Plastic pollution, eutrophication, climate change and conservation

Plastics in the sea: persistence, fragmentation and microplastics

Plastics are synthetic polymers made from oil and gas, so this section follows directly from the work on oil under the seabed: the same crude oil that is burned as fuel is also the raw material for bags, bottles, fishing nets and clothing fibres. The property that makes plastic useful is the property that makes it dangerous once it reaches the sea. Its long chain-like molecules are held together by strong carbon-carbon bonds that most bacteria and fungi cannot break, so plastic is not biodegradable in any useful sense.

A fish bone or a cotton shirt is broken down by decomposers in weeks or months; a plastic bottle is not.

The syllabus phrase is that plastics are very slow to break down, and every word of that matters. They do eventually break down, but over very long time periods, and by physical and chemical processes rather than biological ones. Ultraviolet radiation in sunlight makes the polymer brittle, waves and sand grind it against rock and against other plastic, and the fragments are abraded again and again.

The product is not harmless molecules but smaller and smaller pieces of the same polymer, called microplastics: fragments less than about 5 mm across, down to particles too small to see without a microscope.

Fragmentation releases toxins in two separate ways, and exam answers should give both. First, plastic is not a pure material: it contains additives such as plasticisers, stabilisers and flame retardants, which can leach out of the fragment into the water and into any animal that swallows it. Second, plastic surfaces adsorb oily pollutants that are already present in seawater, such as PCBs and some pesticides, concentrating them on the fragment to levels far above those in the surrounding water.

The fragment then carries that concentrated load into the food chain.

Most marine plastic begins on land. Litter dropped inland is blown or washed into drains, streams and rivers and carried to the coast; poorly managed landfill leaks; synthetic clothes shed microfibres in the wash which pass through sewage works; and tyres shed particles onto roads that reach the sea in run-off. The rest is fishing gear - nets, ropes, lines and pots that are lost overboard or deliberately dumped - known as ghost gear because it goes on catching animals long after the boat has left.

Ships add waste of their own, and although the international convention on pollution from ships (MARPOL) forbids dumping plastic at sea, enforcement on the open ocean is difficult.

• Not biodegradable: the carbon backbone of the polymer resists bacterial and fungal attack, so plastic persists for decades to centuries.
• Breaks down very slowly by ultraviolet light, wave action and abrasion with sand and rock, not by decay.
• Microplastics: fragments under about 5 mm, produced by that fragmentation - they are the same plastic, only smaller.
• Toxins are released because additives leach out of the polymer and because pollutants such as PCBs adsorb onto plastic surfaces and are concentrated there.
• Sources: land-based litter, landfill, storm drains, rivers, microfibres from washing, tyre particles, and lost or dumped fishing gear (ghost gear).
• MARPOL prohibits the disposal of plastics from ships, but it has to be enforced port by port and sea area by sea area.

The impacts of plastics on marine ecosystems

Microplastics are taken up by plankton and enter food chains. Filter-feeding plankton, and the larvae of many fish, crustaceans and molluscs, cannot tell a plastic particle from a food particle of a similar size, so they ingest it. Because plankton sit at the base of almost every marine food chain, the plastic - and the toxins stuck to it - passes to the animals that eat the plankton, then to the fish that eat those, then to seabirds, turtles, seals, whales, and eventually to people eating seafood. This is the route that turns a local litter problem into an ecosystem-wide one, and it is the reason microplastics matter even though the particles are invisible.

Large plastics are ingested by animals and can cause starvation. Turtles mistake floating bags for jellyfish; seabirds pick fragments off the surface and feed them to their chicks; whales, dolphins, seals and fish swallow bags, sheeting and packaging. Once swallowed, plastic fills the stomach so that the animal feels full and stops feeding, blocks the gut so that food cannot pass through, or tears the lining and causes infection. The animal then starves with a full stomach, which is the phrase that earns the mark, and it may also absorb the toxins carried by the plastic.

Indigestible plastic can remain in the gut for months and be passed up to a chick or a calf.

Plastics can entangle animals and can cause death. Lost nets, ropes, line and packing loops wrap around the necks, flippers, mouths and beaks of seals, turtles, dolphins, whales and seabirds. Entanglement can drown an air-breathing animal that cannot reach the surface, cut into skin and cause a slow infection, drag on the animal so that it cannot swim or feed efficiently, or snag on a reef and hold it there until it dies. Ghost gear is the worst offender because it is designed to catch animals, it is almost invisible in water, and a single lost net can keep killing for years as it drifts and snags.

