Important effects of calcium and magnesium in Aquaculture

Several other effects of calcium and magnesium deserve mention. In ponds, calcium functions to minimize the rise in pH that can occur when photosynthesis rates are high. After plants deplete the water of free carbon dioxide, they can use bicarbonate as a carbon source. But when using bicarbonate, plants release carbonate that hydrolyses and causes pH to increase. Calcium ions react at elevated pH to precipitate carbonate ions as calcium carbonate, and this reaction minimizes the amount of carbonate in the water to hydrolyze and increase pH.

There are some pond waters in which the alkalinity is high and calcium concentration is low. This combination can lead to dangerously high afternoon pH when photosynthesis is proceeding rapidly. Calcium sulfate can be applied to increase the concentration of calcium ion. As a general rule, it is desirable to have a hardness similar to or greater than the alkalinity — roughly 2 mg/L of calcium sulfate are required to provide 1 mg/L of hardness.

Hardness in water also facilitates flocculation and precipitation of suspended clay particles to lessen turbidity. An abundance of calcium and magnesium ions tends to neutralize the negative charges on suspended clay particles, allowing them to floc together and create a mass great enough to precipitate. Calcium sulfate often is applied to ponds to clear turbidity from the water. The recommended treatment rate usually is 1,000-2,000 kg/ha.

Calcium ions affect the toxicity of trace metals to fish and other aquaculture species. The presence of calcium blocks the uptake of metal ions across the gills, thereby increasing the dissolved concentration of metals required to cause a toxic effect.

The lethal concentrations of metal ions such as copper, zinc, lead, cadmium and chromium usually are considerably greater in harder water than in softer water. To illustrate, the 96-hour lethal concentration 50 (L.C.50) of copper to channel catfish was reported by Drs. David Straus and Craig Tucker to be 0.051-0.065 mg/L in water with a hardness of 16 mg/L, but 1.040-1.880 mg/L in water with a hardness of 287 mg/L. L.C. 50 is a standard measure of the toxicity of a medium that will kill half of the sample species population in a specific period of exposure.

Calcium also is important in fish hatchery water supplies. Eggs tend to hydrate at low calcium concentrations and do not develop and hatch normally. The minimum concentrations of calcium ions for good development and hatchability have been reported as 10 mg/L for eggs of brown trout and 4 mg/L for those of channel catfish. A recent study suggested the minimum calcium concentration for channel catfish hatcheries should be 10 mg/L, and best hatchability and fry survival were achieved at around 30 mg/L.

High concentrations of alkalinity and hardness can lead to precipitation of calcium carbonate from the water. This is especially common when groundwater that has high alkalinity and hardness, as well as elevated carbon dioxide concentration, is brought into contact with the atmosphere. For example, the alkalinity and hardness in well water used to supply ponds at an inland shrimp farm in Alabama, USA, were 275 and 325 mg/L, respectively. Once put into ponds, the water equilibrated with atmospheric carbon dioxide, and alkalinity and hardness dropped to 120 and 168 mg/L, respectively, as a result of calcium carbonate precipitation.

Although calcium carbonate precipitation is not usually of great concern in ponds, it can be troublesome in hatcheries. The author has observed that at a shrimp hatchery supplied with saline groundwater with initially high concentrations of carbon dioxide, alkalinity and hardness, calcium carbonate precipitated onto the larvae and resulted in high mortality. The same phenomenon probably can occur with eggs in a fish hatchery.

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Công ty TNHH Khoáng Sản Xanh / 0 Bình luận / 17/ 04/ 2017

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