Bioaccumulation, Biomagnification & Human Health

You’ll be able to

Distinguish bioaccumulation from biomagnification
Interpret a dose-response relationship and the meaning of LD50
Calculate how a persistent toxin concentrates up a food chain

Bioaccumulation vs. biomagnification

Two related processes concentrate toxins in living things. Bioaccumulation is the buildup of a persistent substance within a single organism over its lifetime, because the organism takes in the toxin faster than it can excrete it. Biomagnification is the increase in the toxin’s concentration at each higher trophic level of a food chain: a predator eats many contaminated prey and accumulates all of their toxin load, so top predators end up with the highest concentrations. Both require toxins that are persistent (slow to break down) and fat-soluble (stored in tissue rather than excreted).

The classic culprits

Several notorious pollutants biomagnify. DDT, a pesticide, thinned the eggshells of top predators like the bald eagle and peregrine falcon, nearly driving them extinct; Rachel Carson’s book Silent Spring (1962) exposed the problem, and DDT was banned in the U.S. in 1972. Mercury (from coal burning and mining) biomagnifies in aquatic food chains, which is why pregnant women are warned to limit tuna and other large predatory fish. PCBs, industrial chemicals now banned, persist and biomagnify similarly. All accumulate to their highest levels in top predators, including humans.

Dose-response and LD50

Toxicology asks how the dose of a chemical relates to its harmful effect — the dose-response relationship, usually shown as an S-shaped curve of effect versus dose. A key measure is the LD50 (lethal dose, 50%): the dose that kills half of a test population. A lower LD50 means a more toxic substance, because it takes less to be lethal. Dose is often expressed relative to body mass (mg per kg of body weight), so the same dose is more dangerous to a smaller body — which is why children and small animals are more vulnerable.

Biomagnification up the food chain

concentration at a level ≈ base concentration × (magnification factor)^(number of levels up)

If a toxin roughly multiplies by 10 at each trophic level, a predator three levels above the base carries about 10³ = 1,000 times the base concentration. LD50: lower value = more toxic.

Worked example

A persistent pesticide is present in phytoplankton at 0.04 ppm. It biomagnifies about tenfold at each trophic level. Estimate its concentration in a large fish three trophic levels above the phytoplankton.

1.Identify the base concentration and factor: 0.04 ppm in phytoplankton, ×10 per level.
2.Count the levels of magnification to reach the large fish: 3 levels up.
3.Apply the factor three times: 0.04 × 10 × 10 × 10 = 0.04 × 1,000.
4.Compute: 0.04 × 1,000 = 40 ppm.

Answer: About 40 ppm in the large fish — a thousandfold increase over the phytoplankton. This is why top predators, including people who eat large fish, face the greatest exposure.

Checkpoint

A toxic chemical has a concentration of 0.01 ppm in plankton and increases tenfold at each of two trophic levels up to a small fish. What is its approximate concentration in that fish?

Watch out

Don’t confuse the two "bio-" terms. Bioaccumulation = buildup within one organism over time. Biomagnification = increasing concentration across trophic levels up the food chain. Top predators suffer most from biomagnification because each meal delivers the accumulated toxin of everything below.

Checkpoint

A pesticide has an LD50 of 5 mg per kg of body weight. Based on this and the meaning of LD50, which statement is correct?

On the exam

Two quantitative anchors here: biomagnification multiplies at each level (×10 per level → 10ⁿ over n levels), and LD50 dose = (mg/kg) × body mass in kg. Remember the counterintuitive LD50 rule — a lower LD50 is more toxic, because less of it is needed to kill.

Answer the 2 checkpoints as you read.