Ocean gyres and plastic garbage patches

An ocean gyre is a large, roughly circular system of surface currents rotating around an ocean basin. It is driven by the prevailing winds - the trade winds near the equator and the westerlies further from it - which push the surface water, and it is shaped by the Coriolis effect, the deflection of moving water to the right in the northern hemisphere and to the left in the southern hemisphere, and by the edges of the continents. There are five main subtropical gyres: the North Pacific, South Pacific, North Atlantic, South Atlantic and Indian Ocean gyres.

Inside a gyre the surface water converges. The currents around the edge of the gyre push water, and anything floating in it, towards the slow, gently turning centre. Floating plastic is therefore not carried out of the gyre and dispersed; it is carried into it and held there.

Most plastics are less dense than sea water, so they float, and they do not rot, so nothing removes them. A fragment that enters the gyre stays in the slow central water for years, breaking into smaller pieces but never disappearing. Over decades, plastic from every coast and river that feeds the gyre collects in that one region.

The result is a plastic garbage patch: a diffuse region of the ocean surface where fragments are far more concentrated than elsewhere. It is important to be accurate about what it is. A patch is not a solid island and not a raft of bags you could stand on.

Most of it consists of small fragments and weathered fishing gear suspended in the upper few metres, spread over a very large area, and a boat can sail through much of a patch without noticing anything unusual. That is exactly why cleaning it up is so difficult: there is no mass to scoop up, only a thin soup of particles over thousands of square kilometres.

The Great Pacific Garbage Patch is the largest of these patches, lying in the North Pacific gyre between Japan and the west coast of North America, with Hawaii near its southern edge. It is the largest because the North Pacific gyre is the largest of the gyres and because it is fed by the densely populated and heavily industrialised coasts of eastern Asia and western North America, and by the rivers draining them. Comparable patches exist in the other gyres; they are less famous, not absent.

• Gyre: a large circular system of surface currents, driven by prevailing winds, deflected by the Coriolis effect and bounded by continents.
• Convergence: water and floating debris are pushed towards the slow centre of the gyre, where they are trapped rather than dispersed.
• Why plastic stays: most plastics float, and because they do not break down biologically they accumulate and fragment instead of disappearing.
• What a patch is: a diffuse region of concentrated fragments in the upper few metres - not a solid island and not visible as a mass from space.
• Great Pacific Garbage Patch: the largest, in the North Pacific gyre between Japan and North America, fed by the large population and industry around that ocean.

Reducing the size of garbage patches, and limiting the impacts of plastics

These are two different exam questions and they need different answers. Reducing the size of a garbage patch means getting plastic out of the ocean, or stopping it entering. Limiting the impacts of plastics means changing what people produce, buy, use and throw away, so that less plastic reaches the sea and less harm is done by the plastic already there.

Removing plastic from the sea. Skimming systems use long floating booms towed slowly between two vessels to concentrate surface plastic, which is then lifted out; dedicated clean-up vessels collect debris from known accumulation zones; and beach and river-mouth clean-ups intercept plastic before it is carried offshore. Intercepting it in rivers is far more efficient, because rivers deliver most land-based plastic to the sea and the plastic is still concentrated in one channel. Monitoring by satellite, aircraft and ocean models is used to find where debris is concentrated so that collection effort is not wasted.

Evaluating those strategies. Recovery at sea is expensive, energy-hungry and slow, and it can only remove floating macroplantic; microplastics and plastic suspended below the surface cannot be collected at all. The gear can catch plankton, fish and turtles (bycatch), and the patch refills from the same currents, so a clean-up has to keep running for ever just to hold the size steady. Once plastic is in international waters, no single government is responsible for it, and once it has fragmented it is effectively impossible to remove.

The conclusion is unavoidable: prevention at source is cheaper and more effective than removal, and removal is worth doing mainly where plastic is concentrated and accessible, such as beaches, river mouths and known accumulation zones.

