DISSOLVED OXYGEN IN AQUACULTURE
Aquaculture is a form of agriculture which involves the propagation, cultivation, and marketing of aquatic plants and animals in a controlled environment. World fish farming started as early as 2000 B.C. in China, but United States aquaculture started in the late 19th century.
Water quality is the single most important factor in selecting the proper location for an aquaculture facility. Wells and springs are the best sources of water, but other sources are acceptable if the quality and quantity are adequate. Important water quality characteristics to consider are temperature, dissolved oxygen, ammonia, pH, alkalinity, and hardness. In respect to temperature, fish may be classified as warmwater, coolwater, and coldwater species, with each type having an optimal temperature range.
To a great extent your water quality determines the success or failure of a aquarium livestock keeping. Physical and chemical characteristics such as suspended solids, temperature, dissolved gases, pH, mineral content, and the potential danger of toxic metals must be considered in the selection of a suitable water source. Of these many water quality characteristics, only temperature, dissolved oxygen, ammonia, pH, and alkalinity will be discussed. In any closed existing systems such as aquariums, a close watch should be kept on these critical characteristics.
As mentioned, fish are cold-blooded organisms and assume approximately the same temperature as their surroundings. Metabolic rates increase rapidly as temperatures go up. Many biological activities such as spawning and egg hatching are geared to annual temperature changes in the natural environment. These temperatures vary according to the particular species. Fish are generally categorized into warmwater, coolwater, and coldwater based on optimal growth temperatures. Channel catfish are an example of a warmwater species, with a temperature range for growth between 70° and 85°F. A temperature of 82°F is generally considered optimum for growth. This explains, in part, why catfish farming in the southern states, with their longer growing season, has been so successful.
Striped bass, hybrid striped bass, walleye, and yellow perch are examples of coolwater species. Ranges for optimum growth fall between 55° and 75°F. Temperatures in the upper end of this range are generally considered best for maximum growth for all coolwater species.
Coldwater species include all species of salmon and trout. Two of the more commonly cultured coldwater species in the North Central Region are rainbow trout, and to a lesser extent, brown trout. Their optimal temperature range for growth is 48°- 60°F.
Ideally, species selection should be based partly on temperatures of the water within the aquarium. Any attempt to match the fish with improper water temperatures will involve expenditures for heating or cooling to within the desired range.
Like humans, fish require oxygen for respiration. Fish absorbs the oxygen dissolved in the water directly through the 1 or 2 cell thick blood vessels of the gills and skin. Dissolved oxygen (DO) concentrations are expressed in parts per million (ppm) or milligrams per liter (mg/l). Both methods are the same since 1 mg/l is equal to 1 ppm. Some fish such as tilapia and carp are better adapted to withstand periodic low DO concentrations. However, concentrations greater than 5 to 6 ppm (70% of saturation) are required for good growth in fish.
Oxygen enters the water in three ways: (1) through air diffusing into the water at the surface, (2) through the photosynthesis of microscopic plants (algae) in ponds. (In this process carbon dioxide is converted into food by plants and oxygen is released as a by-product.) (3) through wind and wave action. Wave action can be mechanically generated with such equipment as a paddle wheel or large volume pumps.
Photosynthesis is the major source of oxygen in most ponds. Oxygen is produced during the day when sunlight shines on the plants near the surface of the water. Oxygen levels drops at night when respiration continues but photosynthesis does not. These predictable changes in DO that occurs every 24 hours are called the diurnal oxygen cycle. DO levels are highest at dusk and lowest at dawn.
Oxygen depletion refers to low levels of DO that results in fish mortality. Concentrations of 5-6 ppm is recommended for optimum fish health. When DO concentration drops below 2 ppm, fish are severely stressed, and when concentrations fall below 1 ppm they begin to die. The number of fish that die is dependent on how low the concentration gets and how long it remains low. Usually large fish are affected by low DO before small fish are.
Oxygen depletion occurs when oxygen consumption exceeds oxygen production. Increases in oxygen consumption can be caused by overabundance of aquatic plants or algae, thermal stratification of the water, increased organic waste entering the water (e.g. waste from the fish and excess fish feed), death and decay of organic matter (e.g. plant or algae) or by certain chemicals that removes oxygen directly from the water.
