Oxygen and Carbon Dioxide

a) Describe the physico-chemical qualities of water that are important to aquaculturists.
Aquaculture can be defined as the high-density production of fish, shellfish and plant forms in a controlled environment. Stocking rates for high-density aquaculture are typically thousand fold greater than wild environments. Modern fish culturists employ both open and close systems to raise fish. Open systems, such as, the raceways are characterized by rapid turnover of water. Closed systems are commonplace in pond culture. Closed aquaculture systems do not have rapid turnover of water, but do not have a high surface to volume ratio facilitating exchange of gases, nutrients, energy etc. with the surroundings. Water quality for aquaculturists refers to the quality of water that enables successful propagation of the desired organisms.
Physico-chemical parameters of water include: 1. Alkalinity
Alkalinity relates to the capacity of the water to accept protons and is a measure of the water’s buffering capacity. There are no direct effects of alkalinity on fish and shellfish, however, it is an important parameter due to its indirect effects, including the protection of aquatic organisms from major changes in pH. In low alkalinity waters, where CO2 and dissolved carbonates are at low concentrations, photosynthesis may be inhibited, thus restricting phytoplankton growth. Levels above 175 mg CaCO3/L reduces natural food production in ponds which, in turn, leads to a decrease in optimal production. Salt water is slightly alkaline and has a strong buffering capacity so alkalinity is not usually of concern for most seawater and brackish water aquaculturists.

2. Biochemical oxygen demand ( and COD)
It is a measure of the amount of oxygen required by bacteria, algae, sediments and chemicals over a set period of time. BOD is of importance in aquaculture because microbial degradation of organic matter is a major sink for dissolved oxygen, a highly important parameter for aquaculture. Aquaculture operations should not utilise waters which are polluted with chemicals and/or excessive nutrients. Increasing levels of BOD indicate organic pollution which is a cause of concern for aquaculturists. The amount of BOD needed for a particular system can be estimated by taking into account factors such as dissolved. Oxygen requirements of the culture species, the degree of pond aeration, seasonal temperature fluctuations, expected photosynthetic activity, and oxygen solubility.

3. Carbon dioxide
Their presence is important for the buffering capacity of the water. The level of carbon dioxide in the water is related to photosynthetic activity of aquatic plants and respiration of these plants and aquatic animals, as well as bio-oxidation of organic compounds. Dissolved carbon dioxide forms carbonic acid, causing a drop in pH. At equilibrium, freshwater contains about 2.0 mg/L CO2 and seldom rises above 20 to 30 mg/L. High concentrations of carbon dioxide have a narcotic effect on fish and even higher concentrations may cause death; however, such concentrations seldom occur in nature. The direct adverse effects can occur when there is an excess of free CO2, especially in waters low in dissolved oxygen. This latter situation can occur when too much free CO2 is utilized for photosynthesis of phytoplankton, or when water is vigorously aerated with CO2 free air. Free CO2 concentrations below 1 mg/L affect the acid-base balance in fish blood and tissues and cause alkalosis. Most aquaculture species will survive in waters containing up to 60 mg/L carbon dioxide provided that dissolved oxygen concentrations are high.

4. Color and appearance of water
These are not highly objective measurements but many fish farmers and crustacean farmers attach a lot of significance to these two properties of pond water. Color is a result of the interaction of incident light and impurities in the water .There are three common causes of water coloration and variations in water appearance: * suspension of silt and clay particles * significant growth of plankton, particularly microalgae * suspension of humic acids and other organic acids
The ‘color’ of the water, actually refers to turbidity due to significant silt and clay particle accumulation, or growth of phytoplankton and zooplankton.
This type of water coloration may be beneficial in tank and cage culture as it shades fish and prevents sunburn as well as reducing plant biofouling. It is reported that impending oxygen shortages in the water can often be detected by changes in colour.
Although high colour may shade fish and impede algal growth, it is usually due to tannins.
These are phenols which bind with protein and at high levels may affect fish respiration, particularly with sensitive fish species.

5. Dissolved oxygen
Dissolved oxygen is the most critical water quality variable in aquaculture. Anoxia occurs when dissolved oxygen levels in the environment decrease to the point where aquatic life can no longer be supported. Some species are more resistant to low levels of oxygen than others. It was noted that the amount of oxygen required by aquatic animals is quite variable and depends on species, size, activity (levels increase with activity), water temperature (doubles with every increase of 10°C), condition (lean fish consume less than fat fish), DO concentration, etc. The DO concentration can fluctuate in response to photosynthesis of aquatic plants and respiration of aquatic organisms. The amount of DO required also depends on partial pressure of dissolved oxygen in the water and its ability to exchange across gill membranes. DO level in water should be above 5mg/L In ponds, tanks and other enclosed culture systems, mechanical aeration can be used to lift dissolved oxygen levels, while water movement from currents and tides assists in open culture systems. Pure oxygen (oxygenation) may be used to supplement dissolved oxygen levels, particularly in intensive culture systems.
The most common cause of low DO in an aquaculture operation is a high concentration of biodegradable organic matter in the water, resulting in a high BOD. This problem is further exacerbated at high temperatures.

