Ecological impacts, global warming, acidification and conservation

Oil and agriculture: damage that starts on land

Two of the largest sources of damage to the sea begin on land or at the surface. Oil reaches the sea from tanker accidents and well blowouts, which are the events on the news, but most oil arrives quietly from routine operations: drilling muds and rock cuttings, produced water brought up with the oil, bilge and fuel from ships, and small leaks from terminals and pipelines. Because oil floats, it spreads into a thin slick that coats the feathers of seabirds and the fur of seals and otters, so they lose their waterproofing and insulation and die of cold or drown, while the toxic hydrocarbons in the lighter fractions damage the gills and eggs of fish and kill the plankton and fish larvae in the surface layer where most eggs develop.

Agriculture is the largest source of diffuse pollution, because fertiliser, pesticide and soil wash off fields into rivers and are carried to the sea. Nitrate and phosphate fertilisers cause eutrophication. The nutrients allow a rapid bloom of phytoplankton and of seaweeds such as Ulva, the water turns green, and when that growth dies the bacteria that decompose it use up the dissolved oxygen. Oxygen levels fall sharply (a hypoxic or low-oxygen zone), fish, crustaceans and shellfish suffocate or swim away, and the result is a dead zone in which almost nothing that needs oxygen can live.

Pesticides add a second problem, because many are persistent and fat-soluble, so they are stored in the tissues of organisms and passed up the food chain instead of being broken down and excreted. Soil eroded from fields makes the water turbid, and the fine sediment settles on seagrass beds and coral, cutting out the light that the plants and their symbiotic algae need and smothering the small invertebrates that live between the blades and the branches.

• Oil industry: exploration, drilling, produced water, tanker accidents and routine shipping losses.
• Oil floats, so seabirds and marine mammals lose waterproofing and insulation; hydrocarbons are toxic to plankton, eggs and larvae.
• Agriculture: nitrate and phosphate fertiliser -> algal bloom -> death and bacterial decomposition -> oxygen depletion -> hypoxic dead zone.
• Pesticides are persistent and fat-soluble, so they bioaccumulate and pass along the food chain.
• Soil erosion raises turbidity and silt smothers seagrass and coral, so the primary producers and the nursery habitat are lost.
• Both sources damage food webs from the bottom: remove the phytoplankton, the seagrass or the grazers and every level above them loses its food supply.

Sewage, refuse, desalination and renewable energy

Sewage carries three separate problems in one pipe. Untreated or poorly treated sewage contains pathogens, the bacteria and viruses from the human gut, which contaminate shellfish beds and bathing water and cause diseases such as cholera, typhoid and hepatitis. It also carries a large load of organic matter, which is broken down by bacteria that use up dissolved oxygen, so the biochemical oxygen demand of the water rises and oxygen concentrations fall for some distance downstream of the outfall. And it carries nitrate and phosphate, so it causes the same eutrophication as agricultural run-off.

Refuse disposal adds litter, plastics, metals and leachate. Dumping rubbish at sea is prohibited under the London Convention, but waste still reaches the sea from rivers, from landfill sites built on the coast and from ships. Rain percolating through a landfill produces leachate, a liquid rich in ammonia, heavy metals and organic chemicals, which can seep through the ground into the sea, and the plastics in the waste stream are the most persistent part of it.

Desalination plants remove salt from sea water to supply drinking water, and their waste is brine: water at roughly twice the salinity of the sea around it, warmer, and carrying the antiscalants and chlorine used to keep the membranes clean. Brine is denser than sea water, so it sinks and spreads along the seabed, where it dehydrates and kills seagrass, corals and bottom-living invertebrates that cannot move out of the way. The intake also draws in plankton, fish eggs and larvae with the water and kills them on the screens, which removes the base of the local food web.

