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 are plastics (the biggest by volume), oil, chemicals and nutrients (sewage, farm fertiliser), and the dissolved CO2 that drives ocean acidification.
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.
How this is tested: Paper 1 Option B opens with a data-response on a pollution figure or table — you Identify, Estimate or Describe a value off it. It also asks short Outline [2] questions, where you give one reason + develop it. A favourite is why plastic pollution accumulates along coastlines. Always read the units and quote a figure.
| 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.
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.
| 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.
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.
Why the oceans are becoming more acidic: The ocean absorbs about a quarter of the CO2 humans emit. When CO2 dissolves in seawater it forms carbonic acid, which lowers the pH — this is ocean acidification.
Lower pH means fewer carbonate ions, the building blocks corals and shellfish use to make their calcium-carbonate skeletons and shells. So acidification makes it harder for reefs to grow and easier for them to dissolve.
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.
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.