If these gases exceed the standard, they will cause irreparable losses to aquaculture!

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Dissolved gases

There are a variety of gases dissolved in water, and their main sources are twofold:

One is directly dissolved into water by the air; The second is the gas produced in the water body by the life activities of aquatic organisms and the chemical changes in the substrate or substances in the water.

The amount of gas dissolved in water varies according to the water environment, and is generally inversely proportional to the temperature of the water body.

Proportional to the atmospheric pressure, the air pressure increases, and the solubility of the gas increases accordingly; Inversely proportional to the concentration of impurities in the water body, such as hard water or water with a high salt content, will reduce the solubility of the gas.

Of the dissolved gases in the water, oxygen has the greatest impact on fish, followed by carbon dioxide and hydrogen sulfide.

1. Oxygen Oxygen dissolved in water is called dissolved oxygen. Oxygen is one of the necessary conditions for the survival of various organisms. Fish, shrimp, shellfish, and algae also rely on dissolved oxygen to sustain their vital activities.

The amount of dissolved oxygen in water is small and variable. The saturation of dissolved oxygen in freshwater is only 8~10 mg/L, which is less than 1/20 of the oxygen content in the air. There is less dissolved oxygen in seawater.

This indicates that the fish, shrimp, shellfish, and algae in the water have poor breathing conditions and face the threat of hypoxic suffocation from time to time. Fish deaths due to direct and indirect hypoxia sometimes account for up to 60 percent of all fish deaths in farmed fish.

It can be seen that it is very important to grasp the dynamic law of dissolved oxygen in water, be familiar with the causes of hypoxia and the countermeasures to solve hypoxia, and correctly organize aquaculture production and improve technology to achieve high yield.

There are two sources of dissolved oxygen in water:

First, the oxygen in the atmosphere dissolves in water in contact with the water surface, and this dissolution is very slow, especially on the still water surface, if the water surface is stirred, the dissolution rate of oxygen will be accelerated.

The second is the oxygen released by aquatic plants during photosynthesis, which is the main source of dissolved oxygen in water.

As a result of photosynthesis, dissolved oxygen in the upper water body can often be saturated or even more saturated.

The photosynthesis of plants can only be carried out when there is light, so in the same water surface, due to the different light times, the number distribution of aquatic plants is different, and the plane distribution of dissolved oxygen is also different;

The vertical distribution of dissolved oxygen at different depths of the same water area is different due to different light intensities and different numbers of aquatic plants.

On the same day, the oxygen released by photosynthesis by aquatic plants during the day far exceeds that consumed by fish and other aquatic organisms.

Especially in the evening, it is the peak time for dissolved oxygen in the water body, and sometimes there are even small bubbles adsorbed on the branches and leaves of aquatic plants;

At night, the morning is the time when the amount of dissolved oxygen in the water column is lowest, because aquatic plants cannot photosynthesize and produce oxygen, and fish and aquatic plants continue to consume oxygen for respiration.

This is the day-night difference in the amount of dissolved oxygen over the course of the day. The diurnal difference in dissolved oxygen in lakes and reservoirs is similar, but the amplitude is much smaller than that in ponds. Over the course of the year, there is a clear seasonal variation in dissolved oxygen in the water.

There are three aspects to the depletion of dissolved oxygen in a body of water:

One is the oxygen consumption of fish respiration; The second is the oxygen consumption of "water respiration", which refers to the oxygen consumed by the oxidation of suspended matter, plankton, dissolved inorganic matter and organic matter in the water body; The third is the oxygen consumption of the sediment.

Among them, the oxygen consumption of fish only accounts for 5% ~ 20%, which is only a small part; The oxygen consumption of the sediment accounts for about 10%; "Water respiration" consumes about 70% of oxygen.

In the suitable temperature range for fish growth, the oxygen consumption of fish respiration increases with the increase of temperature, and when the water temperature is high, fish respiration becomes one of the important reasons for the consumption of dissolved oxygen in water.

For example, at 15 °C, carp per kilogram of body weight needs to breathe 58~75 mg of oxygen per hour; When the water temperature is at 30°C, it increases to 200 mg.

In order to maintain normal life activities, fish must constantly breathe and consume oxygen.

The rate of oxygen consumption is related to the age, weight, length, sex and food quality of the fish. It is related to dissolved gases, salt content, pH, temperature, etc. in water.

The lethal limit of dissolved oxygen in freshwater fish varies with the species of fish and the many physical and chemical factors in the water body.