Renewable energy installations avoid the carbon emissions of fossil fuels, but they are not impact-free. Offshore wind turbines are fixed to the seabed by monopiles driven in with hydraulic hammers, and the noise can deafen or displace marine mammals such as harbour porpoises and disturb fish, while the power cables generate weak electromagnetic fields and the foundations and their scour protection change the seabed. The structures do act as artificial reefs, and the exclusion zone around a wind farm stops trawling, so a wind farm can work as a small protected area. Tidal barrages and lagoons are more disruptive, because they change the flow, the sediment supply and the intertidal habitat of an estuary, which is the nursery ground for many fish and the feeding ground for wading birds.

Fishing practices: dredging and blast fishing

Most fishing removes fish. These two methods remove the habitat as well, and the damage lasts far longer than the catch, which is why these two methods are singled out for study.

Dredging for scallops, oysters and clams drags a steel-framed dredge with a toothed bar across the seabed. The teeth dig into the sediment and the heavy frame scrapes off whatever is attached to it, so the sponges, sea fans, hydroids, maerl beds and mussel beds that give a seabed its physical structure are destroyed, and the sediment is thrown into suspension. That turbidity cuts the light reaching seagrass and smothers filter feeders, the fine material settles elsewhere and buries the animals there, and contaminants and nutrients stored in the sediment are released back into the water. The gear is unselective, so much of the catch is dead or dying bycatch, and because a complex seabed community takes years or decades to recover, repeated dredging keeps the habitat permanently simplified.

Blast fishing uses explosives, commercially made or homemade from fertiliser, dropped onto a reef. The shock wave ruptures the swim bladder of every fish within range, so they float up dead, and the method takes everything in the area: juveniles, herbivores, predators, turtles and invertebrates. The blast also shatters the coral itself, so a branching reef with three-dimensional structure becomes a flat field of loose rubble, and the fish that used the coral for shelter, for nursery space and for food have nowhere left to live. Rubble is unstable and is quickly colonised by algae, which prevents coral larvae from settling, so recovery is slow and the catch per unit effort falls until the fishers must move to another reef.

The method is illegal in most countries, but it is cheap, needs nothing more than a small boat, and is very hard to police on remote reefs.

• Dredging: toothed steel frame scrapes the seabed -> attached epifauna and reef-building species destroyed -> sediment resuspended and redeposited.
• Turbidity reduces light for seagrass and clogs the filter feeders; resuspended sediment can release stored nutrients and contaminants.
• Dredging is unselective, so bycatch is high and the benthos that fish feed on is removed.
• Blast fishing: shock wave ruptures swim bladders and kills all fish and invertebrates in range, including juveniles.
• The blast converts coral into unstable rubble, so shelter and nursery habitat are lost and coral larvae cannot settle.
• Both methods simplify the food web and reduce biodiversity, and both make the habitat less able to support a fishery in future.

Bioaccumulation and biomagnification

Bioaccumulation is the build-up of a substance inside an organism over its lifetime, and it happens when the organism takes the substance in faster than it can excrete or break it down. It affects substances that are persistent, because they are not decomposed by the organism or by the environment, and fat-soluble, because a fat-soluble substance is stored in fatty tissue instead of dissolving in the body fluids and being flushed out.

Biomagnification is the increase in the concentration of that substance at each successive trophic level. A predator eats many prey and retains all the contaminant that was in each of them, so the concentration in its body is higher than the concentration in any one of its prey. Concentrations can therefore rise by orders of magnitude from plankton to a top predator, and the organisms most at risk are those at the top of long food chains, those that live longest, and those with a high fat content. This is why the same water can be legally safe to swim in and still produce fish that should not be eaten often.

Mercury is the classic example from the combustion of fossil fuels. Burning coal and other fossil fuels, and incinerating waste, release mercury vapour into the atmosphere, and it is deposited on the sea surface and in the sediment. In muddy, low-oxygen sediment, bacteria convert it into methylmercury, which is fat-soluble, so it enters the food chain through the plankton and passes up: phytoplankton -> zooplankton -> small fish -> predatory fish such as tuna, shark and swordfish -> people. In people methylmercury damages the developing nervous system, so unborn children and young children are most at risk, which is why official advice limits how much tuna and swordfish some groups should eat.

