Toxic chemicals and heavy metals flow into the ocean when industrial, agricultural, and human wastes runs off or is deliberately discharged into rivers that then empty into the sea. These pollutants cause disease, genetic mutations, birth defects, reproductive difficulties, behavioral changes, and death in many marine organisms. But the severity of the damage varies greatly between species. In many cases, animals near the top of the food chain are most affected because of a process called biomagnification.
Many of the most dangerous toxins settle to the seafloor and then are taken in by organisms that live or feed on bottom sediments. Because these compounds aren't digested, they accumulate within the animals that ingest them, and become more and more concentrated as they pass along the food chain as animals eat and then are eaten in turn. This is biomagnification, and it means that higher-level predators-fish, birds, and marine mammals-build up greater and more dangerous amounts of toxic materials than animals lower on the food chain.
In this activity you will explore the biomaginification of toxic chemical, mercury, through a simple marine food chain. In the simulation below, the marine environment is contaminated with mercury. Although all animals are exposed to this toxic chemical, seabirds are more severely affected than other organisms.
Feed the bird by dragging clams and fish into it until it is full. First, feed the bird clams only, then just fish, and finally feed it a mixed diet. Try each feeding strategy several times.
Only after you understand how diet affects the seabird, click on the "show toxins" button to reveal the relative environmental distribution of mercury, represented by the orange dots. [The "reset" button on the "show toxins" screen will return you to the activity if you want to try feeding the duck again.] Then, answer the questions that follow.
Describe the food chain illustrated in the simulation. How do toxic chemicals like mercury move from land sources through this chain?
Erosion breaks down rocks and releases their chemical components into streams and groundwater. Chemicals used on farms, or around the house, are spilled or spread on the ground where they too are washed into rivers by the rain or soak into the soil to mix with groundwater. Insufficiently treated industrial wastes and sewage are discharged directly into rivers. Much of this material eventually settles to the bottom of the sea. Bottom dwelling clams ingest the chemical as they burrow into the mud. When fish and birds eat the clams, or birds eat the fish that ate the clams, they also eat the mercury stored within their prey.
What happens to the seabird if it feeds exclusively on clams? On fish? If it has a mixed diet? Explain these results.
The bird usually survives on an all clam diet and dies on an all fish diet. On a mixed diet, the bird has about an equal chance of living or dying. This pattern is due to biomagnification. Mercury is stored within an animal's tissues. Clams have fairly low concentrations of the substance. Clam-eating fish accumulate all the mercury from all the clams they consume, so they have much higher concentrations. Birds, in turn, swallow all the mercury in all the fish and/or clams they eat. If their diet is mainly of fish, which are high in mercury, the birds will quickly accumulate fatal levels of the substance. If they eat lower on the food chain, their mercury level will be much less.
Why don't all the clams or all the fish in the simulation have the same concentration of mercury?
The concentration of mercury in a particular organism will vary with its age and location relative to pollution sources. In general, older and larger animals and those that dwell closest to a pollution source will accumulate higher concentrations of contaminants.
What feeding strategies minimize the build-up of toxic chemicals in the seabird? How might this affect the population structure of clams, fish, and birds over time?
Because biomagnification concentrates mercury up the food chain, seabirds that eat more clams than fish will be exposed to lower concentrations of toxins and have a better chance of survival. If fish eating birds are less likely to survive than clam eaters, over time clam eaters will predominate, which could cause the clam population to dwindle and the fish population to expand.
The greatest concentration of mercury is in sea floor sediments. How might the presence of mercury in the food chain shown be affected if sea level rises or falls? What about if the harbor is dredged so ships can dock right at the factory?
When mercury is buried within the sediments, it is kept away from marine life. If sea level falls and exposes sediments to wave action and erosion, or if dredging disturbs the seafloor, then mercury will be reintroduced into the environment and its concentration in animals will increase.
Sea life is an important human food resource. How does biomagnification impact the safety of seafood? How can humans who eat clams and fish minimize their exposure to chemicals such as mercury?
Humans tend to eat fairly high on the marine food chain. Predatory fish - fish that eat other fish that have eaten still other fish and animals - will have some of the highest concentrations of biomagnified chemicals. Many popular fish are also rich in fat and fish oils, which is where many toxins are stored. People can minimize their exposure to toxic chemicals by eating species from lower trophic levels, such as shellfish, and by avoiding seafood harvested from more polluted waters.