The big idea: Ocean and coastal pollution is the build-up of harmful waste in seawater and along shorelines. Most of it comes from the land and is carried out to sea by rivers, run-off, winds and currents.
The main types
- Plastics — the biggest by volume.
- Oil.
- Chemicals and nutrients — sewage, farm fertiliser.
- Dissolved CO2 — drives ocean acidification.
Why it is hard to manage: It is a classic Option B theme because the oceans are a shared, open resource. Pollution dumped in one place spreads across borders, which makes it hard to manage.
Key terms for this micro
- Marine pollution — harmful waste entering the ocean (plastic, oil, chemicals, nutrients).
- Plastic pollution — durable plastic litter, from large items down to microplastics (tiny fragments under 5 mm).
- Eutrophication — nutrient pollution (sewage, fertiliser) that triggers algal blooms and dead zones of low oxygen.
- Ocean acidification — falling seawater pH as the ocean absorbs atmospheric CO2, weakening shell- and reef-building.
- Aquaculture — farming fish or shellfish in the sea; a fast-growing source of local pollution.
- Bioaccumulation — pollutants building up in living tissue, concentrating up the food chain.
Land first, then the sea: Around 80% of marine pollution starts on land and reaches the ocean through rivers and run-off.
That is why coastlines — where the land meets the sea — take the heaviest hit, and why managing pollution means tackling land-based sources, not just cleaning the water.
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How this is tested: The stimulus here is usually a bar chart or table breaking marine plastic down by source.
What the marks ask
- The first marks go to reading it: Identify the biggest source, or Estimate a share off the axis.
- Short Outline [2] questions then ask for one reason + its development (a classic being why plastic accumulates along coastlines).
Whatever the figure: Always check the units and quote a number back.
Read the key first. Which source is the largest share, and roughly what % is it?
Interactive diagram
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Estimate the percentage of ocean plastic that comes from land-based litter and rivers.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Where marine plastic pollution comes from (illustrative share of input)
| Source | Share of plastic entering the ocean | Mostly enters via |
|---|---|---|
| Land-based litter and dumping | ~80% | Rivers and coastal towns |
| Rivers carrying inland waste | ~50% of the land total | A few large polluted rivers |
| Fishing gear (nets, lines, traps) | ~10% | Lost or dumped 'ghost gear' |
| Shipping and offshore industry | ~10% | Spills and deliberate dumping |
Read the share off the correct row: For a 'biggest source' question, find the largest share in the table (land-based litter, ~80%). For an Outline of why plastic reaches the coast, link the source (land/rivers) to a transport process (waves, currents, onshore winds hold it near shore).
Why plastic accumulates along coastlines
- Closest to the source — most plastic is land-based, and coasts are right next to the towns, beaches and river mouths producing it.
- Rivers act as conveyor belts — they funnel inland waste straight to the coast, dumping it where the river meets the sea.
- Waves, currents and onshore winds push floating plastic back onto beaches and trap it in bays, so it does not all drift out to the open ocean.
- Dense coastal settlement — billions live near coasts, adding litter and sewage directly to the shoreline.
Outline one reason why plastic pollution builds up along ocean coastlines, and develop your answer.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
The Great Pacific Garbage Patch: Between California and Hawaii, rotating currents (a gyre) concentrate floating plastic into the Great Pacific Garbage Patch — an area of unusually high plastic and microplastic density.
Why it matters: it shows pollution dumped on distant coasts is carried by currents and trapped far out at sea, where no single country is responsible for clearing it.
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Plastic is the most visible pollutant, but nutrients, chemicals and oil do serious damage too. Examiners want the mechanism — name the source, then explain the chain of harm to the marine ecosystem.
