• 5. Dissolved oxygen vertical distribution law

About dissolved oxygen in water – Part 2

5.1 Distribution characteristics in aquaculture water at noon and afternoon during the day:

There is a lot of dissolved oxygen in the surface layer, and the saturation is as high as 200% or more. The dissolved oxygen in the bottom water is very little, and the saturation is about 40-80%, or even lower. In the middle water, dissolved oxygen decreases sharply with depth, forming a transition layer. The general tendency is that the dissolved oxygen content decreases sharply with increasing water depth.

The reason for this distribution characteristic is:

• After the sun comes out, the phytoplankton in the true photosphere perform photosynthesis to produce a large amount of oxygen, which makes the oxygenation in the surface water layer > oxygen consumption. The oxygen-enhancing effect exceeds the oxygen-consuming effect, and the dissolved oxygen content increases continuously, and the accumulation reaches a maximum value before sunset. In actual investigations, it is often found that the maximum value of dissolved oxygen does not appear in the surface water layer, but in the subsurface area. The reason, in addition to the escape into the air, is mainly related to the light intensity. If the light intensity in the surface water layer is too high, the photosynthesis of phytoplankton will be inhibited, and the oxygen production will be reduced. At this time, the light intensity of the surface water layer is suitable and the oxygen production is also high, so the maximum value occurs in this water layer.
• At the same time, the surface water absorbs sunlight energy and the water temperature rises. The specific heat of water is large and the thermal conductivity is small, so the thermocline appears in the surface and bottom water layer. If there is no component stirring and other factors to break this divided state. Then a large amount of oxygen in the surface water layer cannot be directly brought to the bottom layer of the water through the convective mixing of water. It can only rely on diffusion to slowly replenish downwards, so that the dissolved oxygen in the bottom water layer is much lower than that in the surface water layer.
• In the thermocline, although the depth is not much different, the temperature drops more rapidly with depth. Correspondingly, the density and buoyancy of water increase rapidly and rapidly. In this way, once the plankton debris, organic debris, etc. that sink from the surface water layer enter the thermocline, the sinking speed is greatly reduced due to the increase of buoyancy. Some of the fine debris is almost completely blocked by the clamshell, allowing a large amount of organic matter to accumulate there. With the bacteria also multiplying, quickly decompose organic matter, consume a lot of oxygen, and finally form a dissolved oxygen cline.

The depth of the dissolved oxygen cline is roughly the same as that of the thermocline, which is mainly determined by factors such as the heating rate of the surface water layer and the strength of wind agitation. When the temperature rises quickly, the time is short, and the wind agitation is weak, the climacteric layer is shallower from the water surface, and the change is more rapid. Deep, the change is gentler.

If the heating and cooling are staggered in a period of time, there may also be a complex situation of several transition layers. The difference between the maximum value and the minimum value of the vertical distribution of dissolved oxygen is called “water layer difference”. Its size depends on the water production performance and stratified flow. During the summer stagnation period, the higher the primary productivity of the water body, the greater the water layer difference, and the bottom water layer is often anoxic. The vertical convection of the water reduces the water layer difference and eliminates it.

5.2 At night, especially in the second half of the night, the concentration of dissolved oxygen continued to decrease, and the vertical distribution tended to be uniform. The reason is that there is only respiration and oxygen consumption after sunset. In addition, the air temperature drops after nightfall, the surface water temperature drops, the density increases, and the density difference between the surface and bottom disappears. Circulate, and finally mix evenly. Homogenize the vertical distribution of dissolved oxygen. It happens that the speed of heat dissipation and cooling on land and water is different, and there is a temperature difference and density difference between the water surface and the ground, so wind often blows from the land to the water surface at night, especially on large water surfaces. Some water bodies are either too deep or complicated, and even if there is wind at night, the separation state cannot be completely destroyed, and the bottom water layer is often in an oxygen-deficient reduction state.

6. Dissolved oxygen level distribution law

The distribution of dissolved oxygen in the pond mainly depends on the wind direction and wind. When there is no wind, the vertical distribution is uneven, and the horizontal distribution is generally uniform. Later, under the action of wind, the surface water with high dissolved oxygen content moved to the leeward coast. Those with low dissolved oxygen levels float upwind along the coast. Dissolved oxygen level distribution produces a non-uniform state. If there is very little dissolved oxygen in the bottom water, the fish and shellfish raised in the upwind coastal water will die of lack of oxygen.