Legislation works by banning, charging for or taxing single-use plastics. Bans have been applied to carrier bags, drinking straws, cutlery, plates, cotton buds and microbeads in rinse-off cosmetics; charges and levies on carrier bags cut their use sharply wherever they have been introduced; deposit return schemes put a value on bottles so that they are returned rather than littered; extended producer responsibility makes manufacturers pay for the collection and recycling of their packaging; and taxes on virgin plastic make recycled material more competitive. MARPOL bans the dumping of plastic from ships, and gear-marking rules make it possible to identify the owner of lost nets.

Legislation is powerful because it changes what is manufactured and sold, applies to everyone at once, and is cheap to administer once in place.

The limits of legislation are just as examinable. A national ban only controls that country, while plastic drifts across borders; enforcement at sea is very difficult; bans can push producers towards substitute materials whose own impacts may be no better; and in countries without reliable waste collection, most plastic never reaches a bin in the first place, so a ban on bags does little about the waste stream.

Education works on behaviour: teaching plastic pollution in schools, public campaigns, labels explaining what can be recycled, beach clean-ups that show people the scale of the problem, and outreach to fishers about returning gear. It changes habits for a lifetime, is cheap, and builds the public support that makes legislation enforceable, but it is slow, it has no immediate effect on the ocean, and it cannot help where there is no waste collection service at all.

Reduce, reuse, recycle. Reduce means designing out unnecessary packaging and buying fewer single-use items; reuse means refillable bottles, bags and containers; recycle means collecting plastic and processing it into new products. Together they cut the amount of new plastic made and the amount of waste that can leak into the sea, and recycling creates jobs and saves the oil and energy used to make virgin plastic. Their weakness is that recycling is really downcycling - each cycle gives a lower-grade material - mixed and contaminated plastics are difficult and uneconomic to recycle, and only a small fraction of all the plastic ever made has actually been recycled.

Recycling also does nothing about the plastic already in the ocean.

Eutrophication: from fertiliser to dead fish

Modern land farming techniques increase food production through the use of fertilisers to improve yields. Fertilisers supply the nutrients that limit plant growth, mainly nitrogen (as nitrates) and phosphorus (as phosphates), so a field that is fertilised grows a bigger crop. The problem is that these fertilisers are usually water soluble.

Rain that falls on a fertilised field dissolves the unused nutrients, and because they do not stick to soil particles they are washed downwards and sideways - leached through the soil and run off across the surface - into ditches, streams and rivers, and from there into estuaries and the sea. The same nutrients that feed a crop also feed whatever is growing in the water.

Untreated sewage does the same thing directly. Sewage is rich in nitrogen and phosphorus as well as in organic matter, and where it is discharged into the sea without treatment - which is still common around rapidly growing coastal cities - it adds those nutrients continuously, close to the shore. Sewage can therefore cause eutrophication in an estuary even where there is no farmland in the catchment at all, and it adds pathogens at the same time.

The process has to be learned as a sequence, because the marks are for the links, not the ending. In most coastal waters the growth of producers is limited by the supply of nutrients. Adding nitrogen and phosphorus removes that limit, so phytoplankton and fast-growing macroalgae multiply rapidly.

A sudden dense growth of producers is an algal bloom, and it turns the water green or brown and reduces light penetration, shading and sometimes smothering the seagrass and corals beneath it.

The bloom is self-limiting: the producers use up the added nutrients, shade one another, and then die. The dead cells and dead algae sink. Decomposers - bacteria and fungi - then break the dead material down, and they do it by aerobic respiration, which consumes dissolved oxygen.

A large mass of dead producers supports a large population of decomposers, so the demand for oxygen rises sharply and dissolved oxygen in the water falls. Once it falls low enough the water is hypoxic, and fish, crustaceans, molluscs and other organisms that need dissolved oxygen suffocate or swim away. The area left with almost no oxygen is a dead zone.

In the worst cases the decomposers themselves run out of oxygen, anaerobic bacteria take over, and the water smells of hydrogen sulfide.

Climate change past and present: carbon dioxide, methane and the enhanced greenhouse effect

The climate of different regions on Earth has changed significantly between 335 million years ago and the present day, and the two variables the syllabus asks you to refer to are the carbon dioxide level and the temperature. At 335 million years ago the continents were arranged very differently from today, carbon dioxide levels were falling from the much higher values of the earlier Palaeozoic, and the planet was cooling from a prolonged warm state into a long glacial phase, with ice sheets spreading over the southern supercontinent of Gondwana. The tropical swamp forests of that time were buried and became the coal seams that are being burned today.

