Important Significance of Calcium and Magnesium Ions in Aquaculture
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Calcium and magnesium ions are the main elements constituting water hardness. Natural freshwater is low in calcium and magnesium ions with low water hardness, while seawater is rich in calcium and magnesium ions with high water hardness. In artificial mariculture, calcium chloride, magnesium chloride, sodium chloride and other raw materials are used to prepare artificial seawater.
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Calcium and magnesium are essential components for the growth of aquatic organisms. Calcium is an important constituent of aquatic animal bones and aquatic plant cell walls, as well as an indispensable nutrient in water bodies. Insufficient calcium content will not only affect the reproduction of algae, but also cause calcium deficiency in aquatic animals. Magnesium is a core component of chlorophyll and is required by all kinds of algae. Sufficient magnesium supplementation for aquatic plants keeps their stems firm and lodging-resistant. Magnesium deficiency will lead to disorder of nitrogen metabolism and hinder the absorption of calcium by aquatic plants.
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Calcium and magnesium ions can reduce the toxicity of heavy metal ions and monovalent metal ions. Excessively high concentration of heavy metal ions and monovalent metal ions in water will produce toxic effects on most aquatic organisms. Properly increasing calcium and magnesium content can effectively reduce such toxicity.
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Calcium and magnesium ions can improve the buffering capacity of water bodies and stabilize water quality. Overly soft water has weak pH buffering capacity for aquaculture ponds, resulting in unstable water quality. When the total water hardness is higher than 20 mg/L, phytoplankton can grow massively after the application of inorganic fertilizers. The suitable total hardness for shrimp farming is 120–150 mg/L. In addition, the ratio of calcium to magnesium ions in water should be controlled reasonably, with the optimal ratio generally at 1:3.
Functions of Calcium Chloride in Aquaculture
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Regulate water pH value
The suitable pH range for aquatic organisms is neutral to weakly alkaline (pH 7.0~8.5). Abnormally high pH (≥9.5) will slow down the growth of cultured organisms, increase feed conversion ratio and raise disease incidence. Therefore, reducing excessive pH is an important technical measure for pond water quality regulation.
Calcium chloride is neutral in nature and cannot be directly used as an acid-base regulator. However, its dissociated calcium ions can precipitate hydroxide ions, flocculate and settle phytoplankton, and slow down the consumption of carbon dioxide by algae, so as to steadily reduce water pH value.
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Maintain algae growth balance
During water fertilization before seedling stocking, lack of calcium, magnesium, phosphorus and other trace elements (especially in freshwater shrimp farming) will hinder the growth and reproduction of algae, causing difficulties in water fertilization and clear and thin pond water.
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Maintain water stability and enhance buffering performance
Aquaculture water requires proper hardness. Sufficient calcium and magnesium in water can not only stabilize the pH of water and sediment, improve water buffering capacity, but also benefit the growth and reproduction of microorganisms, accelerate the decomposition and mineralization of organic matter, and promote the recycling of aquatic nutrients.
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Meet calcium demand for shrimp and crab molting and shell hardening
The growth of crustaceans such as shrimps and crabs depends on continuous molting and shell hardening, which requires a large amount of calcium absorbed from water and feed. Intensive farming usually leads to serious shortage of mineral salts in water bodies. Insufficient absorption of calcium and phosphorus will result in failure of normal shell hardening, causing soft shell disease, unsuccessful molting and slow growth, which seriously affect the normal growth and breeding benefit.
Advantages of Calcium Chloride as Aquatic Calcium Supplement
At present, the main calcium source in aquaculture feed is calcium carbonate. Although it has high calcium content (about 40%) and low price, its solubility is extremely low (only 0.56%). It mainly relies on gastric acid for dissolution, which will neutralize gastric acid and produce carbon dioxide, easily causing gastrointestinal flatulence of aquatic organisms and seriously affecting feeding and digestion. In addition, carbonate ions are prone to form insoluble precipitates with zinc, copper, iron and other trace elements, interfering with their absorption, resulting in feed waste and water pollution.
To solve the problems of poor solubility and low absorption rate of traditional calcium sources, applying calcium chloride artificially in shrimp and crab ponds is an efficient calcium supplementation method.