In addition, at the inflow of tributaries of rivers, the inlet and outlet of lake and pond water, the inflow of fresh water in shallow seas, the places where domestic sewage and industrial pollution are located, and even the clusters of fish and shellfish, dissolved oxygen and other water quality characteristics are also closely related to the surrounding areas. The water quality varies considerably and is unevenly distributed horizontally. For example, it has been determined that when the seawater circulation is not good, the dissolved oxygen in the water inside the cage is much less than the dissolved oxygen outside the cage, especially when the cage mesh is blocked due to excessive stocking. Although there is a lot of dissolved oxygen outside the cage, the fish and shellfish in the cage will still die due to lack of oxygen. This level of inhomogeneity is often overlooked.

7. The general law of the appearance of dissolved oxygen extremes

It is well known that dissolved oxygen in water is an important environmental condition for fish survival and growth. The water body for fish farming requires more than 5 mg of dissolved oxygen per liter of water. If it is reduced to 1 mg, the fish may float, and even dead fish will occur in flooded ponds. Supersaturated oxygen in fish ponds is generally not harmful to fish, but high saturation can cause bubble disease in fish. Too low dissolved oxygen will cause fish to float their heads, and in severe cases, suffocate to death. Therefore, by grasping the law of the occurrence of the maximum and minimum values ​​of dissolved oxygen in water, the losses caused by the gain and loss of dissolved oxygen can be reduced.

First of all, it is necessary to understand the source and consumption of oxygen in the pond. There are several aspects of oxygenation in general aquaculture water: (1) the dissolution of oxygen in the air; (2) the oxygenation of plant photosynthesis; (3) the mixed oxygenation of water supply. These three aspects are mainly based on plant photosynthesis and oxygenation. Some investigations have pointed out that under natural conditions, photosynthesis and oxygenation account for about 89% of the total dissolved oxygen revenue in hydrostatic fish ponds, and air dissolved oxygenation accounts for about 7%. , and the remaining 4% is water supplementation and oxygenation. Oxygen consumption in water includes: (1) Diffusion into the air; (2) Respiration of aquatic organisms to consume oxygen; (3) Decomposition of pond sediment to consume oxygen. Generally speaking, the oxygen escaping into the air only accounts for about 1.5% of the total oxygen consumption, and the oxygen consumption of the fish raised only accounts for 5%~15%, and the other 80%

~90% is consumed by biological respiration and organic matter decomposition. It can be seen that the key factor that causes the uneven distribution of dissolved oxygen in water is biological factors (mainly photosynthesis and respiration), and it is not difficult to grasp the law of the maximum and minimum dissolved oxygen. The rules for the appearance of dissolved oxygen extremes are as follows:

(1) The maximum value of dissolved oxygen usually occurs in the surface water before sunset during the summer day. This is because most of the phytoplankton photosynthesize in the upper water after sunrise, releasing a large amount of oxygen, so that the oxygenation of the surface water exceeds the oxygen consumption, so the actual content of dissolved oxygen in the water gradually increases. After the accumulation throughout the day, before sunset , it accumulates to the maximum value. Therefore, when the water quality of the pond is fertile, the biological density is high, and the photosynthesis is strong, the dissolved oxygen may be supersaturated at this time.

(2) Occasions where the minimum value of dissolved oxygen occurs: ①In the surface layer before dawn or sunrise, especially in the bottom water, this is because the phytoplankton in the surface water after sunset not only cannot perform photosynthesis, but release oxygen, but instead respire and consume Oxygen, the effect of oxygen consumption greatly exceeds the effect of oxygen increase, and the actual content of dissolved oxygen decreases rapidly. After a long night of accumulation, it drops to the lowest value before sunrise, so it is the reason that floating heads are prone to appear before dawn. After sunrise, with the strengthening of photosynthesis, dissolved oxygen gradually rises, and the cycle continues. ②The bottom water (i.e. aging water) in the dead pond in summer maintains a stratified state for a long time, and the bottom and middle water along the upwind coast. Because of the wind, the density of plankton in the downwind is higher than that in the upwind. At the same time, the waves caused by the wind are also larger in the downwind, and the mixing effect is strong. Therefore, the oxygen produced by the photosynthesis of phytoplankton in the downwind and the oxygen dissolved in the air are higher than those in the daytime. The upper wind is higher, so the dissolved oxygen on the upper wind coast has the lowest value. However, in the morning, the dissolved oxygen is high in the upper wind, which is the reason why the fish tend to gather in the upper wind in the early morning.

③ Fish ponds with excessively fertile water quality, high stocking density, excessive feeding and fertilization, and thick bottom silt will encounter sultry summer weather and low air pressure, especially after heavy rain and strong winds, the surface water temperature will suddenly drop, and dissolved oxygen saturation will follow. Lowering, resulting in the lowest value of dissolved oxygen in the entire water body, which is likely to cause a large number of deaths of aquaculture objects. This is one of the reasons why Pantang appeared. If the above situation occurs at night, the consequences will be more serious.

