Introduction
Mercury is a silvery, liquid metal at room temperature. It is sometimes referred to as one of the “heavy metals.” Like water, mercury can evaporate and become airborne. Because it is an element, mercury does not break down into less toxic substances. Once mercury escapes to the environment, it circulates in and out of the atmosphere until it ends up in the bottoms of lakes and oceans. Depending on its chemical form, mercury may travel long distances before it falls to earth with precipitation or dust.
Mercury contamination on aquatic environment
Concentrations of mercury in fish and wildlife are high enough to be a risk to wildlife. Once mercury escapes to the environment, it circulates in and out of the atmosphere until it ends up in the bottoms of lakes and oceans. Depending on its chemical form mercury may travel long distances before it falls to earth with precipitation or dust. Mercury gets into lakes from the atmosphere, where it falls with rain or snow into the watersheds that feed the lakes. Approximately one gram of mercury enters a 20-acre lake each year.
When mercury is deposited in lakes or waterways, bacteria convert it to methyl mercury. methyl mercury accumulates in algae and is eaten by smaller fish, which in turn are eaten by larger fish.
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Unfortunately, the mercury in fish also concentrates in the tissue of any human or wildlife eating the fish. If contaminated fish are eaten on a regular basis, mercury concentrations can become high enough to become a serious health threat to humans. Several Great Lakes states issue advisories each year, cautioning people to limit the amount of fish they eat from area lakes.
As long as the fish continue to be exposed to mercury, mercury continually builds up in fish’s bodies. Fish that eat other fish become even more highly contaminated. Thus, the fish most desirable for many additional bigger fishes become the most affected, and larger fish tend to be the most contaminated.
Bacteria and chemical reactions in lakes and wetlands change the mercury into a much more toxic form known as methylmercury. Fish become contaminated with methylmercury by eating food (plankton and smaller fish), which has absorbed methylmercury.
This type of mercury poisoning occurred in the Japanese coastal town of Minamata, on the island of, Kyushu. At first it affected the wild life environment, especially the birds that fed on the aquatic fishes. The birds would loose their coordination or would circle around and fall to the ground; the same occurrence began to happen to the cats living near the Bay, where it was called the “disease of the dancing cats”. This started affecting the fishermen too, who also ate fish from this lake. Those effected lived in a small area, and much of the protein in their diet came from fish in the Minamata Bay.
This state of mercury poisoning had taken place due to a vinyl chloride factory who used a form of inorganic mercury. The mercury was released in the water affluent that flowed in the bay.
The microbial action in the Bay converted the inorganic mercury to methyl mercury, an organic compound that turned out to be a much more harmful form. Inorganic mercury cannot pass through cell membranes readily. However, methyl mercury readily passes through the cell membrane, and is transported by the cell red blood cells through out the body, fish absorb methyl mercury from water 100 times faster than they absorb mercury. Once absorbed methyl mercury is retained two or five times longer than is inorganic mercury. In the case of mercury, it’s chemical form and concentration changed as the mercury moved through the food web. These pathways involve biomagnifications, where the concentration of the mercury increases in food web.
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Conclusion
In order to avoid consumption of mercury in fish the following precautions maybe taken:
1. Younger fish tend to have lower concentrations of mercury than older, larger fish within the same water body.
2. Make sure you buy mercury-free products whenever you can, and recycle mercury-containing products.
3. Prohibit using mercury in dry-cell batteries;
References
vhttp://water.usgs.gov/wid/FS_216-95/FS_216-95.html ( on contamination of aquatics issue)