6. Gas super saturation (total gas pressure)
Super saturation of dissolved gas occurs when the pressure of the dissolved gas (total gas pressure; TGP) exceeds the atmospheric pressure. TGP refers to the sum of the partial pressures of dissolved gases in the water (i.e. oxygen, nitrogen and carbon dioxide).

| Oxygen supersaturation | Nitrogen supersaturation | Carbon dioxide supersaturation | Definition | Total gas pressure is not above saturation level. | Total gas pressure is above saturation level | Condition of higher levels of dissolved gases in water due to entrainment, pressure increases, or heating. | Mechanism | Oxygen displaces nitrogen in liquid | | diffusion | Reason why | Pure oxygen is used to oxygenate | -Situation develops when water and air is mixed under pressure.-Situation develops when water is heated | When there is high phytoplankton activity though respiration at night. | Results | up to 200–300% can be tolerated if oxygen is used directly or duringphotosynthesis (when air is used, nitrogen becomes the main component and problems can occur). It can cause massive distension of the swim bladder of salmonids, although the mortality is usually low. | gas bubble trauma which may cause acute or chronic problems,especially in eggs, larvae and juveniles. | levels above 20 ppm can lead to stress. mortalitymay not occur, even at levels of 30-40 ppm, High carbon dioxide levels in fish transport systems (where ventilation is absent) can inhibitoxygen uptake. |

7. Hardness
Total hardness primarily measures the concentration of all metal cations (usually dominated by calcium and magnesium in freshwater) in the water. Soft water is usually acidic while hard water is generally alkaline. In soft waters, carbonate and bicarbonate salts are in short supply. Hard water has been found to reduce the toxicity of several heavy metals (calcium and magnesium) as well as ammonia and the hydrogen ion. Some aquacultural species have a specific requirement for calcium, for bone formation in fish and exoskeleton formation in crustaceans. Calcium is also necessary for proper osmoregulation, and the calcium ion generally reduces the toxicity of hydrogen ions, ammonia and metal ions. High calcium levels in freshwater can inhibit phytoplankton growth; however, blue-green algae are known to thrive in harder water (high Ca2+) which can influence productivity of the pond water.
Meade (1989) recommended a range between 10 and 400 mg/L for aquaculture.

8. pH
The term pH refers to the hydrogen ion (H+) concentration in water; more generally, pH refers to how acidic or basic water is. In aquaculture, low pH is often a consequence of sulfuric acid formation by the oxidation of sulphide-containing sediments. Note that acidification of highly alkaline water can increase the free carbon dioxide concentration, resulting in CO2 toxicity rather than pH imbalance. In addition, acid water tends to dissolve metals more readily.
High pH in aquaculture is commonly a result of excess photosynthesis in waters with high alkalinity and low calcium hardness. pH can indirectly affect aquaculture species through its effect on other chemical parameters.
Low pH; * reduces the amount of dissolved inorganic phosphorus and CO2 available for phytoplankton photosynthesis. * results in the solubilisation of potentially toxic metals from the sediments
Hugh pH makes the toxic form of ammonia more prevalent.
Meade (1989) recommended that pH be maintained at between 6.5 and 8.0 for all aquaculture species. In freshwater, pH can change quickly due to the amount of carbon dioxide added or removed during plant growth. In culture systems, particularly recirculation systems, the pH may be reduced (more acidic) by the production of metabolites. Buffering is, therefore, important in such systems.
Seawater, in general, resists changes in the pH values.
NOTE: pH can change by the hour as a function of photosynthesis which removes carbon dioxide. This is particularly the case in pond-based culture systems.

9. Salinity (total dissolved solids)
Salinity is the main measure used in aquaculture, as it influences the water and salt balance (osmoregulation) of aquatic animals. Estuarine waters may range from 0.5 to more than 30 ppt often depending on the depth of the sample; marine waters range between 30.0 to 40.0 ppt.
Salinity directly affects the levels of dissolved oxygen: the higher the salinity, the lower the dissolved oxygen levels at given water temperature. Like temperature, salinity is an important limiting factor in the distribution of many aquatic animals. Salinity requirements can vary for particular species depending on their life cycle stage. Salinity also affects the temperature requirements of some species.
Freshwater organisms have body fluids more concentrated in ions than the surrounding water, meaning that they are hypersaline or hypertonic to the environment. These animals tend to accumulate water which they must excrete while retaining ions.
Saltwater species have body fluids more dilute in ions than the surrounding water; they are hyposaline or hypotonic to their environment. They must excrete ions and uptake water continually.
Salinity tolerance varies significantly between species and some species have wider tolerances than others. 10. Suspended solids and turbidity
There are three basic types of suspended solids:
phytoplankton, zooplankton and bacterial blooms
suspended organic and humic acids
suspension of silt and clay particles

All influence the level of turbidity (turbidity increases with suspended solids) and scatter light, restricting penetration into water. In aquaculture ponds, less light penetrating to the bottom inhibits growth of troublesome filamentous algae and aquatic weeds.
This turbidity is often measured in centimetres using a secchi disc. Typically, if the secchi disk reading is below 10 cm water turbidity is excessive. If turbidity is due to the presence of phytoplankton, there is likely to be a problem with dissolved oxygen concentrations when the light level decreases below the photosynthetic compensation level.
Conversely, if turbidity is due to silt/clay or organic matter, planktonic productivity will be low.
Suspended solids can cause gill irritations and tissue damage, which increases the stress levels of aquatic animals. Turbid waters can also shield food organisms and clog filters.
The practice of mechanical aeration tends to create water currents which maintain soil particles in suspension and perpetuates the turbidity of the pond. Problems of off-flavors in fish and crayfish are less common in turbid ponds. (except where algae cause the turbidity).
The effect of this criteria varies considerably between species. Meade (1989) recommended a level below 80 mg/L for aquaculture species. Marine species (e.g. snapper) are generally less tolerant, so the recommended guideline is