Biomagnification is the process by which certain toxic substances increase in concentration as they move up through a food chain. A small fish might carry a trace amount of mercury, but the larger fish that eats dozens of those small fish accumulates a higher dose, and the shark or tuna that feeds on those larger fish ends up with the highest concentration of all. The result: animals at the top of the food chain, including humans, are exposed to pollutant levels far greater than what exists in the surrounding water, soil, or air.
How Biomagnification Works
The process starts with a pollutant entering an ecosystem, typically through industrial runoff, agricultural chemicals, or atmospheric deposition. Tiny organisms like plankton or bottom-dwelling invertebrates absorb the substance from their environment. Because these chemicals resist being broken down by the body and dissolve easily in fat rather than water, they get stored in fatty tissues instead of being flushed out.
When a small fish eats thousands of plankton over its lifetime, it retains the pollutants from every single one. A medium-sized predator then eats many of those small fish, stacking the contamination even higher. At each step up the food chain, the concentration multiplies. Scientists measure this using a biomagnification factor, or BMF: the concentration in a predator divided by the concentration in its prey. A BMF greater than 1 means the substance is magnifying. A BMF less than 1 means it’s actually diluting as it moves upward. For a substance like methylmercury in marine food webs, BMFs can exceed 10 in top predators like swordfish, bluefin tuna, and dolphins.
Which Chemicals Biomagnify
Not every pollutant biomagnifies. The ones that do share a few key traits: they persist in the environment for long periods without breaking down, they dissolve in fats rather than water, and organisms absorb them faster than they can eliminate them. Chemicals classified as persistent organic pollutants, or POPs, are the textbook examples. DDT, PCBs (once used in electrical equipment), and dioxins all fit the profile.
Mercury is the other major culprit. In water, bacteria convert elemental mercury into methylmercury, a form that is easily absorbed by living tissue and extremely difficult for the body to get rid of. Once methylmercury enters the base of an aquatic food chain, it concentrates at every level above. The chemical properties that predict whether a substance will biomagnify in water-breathing organisms differ somewhat from those in air-breathing ones, but the core principle is the same: if an animal absorbs a chemical faster than it can excrete it, and that animal gets eaten, the chemical moves upward and concentrates.
Biomagnification vs. Bioaccumulation
These two terms are closely related but describe different things. Bioaccumulation refers to the buildup of a substance within a single organism over its lifetime. A clam filtering water for years gradually accumulates pollutants in its tissues, even if those pollutants exist at very low levels in the surrounding water. Biomagnification describes what happens across the food chain: each predator ends up with a higher concentration than its prey, so the top predator carries the greatest burden. Bioaccumulation is the engine within each organism; biomagnification is the escalation across trophic levels.
DDT and the Near-Extinction of Raptors
The most famous case of biomagnification nearly wiped out bald eagles, peregrine falcons, and brown pelicans in North America. DDT was sprayed widely as an agricultural pesticide starting in the 1940s. It washed into waterways, was absorbed by aquatic organisms, and biomagnified through fish and into fish-eating birds. By the time DDT (in the form of its breakdown product DDE) reached raptors, concentrations were high enough to disrupt their reproduction.
The mechanism was specific. DDE interfered with the shell gland in female birds, blocking the production of compounds needed to transport calcium into the developing eggshell. The result was eggs with shells so thin they cracked under the weight of an incubating parent. Peregrine falcon populations collapsed across much of the United States and Europe. The U.S. banned DDT in 1972 largely because of this evidence, and it became a founding case study for the Stockholm Convention, an international treaty targeting persistent organic pollutants.
Mercury in Seafood
For most people, biomagnification is relevant in one very practical way: the mercury content of the fish you eat. Fish at the top of the marine food chain carry significantly more methylmercury than fish at the bottom. This is why the FDA categorizes seafood into three tiers based on mercury levels.
Fish to avoid entirely due to the highest mercury concentrations include king mackerel, marlin, orange roughy, shark, swordfish, Gulf of Mexico tilefish, and bigeye tuna. These are all large, long-lived predators sitting at or near the top of their food chains.
Lower-mercury options, labeled “best choices” by the FDA, include salmon, shrimp, cod, tilapia, sardines, anchovies, catfish, crab, and canned light tuna (skipjack). These are generally smaller species, shorter-lived, or lower on the food chain. The FDA recommends eating two to three servings per week from this group. People who are pregnant or breastfeeding are advised to eat 8 to 12 ounces of these lower-mercury fish per week. A single serving is about 4 ounces, roughly the size of your palm.
Methylmercury is a neurotoxin. In adults, chronic exposure at high levels can cause numbness, vision problems, and impaired coordination. The greater concern is for developing brains: prenatal and early childhood exposure to mercury can affect cognitive development, which is why guidelines are stricter for pregnant women and young children.
Do Microplastics Biomagnify?
Given how much attention microplastics receive, it’s reasonable to wonder whether they follow the same pattern as mercury or DDT. So far, the evidence says no. A meta-analysis covering over 400 species and nearly 23,000 individual organisms found no clear increase in microplastic concentration from lower to higher trophic levels in marine food webs. Microplastics do accumulate within individual organisms (bioaccumulation), but they don’t appear to concentrate upward through the food chain the way fat-soluble chemicals do.
The amount of microplastics found in a given species seems to depend more on how that animal feeds (filter feeders tend to ingest more) than on where it sits in the food chain. A few laboratory studies have demonstrated that microplastics can transfer from prey to predator, but the exposure conditions in those experiments don’t reflect real-world levels. This is one area where the distinction between bioaccumulation and biomagnification matters: microplastics are a pollution problem, but they appear to follow different rules than classical biomagnifying toxins.
Why Biomagnification Matters Beyond Fish
Biomagnification doesn’t only happen in oceans. Terrestrial food chains are affected too. POPs deposited on soil are taken up by earthworms and other invertebrates, then passed to the birds and mammals that eat them. In Arctic ecosystems, polar bears and seal-eating orcas carry some of the highest concentrations of PCBs and other persistent chemicals on the planet, despite living thousands of miles from any industrial source. These chemicals travel through the atmosphere, deposit in cold regions, enter the water, and biomagnify through Arctic food webs.
The core lesson of biomagnification is that dilute contamination at the base of a food chain can become a concentrated health threat at the top. Pollutants that seem negligible in water or soil can reach dangerous levels in the animals (and people) that sit at the end of long food chains. This is why environmental regulations increasingly focus on whether a chemical has the potential to biomagnify, not just whether it’s toxic at the concentrations being released.