Once the above rules are mastered, once the season and time when the extreme value of dissolved oxygen is likely to occur, it is necessary to pay special attention to the dynamics of dissolved oxygen, and strengthen the management of Xuntang and water quality. The purpose of opening the aerator at noon on a sunny day in summer is to break the stratification of the water body as soon as possible, and send the water rich in dissolved oxygen on the surface to the bottom water, so that the dissolved oxygen in the upper and lower layers tends to be consistent and prevent floating at dawn. It is turned on in the morning on a cloudy day, and it is turned on in the middle of the night with continuous rainy weather. Therefore, it is very good to start the oxygenation function correctly and timely to prevent the occurrence of pan-ponds. Bubble disease is caused by excessive dissolved oxygen and bubbles appear on the surface and body of the fish, so that the fish cannot sink and die soon. If bubble disease is found, the fish should be moved to clean water with low dissolved oxygen saturation, and the symptoms will be eliminated quickly. . Under natural conditions, this situation is rarely encountered, but it should be paid attention to when transporting fry with pure oxygen. When the fish have floating head symptoms, they should immediately add new water, and at the same time start the oxygen-enhancing equipment or put in the oxygen-enhancing agent.

It can be seen that it is very useful to master the dynamic laws of dissolved oxygen in water, and to be familiar with the causes and countermeasures of hypoxia, for correctly organizing aquaculture production, improving technology, and capturing high yields.

8. The Five Laws of Dissolved Oxygen

1. The higher the temperature, the lower the dissolved oxygen in the water. This is an inverse proportional relationship!

Let me talk more here. In general fresh water bodies, the dissolved oxygen in the water is mainly composed of two parts, one part is the oxygen released by the photosynthesis of underwater plants, accounting for about 60%, and the other part is the movement of water molecules to make the air in the air Oxygen dissolved in water molecules is dissolved oxygen, accounting for about 40%, and it is also an important source of water dissolved oxygen.

,2, the deeper the water, the lower the dissolved oxygen, which is also an inverse proportional relationship!

The deeper the water, the greater the water pressure, and to a certain extent, it will be far greater than the air pressure in the air. Therefore, the deeper the water, the worse the ability of the oxygen in the air to penetrate into the water!

If the depth of seawater exceeds 200 meters, there will be no plants to survive, let alone dissolved oxygen. So where does the oxygen needed by deep-sea creatures come from? This is mainly due to ocean currents!

Generally, the depth of freshwater waters is not that large, and oxygen can penetrate into it. However, the dissolved oxygen in deep waters will be very low. If there is light, it is good. The ability is very poor, too deep water is often very hypoxic!

,3, the lower the air pressure, the lower the dissolved oxygen, which is a proportional relationship!

The lower the air pressure, the smaller the air pressure relative to the water, and the larger the water pressure in disguise. Therefore, the oxygen in the air will encounter greater resistance in the process of penetrating into the water, and the ability of the water to dissolve oxygen will also change. Worse!

At this time, not only the fish on the bottom feel very uncomfortable, but also the human beings will feel stuffy and panic.

For people, it is necessary to have the ability to recognize low-pressure weather. In terms of body sensation, it is generally when the low pressure appears when it is stuffy; in terms of seasons, low-pressure weather is the favorite in summer, and almost none in winter; I love low-pressure weather, etc., but it’s really not good, just look at the weather forecast!

4. The bigger the water surface, the higher the dissolved oxygen. This is a proportional relationship!

The larger the water surface, the larger the contact surface with the air, the greater the chance of naturally dissolving the oxygen in the air, and the water body is not easy to lack oxygen!

This is also the phenomenon of floating heads of fish rarely seen in lakes and reservoirs in hot summer, and in some small ditches, fish may have died due to extreme hypoxia.

,5,The stronger the wind, the higher the dissolved oxygen,This is also a proportional relationship!

This principle is similar to the above one. The strong wind will roll up the water surface, turning the water surface from a static plane to a flowing surface. On the one hand, it increases the contact area between the water body and the air, and the dissolved oxygen naturally increases!

On the other hand, the change of the water surface from static to flowing will also enhance the ability of water to dissolve oxygen. These two factors together make the dissolved oxygen in the water extremely rich in windy weather!

The amount of dissolved oxygen in water has a limit value. Once the dissolved oxygen in the water is saturated, no matter how strong the wind is, it will not increase the dissolved oxygen in the water.