Antifouling paints are the other named example. Paints containing tributyltin (TBT) or copper compounds are applied to hulls to stop barnacles and weed settling, and they work by releasing a toxin slowly into the water around the hull. In the 1980s TBT was shown to cause imposex in dogwhelks, in which females develop male reproductive structures that block the oviduct and make them sterile, and populations collapsed in harbours and estuaries around the world. TBT is now banned internationally on ships, but copper-based paints still release metal into marinas and ports, where it accumulates in the sediment and in the grazers that rasp it off the painted surfaces.

Plastics and microplastics: from a bottle to a plankton

Microplastics are plastic particles with a diameter of less than 5 mm. Primary microplastics are manufactured at that size and released into the environment that way: the microbeads used in face scrubs and toothpastes, the resin pellets (nurdles) that are melted down to make plastic products, the microfibres shed from synthetic clothing every time it is washed, and the plastic beads used to blast-clean paint and machinery. Secondary microplastics are made in the environment by the fragmentation of larger items - bottles, bags, packaging, ropes and fishing nets - into smaller and smaller pieces.

The key point about most plastics is that they are non-biodegradable. No organism produces an enzyme that can break the polymer chains down, so a plastic bottle does not rot the way a paper bag does. What happens instead is physical and chemical breakdown. UV radiation in sunlight supplies the energy that breaks the polymer chains, which makes the plastic brittle so that it cracks and flakes. Wave action grinds the fragments against sand, rock and one another, and wind action carries fragments across the surface and blows them off beaches back into the sea. The fragments become smaller and more numerous, but they do not disappear, so one bag can become thousands of microplastic particles.

Temperature affects the rate of this breakdown, because the chemical reactions of photodegradation and the biological and chemical activity around the fragments are faster when it is warm. In warm, sunlit, shallow tropical water a plastic item fragments quickly, and in the cold, dark water of the deep sea or the Arctic the same item may persist for decades. That is why the highest concentrations of microplastics are not always found where the plastic was released.

Once the particles are small enough, plankton take them up. Zooplankton filter microplastics out of the water in the same way as food, and laboratory studies show that a diet of microplastics reduces feeding, growth and reproduction and can block the gut. Because plankton are the base of nearly every marine food chain, the particles are then transferred along the food chain, from zooplankton to small fish and from small fish to the predators that eat them, including the fish and shellfish that people eat.

Plastics also absorb toxic compounds from the water. Persistent organic pollutants such as PCBs and PAHs, and metals, stick to the surface of the particles and become far more concentrated on the plastic than in the water around it. When an organism ingests the particle, the gut is a warm, oily, acidic environment, so those toxins are released inside the animal and can be absorbed into its tissues. The plastic therefore delivers a concentrated dose of poison rather than an inert mouthful, which is the mechanism behind the risk to humans if plastics and their toxins enter the human food chain through seafood.

Larger items cause injuries that have nothing to do with ingestion. Turtles mistake floating bags for jellyfish, seabirds feed plastic to their chicks, and filter feeders such as whale sharks and baleen whales take in large volumes of water and the plastic in it, so the plastic fills the gut, gives a false sense of fullness and leads to starvation. Entanglement in lost or discarded fishing nets, ropes and packing loops kills by drowning, strangulation and starvation, and ghost nets - nets that have been lost but go on fishing - continue to catch fish, crabs, turtles and marine mammals for years. Entangled animals also drag the gear with them, so the damage spreads beyond the place where it was lost.

Strategies to limit the release attack the problem at each stage. At source, bans on microbeads in cosmetics, charges and bans on single-use bags, deposit return schemes for bottles and restrictions on landfill keep plastic out of the environment. In the waste stream, better collection and recycling, wastewater treatment with fine filters, washing machine filters and changes to textile design capture or prevent microfibres and pellets before they reach a river.

At sea, MARPOL Annex V prohibits the dumping of plastics from ships, ports provide reception facilities for waste and old gear, and gear marking and net retrieval schemes reduce ghost fishing. Clean-up projects remove litter from beaches and the surface, but they can recover only a small fraction of what is there, so they are the last line of defence rather than the solution.