Oxygen depletion can occur anytime of the year, but is more likely in the warm summer months. Warm water is much less capable of holding oxygen gas in solution than cool water--see DO Table. For example, water that is 90°F can only hold 7.29 ppm DO at saturation, whereas water that is 45°F can hold 12.13 ppm DO at saturation. This physical phenomenon puts the fish at double jeopardy because at high water temperature the metabolic rate is increased, hence their physiologic demand for oxygen is increased.
Oxygen depletion can be predicted and, therefore prevented, by monitoring dissolved oxygen levels in a aquarium. The most efficient tool for measuring DO is an electronic oxygen probe and meter. DO Controllers can also be preset to turn on aeration systems if the DO level drops below a predetermined level. A galvanic Dissolved Oxygen Probe generates its own potential (current/voltage) with the Dissolved Oxygen in the water. At saturation, the Dissolved Oxygen probes will generate 30 to 120 millivolts depending on the membrane thickness, cathode, and anode size and materials. The higher the calibration point, the greater the sensitivity and accuracy at low ranges. At zero Dissolved Oxygen in the water, the probe will read zero millivolts. The galvanic probes inherently have no "zero drift". They also have greater stability, providing longer time intervals between maintenance.
The proper mounting of the Probe requires careful consideration. For accurate readings in the aquarium, the water must be moving (although the required movement is very slow—a few inches per second to replenish the Dissolved Oxygen consumed by the Probe in making its measurement). Such movement is often created by the fish. To avoid false readings, the probe should not be mounted near an aeration device. Furthermore, to prevent air bubbles from attaching to the probe and giving false readings, the probe’s membrane should be pointed upward. In some cases, the membrane may require protection from damage by large fish. This can be done by recessing the probe in a 2" pipe or pipe fitting (i.e. Tee or elbow).
Fish excrete ammonia and a lesser amount of urea into the water as wastes. Two forms of ammonia occur in aquaculture systems: ionized and un-ionized. The un-ionized form of ammonia (NH3) is extremely toxic to fish; ionized ammonia(NH4+) is not. Both forms are grouped together as "total ammonia nitrogen." (TAN).
NH3 + H2O = NH4+ + OH-
The temperature and pH of water affects the ratio of (NH4+):(NH3). At lower temperatures and lower pH, the above reaction shifts from left to right, decreasing the percent of toxic unionized ammonia.
In natural waters, such as lakes, ammonia may never reach critically high levels due to the low densities of fish. But the aquarist prefers to maintain high densities of fish and therefore runs the risk of ammonia toxicity. Un-ionized ammonia levels rise as temperature and pH increase. Toxicity levels for un-ionized ammonia depend on individual species; however, levels below 0.02 ppm are generally considered safe. Dangerously high ammonia concentrations usually are limited to water reuse systems, where water is continually recycled.
The quantity of hydrogen ions in water determines whether it is acidic or basic. The scale for measuring the degree of acidity is called the pH scale, which ranges from 1 to 14. A value of 7 is neutral, neither acidic nor basic; values below 7 are considered acidic; above 7 basic. The acceptable pH range for fish culture is normally between pH 6.5 and 9.0.
The controlling factor for pH in most aquacultural operations is the relationship between alga photosynthesis, carbon dioxide (CO2) and the bicarbonate (HCO3-) buffering system:
CO2 + H2O = H2CO3 = HCO3- + H+
At night, respiration by bacteria, plants, and fish results in oxygen consumption and carbon dioxide production, the reaction in the above formula goes from left to right, first producing carbonic acid (H2CO3), then bicarbonate HCO3- and H+ ions. The increase in H+ cause the pH to drop. During sunlight, respiration continues, but algae use CO2 for photosynthesis. Photosynthesis reduces the H+ ions and pH goes up.
CO2 + H2O + sunlight = C6H12O6 + O2
Alkalinity is a system by which wide pH fluctuations are prevented or "buffered." It is a measure of the carbonates (CO3-2) and bicarbonates (HCO3) as expressed in terms of equivalent calcium carbonate (CaCO3). An example of this type of buffering system is the addition of agricultural lime to prevent decline in pH. Hardness is the measure of the calcium and magnesium portion of the buffering system. These two elements can be absorbed by the fish's gills and, in addition to other uses, they help with the bone development in fish. Fish will grow over wide ranges of alkalinity and hardness, but values of 100 to 200 ppm are considered optimum.
pH of an aquarium can be measured using a pH electrode and a meter or controller. The controller can add the alkalinity or CO2 to the aquarium automatically based upon preset pH values.