Eichhornia crassipes will produce almost totally deoxygenated conditions underneath these plants, because they reduce the light penetration and prevent oxygenation of water by blocking the water-air interface.

Native water lilies (Nymphaea spp.) do not appear to have the same negative impact on dissolved oxygen levels as floating mats of exotic vegetation. Water lilies are less effective at blocking oxygen transfer from the atmosphere to the water column. Water lilies attach to bottom sediments and only inhabit certain depths, often preventing them from colonising entire water bodies. Species such as water lilies can occur at relatively high densities without significant adverse impacts on dissolved oxygen (although oxygen concentrations may still be below saturation).

Emergent and floating plants generally release oxygen into the air but all submerged plants release oxygen directly into the surrounding water. Under adequate light, bubbles of oxygen can often be seen on the leaves of aquatic plants. However, since photosynthesis requires light, plants don’t produce oxygen during the night but respire and remove a small amount of oxygen from the water which slightly decreases oxygen levels. As a general rule, submerged aquatic plants produce about six times more oxygen through photosynthesis than they consume through respiration. However, under low light conditions these submerged plants are net consumers of oxygen.

Oxygen is not as abundant in water as it is in air. Air can be regarded as having a constant percentage (approximately 20.9%) of oxygen. Wherever air is exposed to water, the oxygen in the air will dissolve in the water. The amount of oxygen that dissolves in the water depends on many factors: whether there is adequate time and adequate mixing to fully saturate the water, the water temperature, the air pressure, etc. Water that contains the maximum amount of oxygen that can be obtained from the overlying air under the prevailing conditions is said to be saturated and the dissolved oxygen concentration is 100 %Saturation. The concentration of oxygen dissolved in water can be expressed as mg/L or as percentage of air saturation value. Water temperature, atmospheric pressure and dissolved salts in water have to be taken into account when the values in mg/L are converted to %Saturation or vice versa.

There are two main sources of dissolved oxygen in an aquarium: Oxygen diffuses into the water from the air especially when the surface is agitated and also from the photosynthesis of aquatic plants. On the other hand, oxygen is removed by the aerobic degradation of organic substances by bacteria and by the respiration of all the organisms present in the water. Most indoor aquarium systems lack sufficient photosynthesis. Therefore, mechanical means of aeration is the only alternative for supplying oxygen to aquatic animals maintained in these systems. Providing some form of aeration or surface agitation to aquarium water will allow more water to contact air at the surface, increasing dissolved oxygen levels and maintaining oxygen at safe levels.

Different fish species have different requirements for the concentration of oxygen dissolved in water. The oxygen requirements of fish also depend on a number of other factors, including the temperature, pH, and CO₂ level of the water, and the metabolic rate of the fish. The major criteria for the oxygen requirement of fish include temperature, and the average individual weight and the total weight of fish per unit volume of water. Oxygen requirements increase at a higher temperature; a higher total weight of fish per unit volume of water can lead to increased activity and thus increased respiration as a result of overcrowding. Oxygen deficiency causes asphyxiation and fish will die, depending on the oxygen requirements of the species and to a lesser extent on their rate of adaptation.

In general, it is recommended that the dissolved oxygen concentration be kept near to saturation. Typical values in a healthy aquarium system should be around 8 mg/L or 85–95 %Saturation at 24°C. If the level declines below 3 mg/L most aquatic species begin to show signs of suffocation. In aquariums equipped with proper filtration and aeration, insufficient dissolved oxygen is seldom a problem. It is therefore generally unnecessary to test oxygen levels in aquariums, except when carbon dioxide is being added as a fertiliser for aquatic plant growth. However, corrective measures need to be initiated if conditions become unfavourable. Remedial action is to aerate the water. Aeration can be with air or oxygen pumps, by agitating the water surface, or by increasing the input of aerated water.

