Bioaccumulation occurs when toxins build up - or accumulate - in a food chain. The animals at the top of the food chain are affected most severely.
This is what happens:
DDT is an insecticide that was widely used in the 1950s and 1960s. The population of birds of prey at the top of food chains were badly affected because it made the shells of their eggs very thin, causing them to break easily when the birds tried to incubate them.
Mercury compounds were used until quite recently to make insecticides (chemicals that kill the insects that damage crops) and special paints that stop barnacles growing on the hulls of ships.
Unfortunately, when it gets into the food chain mercury damages the nervous systems and reproductive systems of mammals, including humans.
Tiny plankton in the sea absorb the mercury compounds. When the plankton are eaten by small fish, the mercury they contain stays in the fish. As the fish need to eat a lot of plankton, the concentration of mercury in them becomes higher than its concentration in the plankton.
The small fish are eaten by larger fish, and these are eaten by still larger ones, such as tuna fish. This creates a high concentration of mercury in the tuna. People eating contaminated tuna may get mercury poisoning. Mercury is now banned from many chemical products and mercury use in industry is carefully regulated.
When fertilisers are added to farmland in large quantities, plants and animals in nearby rivers and lakes can sometimes be killed, but not by poisoning. The steps in eutrophication are shown in the animation below.