The same species of fish will also have different hypoxic tolerance at different stages of growth.

The lethal limit of dissolved oxygen in carps is 0.7~1 mg/L, and the optimal dissolved oxygen content is about 5.5 mg/L, but it can generally maintain normal growth when it is above 4 mg/L.

The lethal limit of oxygen for tilapia is 0.5 mg/L. Plankton and benthic organisms used as fish feed reproduce normally at an oxygen content of 3 mg/L.

Many aquatic animals, such as Daphnia, Daphnia, and Mosquito larvae, have less oxygen demand and can even be hypoxic for a short time. Rotifers and crustaceans can also grow and reproduce freely in water with an oxygen content of 0.15~0.4 mg/L.

2. Hydrogen sulfide

Hydrogen sulfide is formed by the decomposition of sulfur-containing organic matter under anoxic conditions. Or in sulfate-rich water, sulfate is reduced to sulfide, and then hydrogen sulfide is generated.

Both sulfide and hydrogen sulfide are toxic to fish, with hydrogen sulfide being the most toxic.

Generally, most of the sulfides exist in the form of hydrogen sulfide under acidic conditions, and when the dissolved oxygen in the water increases, the hydrogen sulfide is oxidized and disappears.

The toxic effect of hydrogen sulfide on fish is to combine with the iron in hemoglobin, so that hemoglobin loses its oxygen-carrying capacity, resulting in hypoxia in fish tissues.

Lethal concentrations of hydrogen sulfide in juvenile fish: 0.0087 mg/L for rainbow trout, 0.084 mg/L for goldfish, and the same is true for other aquatic organisms, showing that hydrogen sulfide is highly toxic to fish.

Therefore, special attention should be paid to the presence of hydrogen sulfide in the aquaculture waters.

3. Carbon dioxide

Carbon dioxide in natural water comes mainly from water vigour, plant respiration, and the decomposition of organic matter.

The atmosphere contains only 0.033% free carbon dioxide, and the amount dissolved in water from the atmosphere is very small.

At a water temperature of 25°C, when the carbon dioxide in the atmosphere and water is in equilibrium, the dioxide content per liter of water is only 0.5 milligrams.

The consumption of carbon dioxide in water is mainly absorbed and utilized by photosynthesis of aquatic plants to make organic matter.

The movement of carbon dioxide in water varies with the activity of aquatic organisms and the decomposition of organic matter, and there are also horizontal, vertical, diurnal and seasonal variations.

It generally changes in the opposite way of oxygen. For example, during the day, carbon dioxide in the water is consumed to a minimum by plant photosynthesis.

Photosynthesis stops at night, while animal and plant respiration and the decomposition of organic matter continue, which will cause the carbon dioxide in the water to continue to increase and reach its maximum value before dawn.

High concentrations of carbon dioxide have a paralyzing and toxic effect on fish. For example, juvenile tests on herring, silver carp and bighead carp have shown that the amount of dissolved oxygen in the water remains sufficient.

When the amount of carbon dioxide exceeds 80 mg/L, the fish have difficulty breathing; When it exceeds 100 mg/L, coma supine phenomenon occurs; Above 200 mg/L, it causes death.

Under normal circumstances, however, carbon dioxide does not reach the level that paralyzes fish and kills them. Except in the northern regions, the winter is long frozen backwaters.

or during the transport of live fish, carbon dioxide in the water may accumulate in high concentrations, threatening the life of the fish.

4. Ammonia

It is produced by the decomposition of organic matter when oxygen is insufficient, or by the reduction of nitrogen compounds by denitrifying bacteria.

The end product of aquatic animal metabolism is generally excreted in the state of ammonia, and the same is true for freshwater fish.

Exists in the form of NH3. As the water temperature increases, so does the NH3 ratio. NH3 and NH4+ are converted into each other in aqueous solution, and they are two substances with different properties.

NH3 is toxic to fish and other aquatic organisms, while NH4+ is non-toxic, and NH3 inhibits fish growth even in low concentrations.

The maximum concentration of long-term tolerance of NH3 in fish was 0.025 mg/L, and the allowable limit index was 0.05 mg/L.

NH3 levels are generally low in natural waters, and ammonia excreted by fish and aquatic organisms is diluted by water, which can also be converted into nitrate by nitrifying bacteria.

Therefore, there will be little impact on the fish. However, in water bodies with poor running water and high densities of aquatic life and fish, ammonia concentrations may reach levels that inhibit fish growth and must be closely monitored.