How does mercury enter aquatic food webs and why is it dangerous?

• A. Mercury is released from industrial sources, converted to methylmercury in water, bioaccumulates in organisms and biomagnifies up the food chain, harming nervous systems.
• B. Mercury is released and immediately harmless.
• C. Mercury is not persistent in water.
• D. Mercury is only a solid and cannot dissolve.

Answer: (A)

Mercury becomes dangerous in water when microbes in sediments transform inorganic mercury into methylmercury. This methylmercury is easily taken up by aquatic organisms and tends to accumulate in their bodies. As predators eat prey, the concentration of methylmercury increases up the food chain—a process called biomagnification. Because methylmercury strongly affects the nervous system and can cross the placenta and the blood–brain barrier, it poses serious risks to brain development in fetuses and young children, as well as to wildlife.

Saying mercury is released and immediately harmless isn’t true, since it is converted into a highly toxic form in the environment. Saying mercury isn’t persistent in water isn’t accurate because methylmercury remains in the ecosystem and accumulates. Saying mercury is only a solid and cannot dissolve ignores the dissolved forms that move through water and into organisms.