The importance of iron in aquaculture systems

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The importance of iron in aquaculture systems

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Abundant element serves crucial role on many levels

Iron is an essential element for bacteria, plants, and animals. Many enzymes important in energy transformations contain iron. Iron forms the center of the hemoglobin molecule important in oxygen transport in the blood of vertebrate and some invertebrate animals. Iron also plays an important role in photosynthesis by plants. Thus, iron plays a role in aquaculture.

Iron is the fourth most abundant element in the earth’s crust, but it is present in surface waters and in the ocean at a very low concentration. The reason for the low concentrations of iron in surface waters lies in the insolubility of iron minerals. The main sources of iron are oxides such as hematite and magnetite, but soils contain a variety of iron oxides and hydroxides.

Solubility, concentration

In water containing dissolved oxygen, the solubility of iron is governed mainly by pH. The concentration of ferric iron (the trivalent, oxidized form) will seldom exceed 2 mg/L unless the pH is below 4. Nevertheless, freshwaters may contain up to 1 mg/L or more of dissolved iron, because iron forms soluble hydroxides and ion pairs, and it also forms soluble complexes with dissolved organic matter (chelated iron).

It has been calculated that iron concentrations in natural water and in the soil solution would be too low to support plant life if not for soluble non-ionic forms of ion that are available to plants. Nevertheless, iron is a limiting factor for aquatic plant growth in some freshwaters – especially clear waters that have little dissolved organic matter. Iron is typically not included in fertilizers for freshwater aquaculture.

The sea has an average dissolved iron concentration of 0.01 mg/L (10 mg/L). There is speculation that iron is a major factor limiting phytoplankton productivity in the sea. There have been serious proposals to fertilize the sea with chelated iron to increase phytoplankton productivity and enhance carbon dioxide removal from the atmosphere through sequestration in organic matter.


Lime applications to low pH and alkalinity and acidic bottom soils is a common management measure against possible adverse effects of iron. Photo by Darryl Jory.

The presence of iron oxides in pond bottom soil is beneficial. Sulfide that is produced in the sediment is precipitated by ferrous iron as iron pyrite. This greatly lessens the likelihood of hydrogen sulfide diffusing into the water and causing toxicity to fish or shrimp. In normal soil, the resulting concentration of iron pyrite will not be high enough to create appreciable acidity when ponds are dried between crops. Normal liming procedures will counter any acidity that is released.

Iron is almost never at a high enough concentration to be directly toxic to shrimp or fish. Moreover, attention to liming waters of low pH and alkalinity and acidic bottom soils will counteract the possible adverse effects of iron. It is not clear whether inclusion of chelated iron in fertilizers for coastal ponds is cost effective. Removal of ferrous iron from water used in for hatcheries and intensive tank culture is a necessary procedure at some facilities.

When water and soil analyses are made, it is exceedingly difficult to interpret the iron concentration. Many soils have an abundance of iron oxides and hydroxides, and the soils are typically extracted with acid and iron analyses made on the extract. The acid will extract much more iron than will dissolve in the water. Therefore, pond managers should not worry much about the iron concentration in soils, but apply adequate liming material to maintain bottom soil pH between 7 and 8.

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