In this small town, people eat a lot of fish from the nearby lake. But many people have been feeling unwell lately. They have been experiencing numbness in their fingers, toes, tongue or lips. Many feel very tired and some can’t see very well. These are the signs, or symptoms, of mercury poisoning.
15 years ago, a chemical factory started operating in town, and started releasing toxic substances containing mercury into the lake. Once in the lake, mercury is taken up, or absorbed by tiny organisms, called plankton. Once mercury is in the plankton, it stays in the plankton. Small fish eat this plankton, and the mercury it contains. Each small fish might have to eat as much as ten times more plankton than its own weight, to grow.
So all the mercury from plankton in this lake becomes concentrated in these small fish. The concentration of mercury in each individual small fish, is therefore much larger than in each tiny plankton. Similarly, each large fish in the lake might have to eat as much as ten times its own weight, in small fish. Large fish therefore end up with an even higher concentration of mercury in their bodies, than small fish. Both small and large fish, containing large concentrations of mercury, were eaten by the residents of this town.
This explains the mercury in their bodies. Because our bodies can’t dispose of mercury at all, the amount of mercury in the body increased with every fish a person ate. This is why now, 15 years after the factory opened, many people are suffering from mercury poisoning. This build up of toxic substances in organisms is called bioaccumulation. We usually talk about bioaccumulation when it’s a result of pollution caused by humans.
But bioaccumulation occurs without human involvement too! Monarch butterflies, for example, are poisonous, but not because they produce their own poison. When they are caterpillars, monarch butterflies eat milkweed, a plant that naturally contains toxic substances. The milkweed’s toxic substances don’t harm the caterpillars, but get stored in their cells. So when the caterpillar develops into a butterfly, the butterfly contains these toxic substances too!
This causes many other animals to avoid the monarch butterflies. This protects the butterflies from being eaten. Golden poison frogs protect themselves in a similar way. They bioaccumulate toxins from certain insects they eat. This makes these frogs some of the most poisonous animals in the world.
Bioaccumulation can be advantageous for species, as is the case with the monarch butterfly and the golden poison frog, because it protects them from being eaten. But bioaccumulation of toxins from human pollution, can have seriously damaging effects on different species, food chains and whole ecosystems. It can even cause widespread health problems in humans. That’s why it’s important to make sure harmful substances don’t end up in nature in the first place.