At HL you need more than “the water is polluted”. You need to know which pollutants matter in aquatic systems, what each one does to living things, and why some water bodies run out of oxygen even when nothing obviously toxic has been added.
POPs resist breakdown, persist for decades, bioaccumulate in organisms and biomagnify up food chains.
Harmful algal blooms (HABs) happen when certain algae or microorganisms grow rapidly, producing toxins or stripping oxygen.
Freshwater HABs are usually cyanobacteria releasing cyanotoxins; marine HABs are often dinoflagellates, sometimes producing red tides.
Hypoxic water has very low oxygen; anoxic water has none. Both create dead zones.
Dead zones are driven by eutrophication, sewage, warming water and thermal stratification preventing mixing.
The main aquatic pollutants at HL
Organic matter
Untreated sewage and animal waste add a large mass of biological material. Bacteria break it down aerobically, consuming dissolved oxygen as they respire, and the resulting oxygen shortage kills fish and invertebrates. Rivers running through fast-growing cities without adequate treatment show this pattern clearly.
Dissolved substances
Some chemicals dissolve and act at very low concentrations. A well-known example is tributyltin (TBT), once painted onto ship hulls to stop organisms growing on them. It leached into the water and disrupted the hormone systems of marine animals, causing reproductive failure in molluscs and sharp declines in oyster populations before it was banned.
Why TBT is a favourite exam example: it shows a pollutant that was deliberately designed to be toxic to marine life, worked exactly as intended, and then affected species nobody was targeting. It also shows regulation working — populations recovered after the ban.
Persistent organic pollutants (POPs)
POPs such as polychlorinated biphenyls (PCBs) share three dangerous properties:
Persistent — they resist chemical and biological breakdown, lasting decades in the environment.
Fat-soluble — they are stored in fatty tissue rather than excreted, so they bioaccumulate.
Mobile — they travel long distances in water and air, so they reach places that never used them.
Put those together and top marine predators end up with the highest loads through biomagnification, causing immune and reproductive problems in animals that live far from any factory.
Plastics
Larger items cause entanglement and blockage of digestive systems. Microplastics enter food chains from the bottom and carry adsorbed toxins with them.
Heat energy
Thermal pollution is warm water discharged from power stations and industrial cooling systems. It causes a double problem: warm water holds less dissolved oxygen, while at the same time it speeds up metabolism, so organisms need more oxygen just as there is less available. Cold-adapted species are pushed out.
Harmful algal blooms
Setting
Main organisms
Toxin and effect
Freshwater lakes and reservoirs
Cyanobacteria, often called blue-green algae
Cyanotoxins contaminate drinking water, damage the liver and affect the nervous system of animals and humans
Coastal and marine waters
Dinoflagellates, a type of protist
Neurotoxins accumulate in shellfish and can cause paralytic shellfish poisoning in people who eat them
HABs damage water quality in two separate ways, and a good answer names both:
They produce toxins that poison organisms directly, including humans.
They deplete oxygen when the bloom dies and decomposes, killing everything that needs it.
The economic consequences are real: fisheries close, shellfish beds are shut for weeks, beaches are declared unsafe, and coastal tourism income drops.
A bloom is not automatically “harmful” in the technical sense. HAB is reserved for blooms that produce toxins or cause serious oxygen depletion. Use the term precisely and you look like you know the topic.
Hypoxia, anoxia and dead zones
DefinitionsHypoxic water — oxygen too low to support most aquatic life Anoxic water — oxygen effectively absent Dead zone — an area where marine organisms cannot survive
Dead zones cluster in coastal regions receiving high nutrient run-off, especially where a large river drains intensive farmland into a sea that mixes slowly. They cause severe disruption to marine food chains and can collapse local fisheries.
Why layering makes it worse
This is the part most students miss. Nutrients start the problem, but thermal stratification is what locks it in.
The red cross marks blocked mixing. Oxygen-rich surface water cannot reach the bottom, so the oxygen used by decomposers down there is never replaced.
🧩 The four causes of hypoxia and anoxia
Eutrophication — excess nutrients fuel blooms; the decomposition of dead algae consumes the oxygen.
Sewage disposal — organic matter gives decomposers even more to work on.
Thermal stratification — layers by temperature stop oxygen-rich surface water reaching the depths.
Global warming — warmer water holds less dissolved oxygen and stratifies more strongly, so hypoxic conditions are expected to become more common.
EXAM PRACTICE
Explain why coastal dead zones are usually worst in late summer. [4]
Point 1: less oxygen can dissolve
Warm summer water holds less dissolved oxygen than cold water, so the starting supply is lower.
Point 2: demand is higher
Warmth speeds up decomposition and respiration, so organisms use oxygen faster.
Point 3: no resupply
Strong summer stratification stops oxygen-rich surface water mixing downwards.
Point 4: peak organic load
Spring and summer nutrient run-off has already produced blooms, and that dead algal material is now sinking and decomposing.
Low supply, high demand and no mixing all peak together in late summerWhenever a question says “why then” or “why there”, look for several factors coinciding.
💡 Exam tip
For POPs, always give the three properties: persistent, fat-soluble, biomagnifies. That is usually three marks.
Distinguish the two HAB dangers — toxins and oxygen depletion. Many answers give only one.
Name the organism group: cyanobacteria in freshwater, dinoflagellates in marine systems.
For thermal pollution, remember it works both ways: less oxygen dissolved, more oxygen demanded.
Link climate change to dead zones. It is a strong synoptic point and examiners reward connections across topics.
⚠️ Common mix-up
Calling every algal bloom a HAB. Reserve the term for toxin-producing or seriously oxygen-depleting blooms.
Saying cyanobacteria are algae. They are bacteria; “blue-green algae” is an old common name, not a classification.
Treating thermal pollution as harmless. It has no colour or smell but changes species composition completely.
Confusing bioaccumulation with biomagnification when writing about POPs. One organism vs. one food chain.
Forgetting stratification. Nutrients alone do not explain why deep water stays anoxic for months.
Up next: How Sewage Is Treated (HL) — the three stages, and why the technology existing is not the same as the technology being available.
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