• Microplastic: a plastic particle with a diameter of less than 5 mm.
• Primary microplastics are manufactured at that size - cosmetic microbeads, resin pellets, synthetic microfibres, blasting media. Secondary microplastics are formed by fragmentation of larger items already in the sea.
• Most plastics are non-biodegradable, so they persist rather than decompose.
• UV radiation embrittles and cracks the polymer; wave action abrades it; wind action transports it; warmer temperatures speed the process.
• Uptake by plankton: zooplankton ingest particles instead of food, so feeding, growth and reproduction fall.
• Transfer along the food chain: plankton -> small fish -> predatory fish -> top predators, including the seafood people eat.
• Absorption and release of toxins: POPs and metals concentrate on the particle and desorb in the warm, acidic gut of the consumer.
• Ingestion and entanglement: bags mistaken for jellyfish, gut blockage and starvation, and drowning or strangulation in ghost nets.
• Strategies: bans and charges, deposit return, better waste collection and recycling, wastewater filtration, MARPOL Annex V, port reception facilities, gear marking and net retrieval, clean-ups and education.

Global warming: the greenhouse effect, the evidence and the human contribution

The Earth receives energy from the Sun as short-wave radiation, mostly visible light, which passes through the atmosphere and is absorbed by the land and the ocean. The surface warms and re-emits that energy as long-wave infrared radiation, and certain gases in the atmosphere - carbon dioxide, water vapour, methane, nitrous oxide and the CFCs - absorb part of that outgoing infrared and re-emit it in all directions, including back down towards the surface. This is the natural greenhouse effect, and without it the average surface temperature would be about -18 °C instead of the roughly 15 °C we experience, so it is the reason the Earth has an ambient temperature suitable for life.

The enhanced greenhouse effect is the same process made stronger by extra greenhouse gases. Burning coal, oil and gas releases carbon dioxide that was locked underground for millions of years; clearing and burning forest removes a carbon sink and releases the carbon stored in the trees; rice paddies, livestock, landfill sites and leaking gas infrastructure add methane; fertiliser use and fuel burning add nitrous oxide. The extra gases absorb a larger fraction of the outgoing infrared, so less energy escapes to space and the Earth's energy balance shifts. The extra energy is stored, mostly in the ocean, until the surface and lower atmosphere warm, and feedbacks make it worse: warmer air holds more water vapour, which is itself a greenhouse gas, and melting ice exposes dark ocean that absorbs more sunlight than white ice did.

The evidence comes from several independent directions, and it is their agreement that makes the case strong. Direct thermometer records, corrected for station moves and urban bias, show a rise of about 1.1 °C in global mean surface temperature since the late nineteenth century, with the warmest years on record in the last decade. Carbon dioxide has been measured directly on Mauna Loa in Hawaii since 1958 and has risen from about 315 parts per million to more than 420, and air bubbles trapped in Antarctic ice show that it is now higher than at any time in the last 800 000 years. Sea level, measured by tide gauges and since the 1990s by satellite, is rising; the ocean has absorbed most of the extra heat, so its heat content is increasing; glaciers are retreating almost everywhere; Arctic sea ice is shrinking in extent and thickness; and spring events such as leafing, bird migration and plankton blooms are happening earlier.

The case that human activity is responsible rests on more than the correlation between rising carbon dioxide and rising temperature. The physics is measured in the laboratory: carbon dioxide absorbs infrared, and the absorption at current concentrations is known. The isotope signature of the extra carbon dioxide identifies its source, because carbon from fossil fuels carries less carbon-13 than atmospheric carbon, and the ratio in the air has fallen as the concentration has risen, while the oxygen content of the atmosphere has fallen slightly, which is what burning carbon uses up. The pattern of warming is also a fingerprint: the troposphere warms while the stratosphere cools, which is what greenhouse gases do and the opposite of what a brighter Sun would do, and the warming is greatest at night, in winter and at high latitudes. Climate models that include human emissions reproduce the observed record, and models driven by natural factors alone do not.