Rainbowfishes exposed to oxygen deficient water do not take food, collect near the water surface, gasp for air, gather at the inflow of filters where the oxygen levels are higher, become sluggish, fail to react to irritation, lose their ability to escape capture and ultimately die. The major pathologicoanatomic changes include a very pale skin colour, congestion of the cyanotic blood in the gills, adherence of the gill lamellae, and small haemorrhages in the front of the ocular cavity and in the skin of the gill covers. In the majority of fishes the mouth gapes spasmodically and the operculum over the gills remains loosely open. The only way to know for sure if low oxygen levels have caused fish deaths in an aquarium is to measure the oxygen in the water when the deaths occurred. Indications of oxygen depletion as a probable cause of a fish death include:

All fish die at approximately the same time (often during the night or in the pre-dawn hours).

Large fish may be affected more than small fish.

Moribund fish may be seen at the surface “gasping” for oxygen.

Some species may die with their back arched, gills flared, and mouth open.

Dissolved oxygen can be measured with an electronic metering device or with a chemical titration test. Dissolved oxygen meters can be expensive, so most aquarists will generally use the chemical titration method. Most aquarium test kits do not meet the requirements for precision and accuracy needed for professional quality data. However, most are reliable if used correctly and can provide good results for aquarium use. Specific instructions on how to use kits are provided with the kits and will vary according to the manufacturer. Commercial test kits are based on the “Azide-Winkler” titration method. It is the most reliable method, against which the others are compared to test for accuracy. It’s important to become familiar with water testing and know how to use the associated test kits.

Dissolved oxygen concentrations are commonly reported as milligrams per litre (mg/L) or as percentage saturation (% Saturation). They measure the same thing, but sometimes your test kit will use only one of the measurements. Most oxygen meters can read oxygen concentrations as both mg/L (ppm) and %Saturation. If the water temperature, salinity and barometric pressure are known, either of these can be calculated from the other. There is confusion in the literature regarding which is the better measure to employ in testing aquarium water. In practice both measures are needed to fully interpret data, but %Saturation is the most readily interpreted and ecologically relevant of the two.

Percent Saturation (%Saturation) is the amount of dissolved oxygen in the water compared to the maximum amount that could be present at the same temperature. As temperature increases, the concentration at 100% saturation decreases. These factors affect the percent saturation (the highest dissolved oxygen level possible even in well-aerated water). Living organisms require specific minimum levels of dissolved oxygen to survive. Saturation values less than 60% or over 125% are undesirable. Dissolved oxygen %Saturation values in the range of 80–120% are desirable.

It is the saturation level that directly indicates how much oxygen is available for aquatic organisms to breathe, not the amount that is dissolved in the water. As a rule of thumb, a fish in water that is 100% saturated with oxygen is able to gain access to an amount of oxygen equivalent to that in the overlying air.

If the concentration falls to 50 %Saturation then it can only obtain half the amount of oxygen that is present in the overlying air. The mg/L concentrations of dissolved oxygen required to achieve these saturation levels vary enormously, particularly with temperature, so results expressed in terms of mg/L are much more difficult to interpret. At saturation the partial pressure in the water is equal to that in the thin layer of moisture-saturated air at the surface layer.

Well-aerated water (in free interchange with the air) will usually be 100% saturated. In general, the colder the water the more oxygen it can dissolve, the more saline the water the less oxygen it can dissolve, and the lower the atmospheric pressure (e.g., the higher the elevation), the less oxygen it can dissolve. These generalities come from the gas laws of physics. Oxygen saturation is calculated as the percent of dissolved oxygen relative to a theoretical maximum concentration given the temperature, pressure, and salinity of the water. It is possible to get more than 100% saturation. The water can be supersaturated in an area where there are a lot of plants or algae on a sunny day (due to photosynthetic activity).