Pollution sources and the harm they cause
| Source | Pollutant | How it harms the marine ecosystem |
|---|---|---|
| Sewage + farm fertiliser run-off | Nutrients | Eutrophication: algal blooms then low-oxygen dead zones kill fish |
| Aquaculture (fish farms) | Waste + chemicals | Uneaten feed and faeces enrich the water; escaped fish + sea lice spread disease |
| Industry + shipping spills | Oil + chemicals | Oil coats seabirds and smothers shorelines; toxins bioaccumulate up the food chain |
| Atmospheric CO2 dissolving in seawater | Dissolved CO2 | Lowers pH (acidification), weakening shells and coral skeletons |
Environmental problems caused by aquaculture
- Water enrichment — uneaten feed and fish waste add nutrients, causing local eutrophication beneath the cages.
- Disease and sea lice — crowded cages breed parasites and disease that spread to wild fish nearby.
- Escaped farmed fish — escapees interbreed with or out-compete wild stocks, weakening their genetics.
- Pressure on feeder fish — farming carnivorous fish needs wild fish for feed, depleting other stocks.
Outline one environmental problem caused by the growth of aquaculture, and develop your answer.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Always give the mechanism: Don't just name a pollutant — explain how it harms the ecosystem. Fertiliser run-off -> extra nutrients -> algal bloom -> oxygen stripped -> dead zone.
Scotland's salmon farms and Deepwater Horizon: Scotland's salmon farms show aquaculture's local cost: dense cages have spread sea lice and added waste nutrients to sea lochs, prompting tighter rules.
The 2010 Deepwater Horizon blowout in the Gulf of Mexico released millions of barrels of oil, killing seabirds and marine life and coating the coast — a reminder that one offshore spill can devastate a whole coastline.
Radioactive materials in the ocean: Until the 1990s some countries dumped radioactive waste in drums at sea. The London Convention (1972) controlled dumping, and a 1993 amendment banned dumping radioactive waste at sea altogether. Today the issues are discharges from nuclear sites such as Sellafield (UK) into the Irish Sea and accidents such as Fukushima (2011). Since 2023 Japan has released treated, diluted water from Fukushima into the Pacific; the IAEA judged it within international standards, but China banned Japanese seafood imports.
Managing ocean pollution: initiatives, strengths and weaknesses
| Pollutant | Initiative (scale) | Strength | Weakness |
|---|---|---|---|
| Oil | MARPOL convention (global, 1973/78); double hulls required on tankers after the Exxon Valdez spill (1989) | Large spills from tankers are now far rarer | Does not stop offshore blowouts (Deepwater Horizon, 2010) or illegal discharges at sea |
| Radioactive | London Convention ban on dumping (global); OSPAR rules for the North-East Atlantic (regional) | Deliberate dumping has stopped | Discharges and accidents still happen; trust in monitoring varies between countries |
| Plastic | Bag charges and bans (national: England's 2015 charge, Kenya's 2017 ban); UN talks on a global plastics treaty since 2022 | Bag charges cut use in big shops by over 90% in England | Most plastic comes from many small, scattered sources; the treaty talks had not agreed a deal by 2025 |
Why the oceans are becoming more acidic: The ocean absorbs about a quarter of the CO2 humans emit. That CO2 makes seawater more acidic.
From CO2 to weaker reefs
CO2 dissolves
CO2 dissolves in seawater and forms carbonic acid.
pH falls
Carbonic acid lowers the pH — this is ocean acidification.
Fewer carbonate ions
Lower pH means fewer carbonate ions, the building blocks corals and shellfish use to make their calcium-carbonate skeletons and shells.
Reefs suffer
Reefs find it harder to grow and easier to dissolve.
More CO2, lower pH, fewer carbonate ions, weaker reefs.
Impacts on reefs are NOT uniform: Acidification weakens reef-building everywhere, but the effect is uneven. It often combines with warming-driven bleaching, and some reefs are more exposed (cooler, deeper or better-flushed reefs cope better; shallow, stressed reefs suffer most).
A top essay must recognise this variation — not all reefs respond alike.
Examine why the oceans are becoming more acidic and how this could affect coral reefs.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
The Great Barrier Reef: Australia's Great Barrier Reef is the world's largest reef system and a textbook case: it faces acidification (slower skeleton growth) on top of repeated bleaching from marine heatwaves.
Why it matters: it shows acidification rarely acts alone — its impact depends on how warm, sheltered and healthy each section of reef already is.