1.1.4 What is supersaturated dissolved oxygen

over-saturated[kacise1] Causes of dissolved oxygen:

1. The temperature suddenly rises, and the oxygen dissolved in the water has not yet been released to reach the dissolved oxygen equilibrium state under the current temperature condition. At this time, the dissolved oxygen content in the water is supersaturated.

2. The pressure suddenly decreases, and the oxygen dissolved in the water has not been released in time to reach the dissolved oxygen equilibrium state under the current temperature condition. At this time, the dissolved oxygen content in the water belongs to the supersaturated state. .

3. Mechanical stirring or air blowing, through stirring or air blowing, increase the contact between water and oxygen and accelerate the dissolution of oxygen, resulting in supersaturation. This supersaturated state belongs to a dynamic equilibrium state.

Consequences of supersaturated dissolved oxygen:

in water dissolved oxygen If the content is too saturated, it will cause adverse effects and damage to aquatic organisms, especially fish, resulting in bubble disease in fish.

The content of dissolved oxygen gas in the water body reaches supersaturation. For example, when the oxygen content in the water reaches 14.4 mg/L, that is, when the saturation is 192%, the fry with a body length of 1 cm can develop bubble disease.

Once these gases enter the fish tissue, air bubbles will appear in the intestine of the fish, or there will be many small air bubbles attached to the body surface and gills, which will cause the fish to float or swim out of balance, which will cause various symptoms depending on the location of the embolism. and pathological changes, such as dyspnea, exophthalmos, anemia, and even death.

Extended information

Dissolved oxygen usually comes from two sources: one source is the infiltration of oxygen from the atmosphere into the water body when the dissolved oxygen in the water is not saturated; the other source is the oxygen released by the plants in the water through photosynthesis. Therefore, the dissolved oxygen in the water will be continuously replenished due to the dissolution of oxygen in the air and the photosynthesis of green aquatic plants.

Dissolved oxygen value is a basis for studying the self-purification ability of water. The dissolved oxygen in the water is consumed, and it takes a short time to restore to the initial state, indicating that the water body has strong self-purification ability, or water pollution Nothing serious. Otherwise, it means that the water body is seriously polluted, the self-purification ability is weak, or even loses the self-purification ability.

Due to violent aeration and other reasons, the amount of molecular oxygen in the air dissolved in the water to become dissolved oxygen increases significantly, making the dissolved oxygen in the water bodyappearedsaturation phenomenon. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of water. Under natural conditions, the oxygen content in the air does not change much, so the water temperature is the main factor. The lower the water temperature, the higher the dissolved oxygen content in the water.

1.1.5 The significance of dissolved oxygen in water

1.1.5.1 Significance for microorganisms

Anaerobic bacteria refer to a class of bacteria that can grow and reproduce under conditions of no oxygen or low redox potential. Due to the lack of a complete enzyme system, these bacteria can only metabolize energy by anaerobic fermentation.

Distribution and harm of anaerobic bacteria

Anaerobic bacteria are widely distributed in nature, including the environment, soil, water sources and animals. Among them, there are many anaerobic bacteria in the human body itself, most of which exist on the deep mucosal surface of the skin and cavities of the human body and animals.

Under normal circumstances, the dissolved oxygen concentration in seawater is saturated. By adding fresh seawater, water with a higher dissolved oxygen concentration can be brought into the aquaculture pond, so that the dissolved oxygen in the aquaculture pond can be significantly improved. For aquaculture ponds lacking water source or inconvenient water injection, photosynthetic bacteria, Bacillus and other beneficial microbial preparations can be used to adjust the water quality.

When the water body is polluted by organic matter, the oxygen consumption is serious, the dissolved oxygen cannot be replenished in time, and the Anaerobic bacteria. It will multiply quickly, and the organic matter will make the water black and smelly due to decay.

1.1.5.2 Significance or relationship to aquatic plants

Although phytoplankton has a dual effect on dissolved oxygen in water, under normal sunshine conditions, a pond with abundant plankton can produce 10-2Og of oxygen per cubic meter of water per day, and the respiration oxygen consumption of phytoplankton accounts for about 10% of photosynthetic oxygen consumption. 1/5 of the oxygen production. At the same time, these plankton can also absorb a large amount of nitrogen, phosphorus and other components, thereby accelerating the self-purification ability of pool water; therefore, in the middle and late stages of aquaculture, by appropriately increasing the density of phytoplankton, the dissolved oxygen in the aquaculture environment can be greatly improved. Plenty of phytoplankton also consume a lot of oxygen in the presence of insufficient light. Therefore, it is an important technology for water quality management to seek the proper reproduction of phytoplankton to keep it stable in the whole breeding process.