Calculating oxygen levels in an aquarium can be somewhat complicated. Water saturated with oxygen at 15°C contains about 9.8 mg/L, whereas water at 30°C is saturated at about 7.6 mg/L. A reading of 1 mg/L @ 30°C (13.15 %Saturation) is a higher concentration than 1 mg/L @ 15°C (10.2 %Saturation) and represents more available oxygen. The figures below gives the amount of dissolved oxygen in mg/L (ppm) that represent 100 %Saturation in freshwater at normal pressure and different temperatures. To calculate the percentage of dissolved oxygen, test the amount of oxygen present in your tank (in mg/L or ppm), and divide the number by the mg/L value below that correspond to your tanks temperature. The answer is the percentage of dissolved oxygen in your system. Answers over 100% are perfectly valid, and indicate supersaturation of the water. So if you have a temperature of 25°C and you measure an oxygen level of 5 mg/L (ppm) you divide the ppm (5) by 8.3 = 60 %Saturation.

Freshwater: @ 20°C normal pressure 9.1 mg/L = 100 %Saturation @ 22°C normal pressure 8.7 mg/L = 100 %Saturation @ 24°C normal pressure 8.4 mg/L = 100 %Saturation @ 25°C normal pressure 8.3 mg/L = 100 %Saturation @ 28°C normal pressure 7.8 mg/L = 100 %Saturation @ 30°C normal pressure 7.6 mg/L = 100 %Saturation

Dissolved oxygen levels change according to the time of day, the temperature and the weather. Levels are usually lowest in the morning and highest in late afternoon. The ability of oxygen to remain in the solution decreases as water temperature increases (e.g., dissolved oxygen saturation decreases by about 2% for each 1ºC increase in temperature). Temperature also increases the metabolic rate of aquatic animals, resulting in increased consumption of oxygen. As a result water temperature has a significant influence on dissolved oxygen levels. Therefore, dissolved oxygen in an aquarium must be maintained above levels considered stressful to the fish.

Prolonged exposure to low oxygen may cause a slowing in growth rates, reproductive difficulties, stress, susceptibility to disease, and in severe cases of depletion, premature death. Usually larger fish are more affected by low dissolved oxygen levels than smaller fish.

Problems caused by too much oxygen dissolved in water are seldom encountered. However, it may happen, for example, when fish are transported in polythene bags with an oxygenfilled air space. The critical oxygen level of water is 250 to 300% of the air saturation value; fish may be injured at these higher values. The gills of such affected fish have a conspicuous light red colour and the ends of the gill lamellae fray. When such fish are placed in the aquarium water they may suffer from secondary fungus infections and some of them may die. It is possible that fish adapted to such high oxygen levels need to be progressively acclimatised to more normal concentrations. This condition should not be confused with the supersaturation of water with dissolved gas, which can cause gas bubble disease.

Supersaturation with dissolved gas occurs when the pressure of the dissolved gas exceeds the atmospheric pressure. It occurs when water is equilibrated with air under pressure, e.g., at the bottom of a lake or reservoir, in ground water, or if air is drawn into a centrifugal water pump. It can also occur if cold air-equilibrated water is warmed up without re-equilibration to the higher temperature. A bottle containing such water will show either minute bubbles forming as a cloudy suspension which will clear from the bottom upwards, or larger bubbles forming on the glass wall. This is analogous to that seen in an opened bottle of carbonated drinking water.

If fish are exposed (at a lower atmospheric pressure) to such water, their blood equilibrates with the excess pressure in the water. Bubbles form in the blood and these can block the capillaries; in sub-acute cases the dorsal and caudal fin can be affected, and bubbles may be visible between the fin rays. The epidermal tissue distal to the occlusions then becomes necrotic and cases are known where the fins have become completely eroded. In severe cases, death occurs rapidly as a result of blockage of the major arteries, and large bubbles are clearly seen between the rays of all the fins. A similar effect of gas bubbles forming in the blood can be experienced by deep-sea divers when they return to the surface. The remedy is either to remove the fish to normally equilibrated water or to provide vigorous aeration to strip out the excess gas.

