The spatial and temporal imbalance of dissolved oxygen in ponds, as well as the low efficiency of oxygenation, are important factors limiting the increase in aquaculture yield. Dissolved oxygen is not necessarily higher as it increases, but is supplied adequately when needed.

1： Oxygenation mechanism
The oxygen content in the air is generally 210ml/L, while the saturated content of dissolved oxygen in freshwater is only 8-10 ml/L, which is less than 1/20 of the oxygen content in the air. When gas molecules pass through the gas and liquid films between the gas and liquid phases, transmission resistance is generated. When gases are transferred from gas phase to liquid phase, the resistance encountered by insoluble gases mainly comes from the liquid film, while the resistance encountered by soluble gases mainly comes from the gas film. The resistance encountered by gases with moderate solubility comes from both the gas film and the liquid film.

The transfer rate of oxygen is proportional to the concentration gradient of oxygen in the liquid film. To improve the oxygen transfer rate, two aspects can be considered.
1. Because the total oxygen transfer coefficient KLa value is related to the thickness of the liquid film and the gas-liquid contact area, measures such as increasing the turbulence level of the liquid phase, reducing the thickness of the liquid film, accelerating the renewal of the gas-liquid interface, and increasing the gas-liquid contact area can be taken to improve the KLa value.
2. By increasing the oxygen partial pressure in the gas phase and increasing the ρ s value in the liquid phase (oxygen saturation in the liquid, kg/m3), the oxygen transfer rate can be improved.

2： Changes in dissolved oxygen in ponds
At noon, the maximum dissolved oxygen does not appear in the surface water layer, but in the secondary surface water layer. This is because: ① surface oxygen needs to dissipate some oxygen into the air; ② Excessive surface light can inhibit the photosynthesis of phytoplankton and reduce oxygen production. At this time, the light intensity in the subsurface water is appropriate, resulting in more oxygen production. Therefore, the maximum dissolved oxygen occurs in this water layer; At the same time, the surface water absorbs solar energy, causing the water temperature to rise, resulting in a high specific heat and low thermal conductivity. Therefore, a temperature jump layer appears between the surface and bottom layers.

When there is no wind, pay attention to increasing oxygen in the lower layer of the water body; When there is wind and sunshine during the day, pay attention to increasing oxygen in the upwind area. In the morning, the opposite is true, and oxygen should be increased in the downwind area.
The difference between the highest and lowest values in the daily variation of dissolved oxygen is called the "diurnal range", which reflects the relative intensity of oxygen production and consumption in water bodies. On the premise that the lowest value of dissolved oxygen does not affect fish growth, it is better for fish ponds to have a larger diurnal range. A slight floating head in the early morning during the growth period is more conducive to accelerating the growth of fish.

3： How to observe dissolved oxygen in ponds
What are some soil methods to determine whether the dissolved oxygen in a pond is sufficient, as dissolved oxygen meters are too expensive and unnecessary for ordinary breeders?
1: PH value: In general, pH value and dissolved oxygen change synchronously. When dissolved oxygen is low, pH will not be too high and may even drop below 7.
2: Algae phase in water: The algae species are abundant, the water color is uniform, and the dissolved oxygen is not too poor. On the contrary, there are many harmful algae in the water, and a large number of planktonic animals breed, which inevitably makes it more prone to hypoxia in bad weather.
3: Observing the movement of small fish: Small fish have poor tolerance to low oxygen. Generally, when the pond lacks oxygen, small fish float their heads first.
4: Degree of freedom of water body: When the water body is viscous and organic matter reacts with water molecules due to electric charges, the entire water molecule cluster will become larger, the degree of freedom of the water body will decrease, the dissolved oxygen will not be high, and the oxygenation efficiency will also be compromised. The degree of freedom of water body can be considered by observing water body ripple, aerator spray, foam, etc.

4： Summary
The uncontrollable seedlings, feed, and drugs are the only things that farmers can control: their own pond management: improving water quality and sediment, preventing various diseases, all of which are closely related to dissolved oxygen. The use of a dissolved oxygen meter enables online real-time monitoring of dissolved oxygen and temperature in aquaculture water environments, avoiding the tedious testing process of manually collecting water samples, and possessing precision and timeliness that cannot be compared to manual detection.

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