The case against, and the uncertainties, must be given too. The climate has always changed: ice ages, the Medieval Warm Period and the Little Ice Age happened with no human influence, so natural variation is real. Solar output varies with the sunspot cycle, large volcanic eruptions inject sulphate particles that reflect sunlight and cool the surface for a year or two, and internal oscillations such as El Nino and the Pacific Decadal Oscillation can produce a decade or more of slower warming. Land temperature records can be biased by urban heat islands, some proxy reconstructions of past temperature have been criticised for their statistics, and the largest uncertainties in the models are the feedbacks from clouds and water vapour, which is why projections of regional change differ so much.

The defensible conclusion is that the greenhouse effect itself is not in dispute, because it is measured physics, and that the evidence for a substantial human contribution is very strong, because the isotope, oxygen and stratospheric fingerprints cannot be explained by the Sun or by natural variability. What remains genuinely uncertain is the size of the feedbacks, and therefore how much warming a given emission will cause and how quickly the regional impacts will arrive.

Impacts of global warming on the marine environment

Sea level rise has two causes and you must keep them apart. Water expands as it warms, which is thermal expansion and accounts for a large part of the rise so far; and melt water from land-based ice - mountain glaciers, the Greenland ice sheet and West Antarctica - adds new water to the ocean. Melting sea ice does not raise sea level, because that ice is already floating. The consequences are coastal: beaches and salt marsh are eroded or squeezed against sea defences, low-lying deltas and islands are flooded more often, salt water intrudes into coastal aquifers and estuaries, mangroves and turtle nesting beaches are lost, and people are displaced from the coast.

Coral bleaching is the most visible impact. Reef-building corals live in symbiosis with single-celled algae called zooxanthellae, which live in their tissues, give them their colour and supply most of their energy through photosynthesis. When the water is too warm for too long - a sustained rise of about 1 °C above the normal summer maximum is enough - the coral expels the algae, the colony turns white, and because it has lost most of its energy supply it starves. If the heat passes quickly the coral can recover its algae, but if it persists the coral dies, and the reef loses the three-dimensional structure that fish and invertebrates depend on. Recovery takes a decade or more, so reefs that bleach repeatedly never fully recover.

Changes in the distribution of species follow directly from temperature. Every species has a range of temperature it can tolerate, and as the sea warms the suitable zone moves towards the poles and into deeper, cooler water. Mobile animals such as fish, squid and whales can follow it, so warm-water species appear in temperate seas and cold-water species retreat, and the composition of a fishery changes. Sessile organisms such as corals, kelp and barnacles cannot move, so they die at the warm edge of their range instead. The timing of events also shifts, so spawning, plankton blooms and the arrival of predators can fall out of step with each other, and species that depend on ice - krill, polar cod, ice seals - lose their habitat entirely.

Changes to the global circulation of sea water are the least visible and the most far-reaching. The thermohaline circulation is driven by differences in density, which depend on temperature and salinity: warm, salty surface water flows towards the North Atlantic, cools, becomes dense and sinks, and that sinking drives the deep flow that returns along the ocean floor. Warming and the extra fresh water from rainfall and melting ice both reduce the density of the surface water, so the sinking can weaken and the whole circulation can slow. The consequences would include regional cooling in north-west Europe, a changed supply of nutrients to the surface, shifts in rainfall, faster regional sea level rise along some coasts, and less oxygen reaching the deep ocean. Warming also strengthens the layering of the water column, which reduces the mixing that brings nutrients up from below and lowers the oxygen content of the water, because warm water holds less dissolved gas.

• Sea level rise: thermal expansion of warming water plus melt water from land-based ice; melting sea ice does not raise sea level.
• Consequences of sea level rise: coastal erosion, flooding, saline intrusion, loss of mangroves, salt marsh and turtle beaches, displacement of people.
• Coral bleaching: the coral expels its zooxanthellae when the water stays too warm, loses its colour and its energy supply, and dies if the heat persists.
• Distribution: the tolerable temperature zone moves polewards and deeper; mobile animals follow it, sessile organisms die at the warm edge, and the timing of life cycles falls out of step.
• Circulation: freshening and warming reduce surface density and can slow the thermohaline circulation, with effects on regional climate, nutrients and deep-ocean oxygen.
• Warmer water holds less dissolved oxygen and mixes less, so stratification and deoxygenation add to the stress on marine life.