Since then carbon dioxide has risen to very high levels in the Mesozoic, fallen through the Cenozoic, and temperature has broadly risen and fallen with it. There have been long periods with no polar ice at all, and ice ages during which ice reached far south into Europe and North America.

The evidence for that history comes from rocks, fossils and ice. Coal seams and fossil reef corals show where warm, wet climates and shallow tropical seas once were; glacial deposits show where ice sheets lay; fossils of warmth-loving plants and animals are found today in polar latitudes; and air bubbles trapped in Antarctic ice cores record carbon dioxide concentration and temperature directly for the last few hundred thousand years. Isotope ratios in shells and deep-sea sediments push the record much further back.

The pattern that matters for the exam is that carbon dioxide and temperature have moved together.

Present-day pollution of the air by carbon dioxide and methane strengthens the greenhouse effect. Short-wave radiation from the Sun passes through the atmosphere and warms the Earth's surface. The warm surface then radiates energy back outwards as long-wave infrared radiation, and greenhouse gases in the atmosphere absorb that infrared radiation and re-emit it in all directions, so some of it returns to the surface instead of escaping to space.

That natural greenhouse effect is what keeps the planet warm enough for liquid water. Adding carbon dioxide and methane increases the amount of infrared radiation that is absorbed and returned, so the Earth retains more heat: this is the enhanced greenhouse effect, and its consequence in this syllabus is increasing temperature.

Carbon dioxide comes mainly from burning fossil fuels - coal, oil and gas - in power stations, vehicles, industry and homes, and from deforestation and cement manufacture. Methane comes from livestock, rice paddies, landfill and leaks from gas extraction and pipelines. Methane absorbs infrared radiation far more strongly per molecule than carbon dioxide, but there is much less of it in the atmosphere and it breaks down more quickly, so carbon dioxide remains the main long-term driver of the enhanced greenhouse effect.

• 335 million years ago to now: carbon dioxide levels and global temperature have both changed greatly, and they have risen and fallen together.
• At 335 million years ago: carbon dioxide was higher than today, the planet was cooling into a glacial phase and ice sheets spread over Gondwana; the coal we burn formed in tropical forests then.
• Evidence: coal seams, fossil corals, glacial deposits, fossils in the wrong latitudes, ice cores and isotope ratios in sediments.
• Greenhouse effect: short-wave solar radiation warms the surface; the surface emits long-wave infrared; greenhouse gases absorb it and re-emit some back towards the surface.
• Enhanced greenhouse effect: extra carbon dioxide and methane from human activity absorb and return more infrared radiation, so more heat is retained and global temperature rises.
• Sources: carbon dioxide from fossil fuel combustion, deforestation and cement; methane from livestock, rice paddies, landfill and gas leaks.

The ocean's role: heat absorption, thermal expansion and falling pH

The ocean has absorbed most of the extra heat trapped by the enhanced greenhouse effect - about 90 per cent of it - because water has a very high specific heat capacity and the ocean covers roughly 71 per cent of the planet. Currents and the great overturning circulation carry that heat from the surface into the deep water, where it is stored for centuries. The effect on the atmosphere is that the ocean has maintained the stability of global temperature: it has acted as a buffer, so air temperatures have risen more slowly than they would have done if the heat had stayed in the atmosphere.

That is a benefit to us now and a debt to the future, because the heat is still there.

The effect on the ocean itself is that sea water warms and expands. Water expands as its temperature rises, so the same mass of water occupies a larger volume, and sea level rises. This is thermal expansion, and because the deep ocean takes a very long time to warm and to give the heat back, sea level continues to rise for centuries after atmospheric temperature has stabilised.

Warming also makes the surface layer less dense and more stable, so stratification strengthens, mixing between surface and deep water weakens, and the ocean takes up oxygen less effectively - which is one reason dissolved oxygen is falling.

The ocean also absorbs carbon dioxide: roughly a quarter of the carbon dioxide released by human activity has dissolved in the sea. Carbon dioxide dissolves in water to form carbonic acid, which dissociates and releases hydrogen ions, so the concentration of hydrogen ions rises and the pH falls. The ocean is still alkaline, but it is measurably less alkaline than it was before industrialisation, by about 0.1 pH units.