There is little information available on how rainbowfishes are affected by low levels of oxygen. The minimum dissolved oxygen level that rainbowfishes can safely tolerate depends upon individual species and temperature. Different life stages (i.e., eggs, larvae, juveniles and adults) may also have different oxygen needs. Several experiments have been conducted on the effects of low oxygen on rainbowfishes and other freshwater species. One such study

(Flint, 2003) reported that Melanotaenia splendida and

Melanotaenia utcheensis died when dissolved oxygen saturation reached 7%. Lower egg production was also noted over the duration of the study in tanks with lower oxygen levels. However, the eggs of Melanotaenia utcheensis and Melanotaenia splendida were found to be remarkably tolerant to low dissolved oxygen. Eggs were able to survive and produce viable larvae at dissolved oxygen saturations levels lower than those that killed their parents (lowest tested was 5% at 28°C).

In another study, experiments were conducted to identify acute threshold values for Melanotaenia splendida exposed to various low dissolved oxygen levels. The rainbowfishes exposed to oxygen levels of 25–35 %Saturation or higher for 5 days experienced negligible mortality. In contrast, all the fish exposed to 1–10 %Saturation died within 24 hours after the start of the experiment. Five-day exposure to dissolved oxygen levels of 25–35 %Saturation did not affect the survival, and did not appear to affect the feeding behaviour, of adult Melanotaenia splendida. However, they appeared to be more lethargic following exposure to 25–35 %Saturation than individuals exposed to higher oxygen treatments. Relative to normoxia (approximately 100 % Saturation), breathing rates of Melanotaenia splendida had doubled by the time oxygen levels had declined to 55 % Saturation, nearly tripled by 40 %Saturation and quadrupled at approximately 35 %Saturation.

Furthermore, a 24-hour experiment on survival of Melanotaenia splendida by Pearson et al. (2003) revealed a fine line between survival and mortality. The data suggested that Melanotaenia splendida exposed to lowered dissolved oxygen levels for 24 hours were able to survive exposure to oxygen levels down to 13 %Saturation. Levels of approximately 12 %Saturation were lethal to 90% of fish, while exposure to oxygen levels of 9 % Saturation was lethal to all the rainbowfishes. However, it was apparent during the experiment that the rainbowfishes were making use of dissolved oxygen gradients in the test tanks that were unable to be measured. Thus, the recorded oxygen measurements are not indicative of the dissolved oxygen concentration that the rainbowfishes were actually exposed to.

Another study (Flint, 2005) tested the tolerance of various life history stages of several native fish species to fluctuating oxygen levels. Water temperature was maintained at 28°C (± 2°C) during the experiments. The rank order of resistance to oxygen levels of each species/life history stage from highest to lowest was: eggs of Melanotaenia splendida and Melanotaenia utcheensis (no immediate lethal level identified), juvenile Melanotaenia splendida and adult Melanotaenia utcheensis (lethal level 6-7% for both). Previous studies show that if oxygen levels drop suddenly the dissolved oxygen saturation that is lethal to rainbowfishes is a much higher threshold (Pearson et al., 2003). Oxygen levels experienced by wild populations of Melanotaenia utcheensis range from 5.7 mg/L (70-77 %Saturation at 25-30°C) to 9.2 mg/L (supersaturated waters of about 112-124 %Saturation at 2530°C).

Rainbowfishes can perform aquatic surface respiration and access higher oxygenated water at the air-water interface to survive lower oxygen concentrations for short periods of time. They stay just below the surface, put their snout at the air-water interface, and breathe in the film of water that is in direct contact with the air. This thin layer of water is comparatively rich in oxygen. Typically, rainbowfishes will wait until a very low threshold of oxygen concentration is before starting surface respiration. This reluctance to breathe near the surface is easy to understand when we consider that, in nature, many predators of rainbowfishes are terrestrial or aerial animals that attack from above, and therefore being close to the surface has some risks.