Ocean acidification: chemistry, carbonate and shells

Carbon dioxide dissolves in sea water until the surface ocean is in equilibrium with the atmosphere, and because the atmosphere now contains more carbon dioxide, more of it dissolves. The ocean has absorbed roughly a quarter of the carbon dioxide released by human activity, which is why it limits the rise in atmospheric concentration and slows global warming. That service has a cost, because dissolved carbon dioxide is an acid-forming gas.

The chemistry happens in three linked steps. Carbon dioxide reacts with water to form carbonic acid: CO2 + H2O -> H2CO3. The carbonic acid dissociates to release a hydrogen ion and a hydrogen carbonate (bicarbonate) ion: H2CO3 -> H+ + HCO3-. The hydrogen ions then combine with carbonate ions already in the water to form more hydrogen carbonate: H+ + CO3 2- -> HCO3-. Adding carbon dioxide therefore pushes all three reactions to the right, so the concentration of hydrogen ions rises, the pH falls, and the concentration of carbonate ions falls at the same time.

Sea water is slightly alkaline at about pH 8.1 to 8.2, and surface pH has already fallen by about 0.1 units since the industrial revolution. Because pH is a logarithmic scale, that 0.1 unit is about a 30 per cent increase in hydrogen ion concentration, and the ocean's capacity to absorb further carbon dioxide without a large change in pH is decreasing as its buffering is used up.

The effect on hard corals and shelled organisms follows from the carbonate. To build a skeleton or a shell an organism precipitates calcium carbonate, which requires carbonate ions from the water; the reaction is Ca2+ + CO3 2- -> CaCO3. When carbonate becomes scarcer, calcification is slower and costs the organism more energy, so corals grow more slowly and their skeletons are less dense, and molluscs, sea urchins and coralline algae build thinner shells. In cold water and at depth, where carbonate is already scarce and pressure favours dissolution, the water can become undersaturated and existing shells start to dissolve. Slower-growing corals build weaker reefs that are more easily broken by storms and less able to keep pace with sea level rise, and because shelled plankton such as pteropods are food for fish, salmon and whales, the damage reaches the food web.

The core practical investigates this directly: the effect of pH on the loss of mass of empty mollusc shells. Clean and dry shells of the same species and similar size are weighed, then each is immersed in the same volume of water at a different pH - for example sea water bubbled with carbon dioxide, or dilute hydrochloric acid at pH 6, 5, 4 and 3, with distilled water as a control - and kept for the same time at the same temperature. The shells are then removed, rinsed, dried and reweighed, and the loss of mass or the percentage loss is calculated for each pH. Mass loss increases as pH falls, because the acid reacts with the calcium carbonate of the shell and releases carbon dioxide, which is why a shell fizzes in acid. The variables to control are the volume and temperature of the solution, the size and species of shell, and the time of immersion.

Conservation: biodiversity, the Red List and the strategies

Conservation is the management of human use of the environment so that it maintains or enhances biodiversity, which is the variety of life at three levels: the number and variety of species, the genetic variation within each species, and the range of habitats and ecosystems. High biodiversity matters because diverse communities are more stable and more resilient to disturbance, because marine ecosystems provide goods and services that people depend on - fisheries, coastal protection, nutrient cycling, tourism - and because species have value in their own right and may hold uses, such as medicines, that have not yet been discovered.

The IUCN Red List of Threatened Species is the global system for assessing how close a species is to extinction. Each species is assessed against standard, quantitative criteria - population size, rate of decline, area of distribution and how fragmented it is - and placed in one of the categories: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild or Extinct, with Data Deficient for species that cannot be assessed. Because the criteria are the same everywhere, the Red List provides an objective, comparable evidence base, and that is how it helps to prioritise: limited money and effort can be directed to the species at greatest risk and to the sites that hold concentrations of threatened species, instead of to whichever species is most attractive. It has limits, though - many marine invertebrates and deep-sea fish are Data Deficient, assessments go out of date, it ranks species rather than ecosystems, and a listing does not by itself protect anything.