A lower pH makes it harder for organisms that build shells and skeletons of calcium carbonate - corals, molluscs and some plankton - to deposit that calcium carbonate, and it dissolves carbonate more readily. The syllabus calls this the effect of increasing atmospheric carbon dioxide on sea water, and the mark is for decreasing pH.

Thermal expansion compared with melting land-based ice. Both raise sea level, but for different reasons and at different rates. Thermal expansion raises sea level because existing sea water takes up more space; no water is added. Melting land-based ice - glaciers, the Greenland ice sheet and the Antarctic ice sheet - raises sea level because water that was stored on land as ice flows into the ocean, so the total volume of water in the ocean increases.

Thermal expansion has been the largest single contributor to the rise observed over the past century, and it is the part that cannot be reversed quickly, because the deep ocean holds its heat. Melting land ice is smaller so far but is the greater long-term risk, because the ice sheets hold enough water to raise sea level by many metres and because their melting can accelerate as the ice surface lowers and outlet glaciers flow faster. Melting floating sea ice does not raise sea level at all, because floating ice already displaces the water it will become.

The impacts of increasing greenhouse gases on marine ecosystems

Sea level rise, with loss of habitat and land. As sea level rises, low-lying coasts are flooded and squeezed: sandy beaches and turtle nesting beaches are eroded or drowned, mangrove forests and saltmarshes are lost where there is no space for them to move inland, and shallow reef flats and seagrass beds lose the light they need as the water deepens. On low-lying islands and densely populated deltas there is simply land loss, and salt water intrudes into freshwater supplies and farmland.

Coral bleaching. Corals live in partnership with single-celled algae (zooxanthellae) that live in their tissues, give them their colour and supply most of their food by photosynthesis. When the water becomes too warm - during a marine heatwave, for example - the partnership breaks down, the coral expels the algae, the white calcium carbonate skeleton shows through, and the coral is bleached. A bleached coral is still alive but is starving; if the heat stress is brief it may recover its algae, but if it lasts for weeks it dies.

Repeated bleaching events, with too little time between them for recovery, convert a diverse reef into a reef of dead skeletons covered in algae.

Decreased dissolved oxygen. Warm water holds less dissolved oxygen than cold water, and warming also strengthens stratification, which reduces the mixing that carries oxygen down from the surface. Combined with nutrient-driven eutrophication, this expands the volume of hypoxic water in the ocean and enlarges dead zones, forcing mobile animals to move and killing those that cannot.

Changing distribution and migration patterns. Organisms are adapted to a range of temperature, so as the sea warms they shift towards the poles and, on reefs, into deeper and cooler water, tracking their preferred conditions. Fish stocks move across national boundaries, which creates disputes over quota; species that cannot move fast enough, such as corals and slow-moving invertebrates, are left behind; and the timing of migrations and spawning shifts, so that predators and their prey may no longer arrive at the same time. That mismatch is often more damaging than the temperature change itself.

Loss of species. Bleached reefs, warmed polar seas, acidified water and shrinking habitats all reduce the number of species that can survive in a place, and species with small ranges or narrow tolerances disappear first. Local extinctions reduce species richness and simplify food webs, making the whole ecosystem less stable. Increase in extreme weather adds to this: more frequent and more intense marine heatwaves, stronger tropical cyclones that break coral and strip mangroves, and heavier rainfall that sends more soil and nutrients into coastal water.

• Sea level rise: loss of beaches, mangroves, saltmarsh and shallow reef habitat, plus loss of land and salt intrusion on low-lying coasts.
• Coral bleaching: heat stress makes corals expel their zooxanthellae, so they lose colour and their main food supply and die if the stress persists.
• Decreased dissolved oxygen: warm water holds less oxygen and mixes less, so hypoxic zones and dead zones expand.
• Changing distribution and migration: species shift towards the poles and into deeper water, stocks cross borders, and the timing of migrations and spawning falls out of step with prey.
• Loss of species: local extinctions reduce species richness and simplify food webs, making ecosystems less resilient.
• Increase in extreme weather: more marine heatwaves, stronger cyclones and heavier rainfall damaging reefs, mangroves and coastal water quality.

