Impacts of water pollution
Big idea: Water pollution affects ecosystems through bioaccumulation (toxins build up in organisms) and biomagnification (toxins concentrate up food chains). Top predators — including humans — receive the highest doses.
Bioaccumulation vs biomagnification
Bioaccumulation
- Toxins build up in ONE organism over time
- Organism absorbs toxins faster than it excretes them
- Example: mercury accumulating in a single fish
- Influenced by fat solubility, persistence, and exposure time
Biomagnification
- Toxin concentration increases at higher trophic levels
- Each level contains a higher concentration than the one below
- Example: plankton → fish → seals → top predators
- Top predators have the highest concentrations
Substances that biomagnify
- Heavy metals — mercury, lead, cadmium
- Persistent organic pollutants (POPs) — DDT, PCBs, dioxins
- Microplastics — absorb other toxins and enter food chains
- These substances are fat-soluble, persistent, and not easily excreted
Be able to clearly distinguish between bioaccumulation and biomagnification, and explain why toxin concentrations are highest in top predators using simple food-chain examples.
Seasonal dead zones (hypoxia)
Key idea: A seasonal dead zone is an area of water where dissolved oxygen becomes very low during certain months, often in summer, so many fish and bottom-dwelling animals die or move away.
What does the term mean?
- A dead zone is an area where very few animals can survive because oxygen levels are too low.
- Seasonal means it forms during particular times of year and may improve later.
- It is strongly linked to eutrophication, which is caused by excess nutrients such as nitrogen and phosphorus.
Why do dead zones happen?
Excess nutrients from agriculture and wastewater increase algae growth. When algae die, decomposers break them down and use up dissolved oxygen. Warm summer water also holds less oxygen, making hypoxia more likely.
- More nutrients enter rivers and seas through fertilizer runoff and sewage.
- Algal blooms increase as eutrophication develops.
- Dead algae sink and are decomposed by bacteria.
- Decomposition consumes dissolved oxygen.
- Oxygen falls to hypoxic levels and animals die or migrate.
Exam tip: In data questions, look for the month with the lowest dissolved oxygen, often July or August. Link it to eutrophication: nutrient input → algal bloom → decomposition → oxygen drops → dead zone.
Key numbers students should know
- Hypoxia is commonly defined as dissolved oxygen below about 2 mg/L.
- Fish and many invertebrates avoid hypoxic water or die if they cannot escape.
- Some organisms, such as certain jellyfish, tolerate low oxygen better and may increase.