Eutrophication, Bioaccumulation, and Biomagnification

Eutrophication is the enrichment of water with nutrients, mainly nitrogen and phosphorus, which causes excessive growth of algae and oxygen depletion, while bioaccumulation is the build-up of a persistent substance in an organism over time and biomagnification is the increase of its concentration at each higher trophic level. In brief: nutrient runoff from fertilizers and sewage feeds algal blooms; when the algae die, bacteria decompose them and use up the dissolved oxygen, and fish and other animals die. Toxins that are fat-soluble and not easily broken down, such as DDT, PCBs and mercury compounds, become more concentrated up a food chain, with the highest levels in top predators.

Both processes show how human releases of chemicals can reach ecosystems far from the source. This guide explains the stages of eutrophication, the conditions for bioaccumulation and biomagnification, the difference between them, gives well-known case studies, compares the processes in a table, works through a biomagnification calculation and corrects common mistakes.

How does eutrophication happen?

Eutrophic means “well-nourished”. Natural eutrophication of lakes takes thousands of years. Cultural (human-caused) eutrophication occurs in years or decades, when excess nitrate and phosphate enter water bodies from agricultural runoff (fertilizers and manure), sewage and detergents that contain phosphates, and the atmospheric deposition of nitrogen. The sequence is:

1. Nutrient enrichment: a limiting nutrient (often phosphorus in fresh water, and nitrogen in seawater) becomes plentiful.
2. Algal bloom: algae and cyanobacteria (blue-green algae) grow rapidly and form a green scum on the surface. This blocks light from reaching submerged plants, which die.
3. Death and decomposition: when the algae die in huge numbers, bacteria decompose them and use up the dissolved oxygen in the water (a high biochemical oxygen demand).
4. Oxygen depletion (hypoxia): fish, shellfish and other aerobic organisms suffocate and die, and anaerobic bacteria produce hydrogen sulfide and methane.
5. Loss of biodiversity: only tolerant species such as some bacteria and worms remain.

What are the effects of eutrophication?

• Loss of fish and other animals, with an effect on fisheries.
• Toxins from some cyanobacteria can make water unsafe for drinking, swimming and animals. They can poison livestock and pets.
• Unpleasant taste and smell of drinking water, and higher treatment costs.
• Dead zones in coastal waters: the zone at the mouth of the Mississippi River in the Gulf of Mexico is typically the size of a small U.S. state, with an average of about 14 000 square kilometers (summer measurements over recent decades), caused by nutrient runoff from the Mississippi basin. The Baltic Sea also has large dead zones.
• Loss of recreational value of lakes and coastal waters.

What is bioaccumulation?

Bioaccumulation is the gradual build-up of a substance in the tissues of an individual organism over its lifetime, when the rate of intake is greater than the rate of loss (by metabolism or excretion). Substances that bioaccumulate share these properties:

• Persistent: they do not break down easily in the environment or in the body (non-biodegradable).
• Fat-soluble (lipophilic): they dissolve in fats and are stored in fatty tissue, and they are not easily excreted in water-based urine. Heavy metals such as mercury and lead are not fat-soluble, but bind to proteins in tissues.

Examples are DDT and other organochlorine pesticides, PCBs (industrial chemicals), dioxins, methylmercury and some plastics additives. The concentration in an organism can be much greater than in the surrounding water, given by the bioconcentration factor (BCF = concentration in organism ÷ concentration in water).

What is biomagnification?

Biomagnification (biological magnification) is the increasing concentration of a persistent substance at successively higher levels in a food chain. It happens because organisms at each level eat many individuals from the level below, and so take in and accumulate all of the toxin they contain. Since only about 10 percent of the energy is passed on, a predator must eat a large mass of prey, so it receives the toxin from many individuals but loses little of it. The top predators, such as eagles, ospreys, seals, orcas and humans who eat large fish, have the highest concentrations.

Eutrophication, bioaccumulation and biomagnification compared

What are some well-known cases?

• DDT: widely used from the 1940s as an insecticide. It biomagnified in aquatic food chains, and caused thin eggshells in birds of prey such as bald eagles, peregrine falcons and brown pelicans, which crushed their own eggs. Rachel Carson’s 1962 book Silent Spring brought attention to the problem. DDT was banned in the US in 1972, and the populations of eagles and falcons recovered. It is still used in some places to control malaria mosquitoes under controlled conditions.
• Minamata disease: in Minamata, Japan, in the 1950s, a chemical factory released mercury into the bay. Bacteria converted it to methylmercury, which accumulated in fish and shellfish eaten by local people, and caused severe nerve damage in thousands of people.
• PCBs in orcas and polar bears in the Arctic, far from where they were used.
• Lake Erie eutrophication in the 1960s, and 1970s; reduced phosphates in detergents and improved sewage treatment led to recovery, but algal blooms have returned since the 2000s.

Worked example: concentrations along a food chain

The table gives illustrative DDT concentrations in an estuarine food chain, in parts per million (ppm) of the wet mass of tissue.

1. Bioconcentration factor of plankton: 0.04 ÷ 0.00005 = 800.
2. Factors between levels: 0.5 ÷ 0.04 = 12.5; 2.0 ÷ 0.5 = 4; 25 ÷ 2.0 = 12.5.
3. Overall magnification from plankton to bird: 25 ÷ 0.04 = 625 times.
4. From water to bird: 25 ÷ 0.00005 = 500 000 times. A concentration in water that is far below any level of concern is a half-million times greater in the bird.

The calculation shows why it can be misleading to measure toxins in water alone. A small concentration may be dangerous to top predators. The values are illustrative, based on the pattern of well-known studies, and not measurements from one site. A safe limit for humans is also set according to the fish consumed: people who eat predatory fish such as swordfish, shark and large tuna often are advised to limit their intake, particularly pregnant women, because of the mercury content.

How can these problems be reduced?

• For eutrophication: reduce fertilizer use and apply it more carefully, plant buffer strips and wetlands along streams, treat sewage to remove phosphates and nitrates, ban phosphates in detergents and restore wetlands.
• For bioaccumulation and biomagnification: ban or restrict persistent toxic chemicals (as in the Stockholm Convention on Persistent Organic Pollutants, adopted in 2001), prevent industrial releases, clean up contaminated sites and use biodegradable alternatives.
• Monitor water quality and the tissues of indicator species such as mussels, birds and top predators.

How were these processes discovered?

Eutrophication of lakes was studied from the early twentieth century, and by the 1960s, researchers such as David Schindler, who fertilized part of an experimental lake in Ontario in the 1970s with phosphorus and showed that phosphorus controls algal blooms, provided strong evidence. George Woodwell and colleagues showed the biomagnification of DDT in a Long Island estuary in 1967. Rachel Carson’s Silent Spring (1962) had a major influence on public awareness, and led to the creation of environmental agencies, such as the US Environmental Protection Agency in 1970.

Which mistakes are common?

Mistake 1: Saying that algae die from lack of oxygen

Fish and other animals die from lack of oxygen. It is the decomposition of the dead algae by bacteria that uses up the oxygen.

Mistake 2: Confusing bioaccumulation and biomagnification

Bioaccumulation occurs in one organism over its life. Biomagnification is the increase along the food chain.

Mistake 3: Believing that all pollutants biomagnify

Only persistent substances that are stored in the body and not excreted easily do so.

Mistake 4: Thinking that nutrients are harmless pollutants

Nutrients are essential, but too much in water causes serious damage.