Conserving species richness: harvesting, aquaculture, marine parks and conservation projects

Species richness is the number of different species that live in an area. It is a count of species, not of individuals: a mudflat with millions of worms of three species has a lower species richness than a reef with a few hundred individuals of fifty species. Richness is one measure of biodiversity, and it is the measure the syllabus asks you to conserve.

Sustainable harvesting of wild macroalgae, plants and animals means taking a catch that the population can replace, so that the same harvest is available next year and the species is not driven down. The tools are quotas (a total allowable catch), minimum landing sizes so that animals can breed before they are taken, closed seasons protecting spawning, gear restrictions to reduce bycatch, licences to limit effort, and no-take zones within which nothing is removed. For macroalgae such as kelp and carrageenophytes, sustainable harvesting means cutting rather than tearing, leaving the holdfast and part of the frond so the plant regrows, and rotating the areas harvested.

Aquaculture - farming commercial species such as salmon, shrimp, oysters, mussels and seaweeds - can conserve species richness indirectly, because farmed produce replaces wild-caught produce and so reduces fishing pressure on wild stocks. It can also conserve directly, where endangered species are bred and reared for release to restock areas where environmental damage has occurred. Its weaknesses are important: carnivorous farmed fish are fed on wild-caught fish, so farming can increase pressure on wild stocks rather than relieve it; waste food and faeces add nutrients and can cause local eutrophication; dense stockings spread disease and parasites to wild populations; escaped farmed fish can interbreed with wild ones; and ponds and cages can destroy the habitat they are built in, especially where mangroves are cleared for shrimp.

National marine parks and Marine Protected Areas (MPAs) are areas of sea where human activity is restricted by law, usually with zones that range from multiple-use (some fishing allowed) to no-take (nothing removed). They conserve species richness because they protect whole habitats rather than single species, so every species within the boundary benefits; because they allow fished populations to recover in size and age structure; and because larvae and adults spill over into the fished water around them, so the fishery gains too. They also protect against the physical damage of trawling and anchoring, support tourism, research and education, and provide a reference area against which the effects of fishing elsewhere can be measured.

Their limits are equally real: a line on a map is not enforcement, so poaching and illegal fishing continue in many MPAs; small or isolated reserves conserve fewer species than large, connected ones; fishing effort displaced from an MPA can simply concentrate elsewhere; and no MPA can protect against warming, acidification or pollution that crosses its boundary.

Conservation projects put those strategies into practice. Coral farming takes fragments of coral from healthy colonies, grows them on underwater nurseries suspended or fixed to frames, and then transplants them onto artificial structures or damaged reef. It is fast, relatively cheap, engages local communities and divers, and can restore structure and diversity on a reef that has been blasted, bleached or broken by storms.

It cannot, however, fix the cause of the damage: if water quality or temperature is still unsuitable, the transplanted corals die too; it works best for branching species that fragment easily, so it restores a fraction of the reef's species; and the area restored is tiny compared with the area of reef lost.

Protecting turtle nest sites and rearing turtles for release works on the beach and on the animal. Nests are patrolled and protected from poaching and from predators, nests laid below the high tide line or in eroding sand are relocated, lighting behind beaches is controlled so that hatchlings head for the sea rather than the road, and hatchlings are sometimes reared in tanks for weeks or months before release to shorten the period when they are most vulnerable. The strengths are that egg and hatchling mortality is the easiest part of a turtle's life to reduce, the work is cheap and local, and it involves communities and tourism.

The limits are that rearing is expensive, that hand-reared animals may not learn natural foraging or navigation, that the sex of hatchlings depends on incubation temperature so hatcheries must control it, and that none of it helps if the adults are still caught in nets, struck by boats or lose their feeding grounds.

Culling invasive species such as lionfish is a different kind of project. Lionfish are native to the Indo-Pacific and were introduced to the western Atlantic and Caribbean, where they have no natural predators and prey heavily on juvenile reef fish. Culling by spearfishing and diver-led removals, supported by lionfish derbies and by promoting lionfish as a food fish, reduces local numbers and allows small reef fish to recover.

It works because lionfish are territorial and easy to approach, and because removing them is a direct, measurable reduction in predation. It is limited by cost and effort, by the fact that lionfish live deeper than most divers can work, by the huge area invaded, and by the need for culling to continue indefinitely: stop, and numbers recover. Culling controls an invasive species; it does not eradicate it.