Two definitions matter. An endangered species is one that the IUCN assesses as facing a very high risk of extinction in the wild. An invasive species is an alien (non-native) species that becomes established in natural or semi-natural ecosystems, is an agent of change, and threatens native biological diversity. Invasive species are a threat because in their new range they usually arrive without the predators, competitors, parasites and diseases that controlled them at home, they often grow and reproduce faster than the natives, and the natives have not evolved defences against them. They outcompete native species for food and space, prey on animals that do not recognise them as a danger, alter the habitat - a mat of Caulerpa smothering seagrass, or lionfish removing juvenile reef fish - and can carry new diseases. The result is local extinction, simplified food webs and economic loss to fisheries and tourism.

The strategies each work in a different way, and each has a limitation that decides whether it is viable in a particular case. Marine protected areas and no-take zones protect the habitat and allow fish to grow larger and breed, and the surplus spills over into the fished area, but they only work if the boundaries are enforced and the site is large enough for the species concerned. Captive breeding and release can rebuild a population that has fallen too low to recover by itself, but it is expensive, the animals may lose the skills they need in the wild, and it does nothing about the cause of the decline. Legislation works at both scales: CITES regulates international trade in listed species and their products, and the IWC moratorium on commercial whaling, agreed in 1986, stopped most commercial hunting, but neither controls habitat loss or domestic use.

UNESCO biosphere reserves combine a strictly protected core with a buffer zone and an outer transition zone where people live and work, so they link conservation with sustainable use rather than excluding people. Marine zoos and aquaria fund research, run breeding programmes and teach millions of visitors, but they are expensive, some collect from the wild and their conservation value is limited. Ecotourism such as whale watching and dive tourism gives local people a reason to keep the ecosystem intact, but it can disturb the animals it depends on if it is not regulated. Control of invasive species - ballast water management, hull cleaning, import bans, physical removal by divers and, cautiously, biological control - is far cheaper at the prevention stage than after establishment.

International cooperation is necessary because of the scale of marine ecosystems: many species are highly migratory or straddle several national waters and the high seas, and pollution, warming and acidification do not stop at a boundary. It is not always possible. Not every state signs or enforces an agreement - Norway and Iceland objected to the IWC moratorium and continued commercial whaling, and Japan withdrew from the Commission in 2019 and resumed commercial whaling - so a species can be fully protected in one ocean and hunted in another, and the effort is displaced rather than reduced.

CITES only covers international trade, so it cannot stop a species being taken or its habitat destroyed within one country. On the high seas, enforcement is expensive, jurisdiction is shared and often weak, and a developing country may lack the patrol boats, laboratories and trained staff to enforce what it has signed. The practical implication is that international agreements tend to be set at the standard the least willing signatory will accept.

• Biodiversity has three levels: species diversity, genetic diversity and habitat or ecosystem diversity.
• Conservation is needed to maintain or enhance biodiversity, for stability, for the goods and services ecosystems provide, and for ethical and future-use reasons.
• The IUCN Red List assesses extinction risk against standard criteria and sorts species into categories from Least Concern to Extinct, so that scarce resources can be prioritised.
• Endangered: facing a very high risk of extinction in the wild. Invasive: a non-native species that becomes established, changes the ecosystem and threatens native biodiversity.
• Invasive species threaten natives because they arrive without natural enemies, outcompete and prey on naive species, alter habitat and carry disease.
• Strategies: MPAs and no-take zones, captive breeding and release, CITES and the IWC moratorium, UNESCO biosphere reserves, marine zoos and aquaria, ecotourism, control of invasive species.
• International cooperation is necessary because species and pollutants cross boundaries, but it fails where states do not sign, do not enforce, or cannot afford to.