1.1.5.3 Significance for aquatic animals

Oxygen is the basic condition for human survival. Similarly, aquatic animals also need oxygen to maintain life in water. In the field of aquaculture, dissolved oxygen is one of the essential elements of aquaculture. When the dissolved oxygen in the water body is sufficient, the organic matter is decomposed at a faster rate under the action of aerobic bacteria, and the decomposed products are CO2, N03-, PO43-, H02 and other substances that are harmless to the aquaculture; the dissolved oxygen in the water body is insufficient. When the bacteria in the water layer is dominant, the decomposition products are mostly substances that are toxic to animals in the water, such as H2S, NH3, CH4, etc. Therefore, it is necessary to maintain sufficient dissolved oxygen in the water to inhibit the chemical reaction of toxic substances. Only by maintaining sufficient dissolved oxygen in the water can we ensure the normal growth of farmed fish and shrimp.

Different fish have different requirements for dissolved oxygen. Generally, it should be higher than 3mg/L. In production, this dissolved oxygen is called the safe concentration, and 2mg/L is the warning concentration.

A study shows that 80% of the dissolved oxygen in purified water comes from photosynthesis, about 7% of the dissolved oxygen in the air, and the remaining 4% is from water supplementation. Of course, different water bodies vary widely, and this ratio is not constant.

Dissolved oxygen is an indispensable condition for the survival of various aquatic organisms such as fish and shrimp. Various aquatic organisms such as fish and shrimp live in water, just like people live in the air, they must continuously breathe oxygen to maintain their normal life activities. The rate of respiration and oxygen consumption of various aquatic organisms such as fish and shrimp is related to various internal factors (such as species, age, body weight, body surface area, sex, food and activity intensity, etc.), and external factors (such as dissolved oxygen, pH, water temperature, etc.) related.

For fish in water, dissolved oxygen needs to be greater than 4mg/L to ensure their normal life activities. The dissolved oxygen content in the water is low, if it is lower than 4mg/L, although it does not reach the suffocation point, it will not cause an acute reaction of the fish, but it will cause chronic harm. The fish and shrimp will swim to the surface and breathe the dissolved oxygen in the surface water. When swallowing air, this phenomenon is called “floating head”. The growth rate and survival rate of fish with light floating heads will decrease, and heavy fish will cause flooding or a large number of deaths. Fish and shrimp live in water with insufficient dissolved oxygen for a long time, their physique will decline, their resistance to disease will be reduced, their morbidity will increase, and they will be more likely to be “poisoned” to death.

We know that people cannot survive if the air is full of poisonous gas, and the same is true for fish. So what are the toxins in the water and how are they produced?

The toxins that are deadly to fish are mainly ammonia and nitrite, which are highly toxic and can cause fish death at very low levels. When the ammonia concentration exceeds 0.012 mg/L, the fish is at risk of poisoning. The nitrite content is below 0.1 mg is safe and healthy water quality, 0.1 mg is slightly polluted, and 0.25 mg or more is serious pollution. start to die.

The production of toxins is as follows: fish’s breath, urine, fish excrement and bait and other organic matter will cause a large number of heterotrophic bacteria in the water, and the metabolite of the heterotrophic bacteria is highly toxic ammonia. In a mature nitrification system, ammonia is immediately decomposed into nitrite by nitrosobacteria, and then into nitrate by nitrifying bacteria. Nitrates are a source of nutrients for algae, and when nitrate concentrations are too high, algae blooms, resulting in lower dissolved oxygen in the water body.

Table 1-2 Adaptation range of several main farmed fish to dissolved oxygen in water　

fish	Normal growth and development/(mg/L)	Respiratory depression/(mg/L)	Oxygen threshold/(mg/L)
crucian carp	2	1	0.1
carp	4	1.5	0.2-0.3
bighead carp	4~5	1.55	0.23-0.4
Dace	4~5	1.55	0.3-0.5
grass carp	5	1.6	0.4-0.57
herring	5	1.6	0.58
Dory	5.5	1.7	0.26-0.6
silver carp	5.5	1.75	0.26-0.79

The dissolved oxygen in the water is high, the fish eats a lot, the feed utilization rate is high, and the growth is also fast. When the dissolved oxygen is insufficient, the fish do not like to eat or move, the food intake decreases, the digestion and absorption are not good, the metabolic intensity decreases, and the disease is prone to disease. At the same time, toxic substances such as ammonia and hydrogen sulfide accumulate in the pond, and the water quality deteriorates.