[SIZE=16pt]Dissolved Gases[/SIZE]
Of the dissolved gases, oxygen is the most important. In ponds, photosynthesis by algae is the primary source of oxygen. A diurnal cycle is established, which coincides with photosynthetic activity. During daylight hours, when photosynthesis occurs, oxygen levels rise and carbon dioxide levels fall. At night, respiration is the driving force, resulting in a decrease in dissolved oxygen (DO) and an increase in carbon dioxide. Most finfish thrive when the DO concentration is >5 mg/L. When DO is <5 mg/L, fish become stressed; depending on species, size, and duration of exposure, a fish kill may result. Cardinal signs of a fish kill caused by hypoxia include sudden, significant mortality, usually noticed early in the morning (when oxygen levels are lowest); often, large fish are affected more than small fish. Fish that are hypoxic often school near the surface of the water and may be seen trying to gulp air, a behavior referred to as “piping.” Differential diagnoses for piping include low DO, high nitrite, and gill disease.
Although low DO is most common early in the morning in outdoor ponds, it can occur at any time. The most common causes in ponds are cloudy weather, death of an algal bloom, and pond turnover. Pond turnover is a common cause of catastrophic mortality in pond fish. It occurs most frequently in deep ponds (>6 ft) and involves a phenomenon referred to as stratification. Water at the bottom of the pond cools, and a temperature gradient, called a thermocline, develops between warm surface water and cool bottom water. The thermocline acts as a physical barrier between the surface water (epilimnion) and bottom water (hypolimnion). Because photosynthesis, and hence oxygen production, occurs at the surface, the hypolimnion becomes hypoxic and develops a biologic oxygen demand. When the pond is mixed, or “turns over,” the oxygen is removed as the biologic oxygen demand of the hypolimnion is satisfied. This sudden removal of oxygen can result in oxygen depletion and a fish kill. The most common cause of pond turnover in the southern USA is a summer thunderstorm, in which energy released from cold rain coupled with wind and wave action is sufficient to mix the pond. Fish kills in Florida have occurred following hurricanes and have been attributed to pond turnover. Pond turnover can also be caused by seining, aeration, or other management practices that result in mixing of the epilimnion and hypolimnion. Fish kills caused by pond turnover can be avoided by performing a weekly oxygen profile during periods of greatest risk (usually during hot, summer weather). If stratification is detected, the pond should be aerated or mixed to break down stratified layers before a significant oxygen demand layer can develop.
When assessing dissolved oxygen and aeration in indoor systems or exhibits in which the primary source of DO is an aeration device, and water is clear, the percent saturation should be considered along with the total DO reading. The amount of oxygen that water can hold in saturation varies with water temperature, salinity, and altitude. Of these 3 factors, water temperature is the most important. As any of these variables increase, the amount of oxygen in solution at saturation decreases. Saturation tables are available to determine percent saturation for a given DO if temperature, salinity, and altitude are known. If oxygen saturation is below 100%, it may indicate inadequate aeration for the bioload or sanitation problems (development of anoxic, organic-rich areas within the system). In either case, an inability to maintain a system at or very near 100% oxygen saturation is a problem that requires correction. Most fish do well if oxygen is >5 mg/L; however, the % saturation should be considered an indicator of the system's health.
Gas bubble disease is caused by supersaturation of water with dissolved gases. In pet fish, it may be associated with the use of well water, which may contain high levels of nitrogen or carbon dioxide. This is easily remedied by aerating the water before it comes into contact with the fish. A common cause of gas bubble disease in public aquaria is the use of cavitating pumps and sometimes excessive turbulence in cold water exhibits. Gas bubble disease is manifest by exophthalmos and the presence of tiny gas emboli within fins, corneas, or other tissue. The presence of gas emboli within gill capillaries is diagnostic. Treatment of gas bubble disease is vigorous aeration to volatilize excess gas. Supersaturation can be assessed using oxygen saturation tables as described above. Permanent correction of the problem includes identification and correction of the source of the excess gas.
Carbon dioxide (CO2) can be toxic to fish when present at concentrations >20 mg/L. Water from affected systems often is acidic (pH <7). A quick field test for excessive CO2 involves vigorous aeration of a bucket of suspect water for 1 hr. A significant increase in pH (ie, >1 unit) over the hour is indicative of excess CO2. Fish exposed to high concentrations of CO2 may be quite lethargic. Hybrid striped bass exposed to toxic levels of CO2 (∼40 mg/L) were observed at the surface with their backs out of the water and reacted dramatically to salt added to the affected tank by trying to leave the water. Nephrocalcinosis and visceral granuloma were reported in salmonid culture, supposedly induced by a high level of CO2 in the water, leading to metabolic acidosis and urinary and tissue precipitation of calcium, around which extensive granulomas develop. Treatment for CO2 toxicity is increased and vigorous aeration. Stocking density should be assessed and may need to be decreased.
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