Eutrophication, Dissolved Oxygen, and Biomagnification

You’ll be able to

Trace the sequence from nutrient loading to hypoxia
Interpret dissolved oxygen and BOD data
Distinguish bioaccumulation from biomagnification

The eutrophication sequence, in order

Free-response questions ask for this chain and award points step by step, so learn it as a sequence rather than a summary. (1) Nutrients — nitrogen and phosphorus from fertilizer runoff, sewage or detergents — enter a water body. (2) Algae and cyanobacteria bloom, since the limiting nutrient has been supplied. (3) The bloom blocks light, killing submerged plants below. (4) Algae and those plants die. (5) Decomposers multiply on the dead organic matter, and their aerobic respiration consumes dissolved oxygen. (6) Dissolved oxygen falls, producing hypoxia and fish kills, and in the extreme a dead zone. The step students skip is (5): the oxygen is consumed by decomposers, not by the algae themselves, and an answer that omits decomposition usually loses the causal point even if it names the right start and finish.

Reading dissolved oxygen data

Two quantities appear in stimulus material. Dissolved oxygen (DO) is oxygen available to aquatic life; it falls as temperature rises, because warm water holds less gas, which is why thermal pollution from power plant cooling water is a genuine problem and why summer fish kills are common. Biochemical oxygen demand (BOD) is the oxygen decomposers will consume in breaking down the organic matter present, so high BOD predicts falling DO. A classic graph shows DO plunging just downstream of a sewage outfall and recovering with distance as the organic load is consumed and the water reaerates — the oxygen sag curve. Where a question gives a DO profile along a river, the minimum locates the pollution source, and the recovery distance indicates the load.

Bioaccumulation versus biomagnification

These are different processes and the exam distinguishes them. Bioaccumulation happens within one organism over its lifetime: a fat-soluble, slowly excreted substance builds up in tissue faster than the organism can eliminate it. Biomagnification happens up a food chain: because each predator eats many prey and retains their accumulated burden, concentration multiplies at each trophic level — often by roughly an order of magnitude per step. The substances that do this share three properties: fat-soluble, persistent, and slowly excreted. The classic cases are DDT, which thinned raptor eggshells and drove the bald eagle and peregrine falcon toward extinction, PCBs, and methylmercury, which is why fish-consumption advisories target long-lived predatory fish such as swordfish, shark and tuna rather than sardines. Water-soluble substances flush out and do not magnify, which is a useful discriminator on a multiple-choice question.

Eutrophication chain

nutrients → algal bloom → light blocked → plants and algae die → decomposers consume O₂ → hypoxia and fish kill

The oxygen is consumed by decomposers, not by the algae. Omitting that step loses the causal point.

Worked example

Mercury measures 0.001 ppm in lake water, 0.05 ppm in zooplankton, 1.2 ppm in small fish and 15 ppm in a predatory fish. Identify the process and explain why the advisory targets the predatory fish.

1.Compare the levels: concentration rises by roughly one to two orders of magnitude at each trophic step, from 0.001 to 0.05 to 1.2 to 15 ppm.
2.Identify the process: this is biomagnification, since concentration is increasing UP THE FOOD CHAIN rather than within a single organism over time.
3.Explain the mechanism: methylmercury is fat-soluble, persistent and poorly excreted, so each predator retains almost the entire burden of every prey item it eats, and a predator eats a great many.
4.Apply to the advisory: the predatory fish carries 15,000 times the water concentration, so a serving of it delivers far more mercury than the same mass of a lower-trophic species.

Answer: This is biomagnification. Methylmercury is fat-soluble, persistent and slowly excreted, so each trophic level retains the accumulated burden of everything it eats and concentration multiplies at each step — reaching 15 ppm in the predator, 15,000 times the water concentration. Advisories target long-lived predatory fish for exactly that reason, while low-trophic species such as sardines carry far smaller burdens.

Watch out

Do not write that the algal bloom itself consumes the oxygen. Algae produce oxygen while alive. The hypoxia comes from decomposers respiring as they break down the dead bloom, and skipping that step is the single most common way this question is lost.

Checkpoint

Dissolved oxygen in a river falls sharply just downstream of a sewage outfall and recovers over several kilometers. The initial drop is caused by

Checkpoint

Which property is required for a contaminant to biomagnify?

Answer the 2 checkpoints as you read.