Marine pollution is the introduction of substances, materials, organisms or forms of energy into the marine environment at levels that cause or are likely to cause harmful effects on living organisms, ecosystems, human health, resources or legitimate uses of the sea. It includes pollution of oceans, coastal waters, estuaries and other connected marine environments.
Pollutants can be chemical, physical or biological and can originate from land based activities, ships, offshore industries, atmospheric deposition, rivers and natural processes intensified by human activity. Common examples include oil, sewage, nutrients, heavy metals, persistent organic pollutants, plastics, agricultural chemicals, industrial waste and radioactive substances. Marine pollution is different from the mere presence of human material in the ocean because environmental pollution generally involves contamination at concentrations, quantities or locations capable of causing harm.
Some substances can be toxic at very low concentrations, whereas others cause problems primarily when present in large quantities or when they alter physical conditions such as oxygen availability. Pollution can be local, regional or global and can remain near its source or be transported long distances by currents, winds and biological processes.
Marine pollution can enter the sea through both point sources and diffuse sources. Point sources include identifiable discharge locations such as sewage outfalls, industrial pipes, wastewater treatment facilities and some offshore installations. Diffuse or non point sources are more widely distributed and include agricultural runoff, urban stormwater, atmospheric deposition and erosion from contaminated land. Rivers are major pathways through which nutrients, sediment, plastics, metals and other contaminants reach coastal waters.
Rainfall can wash pollutants from roads, roofs, industrial areas and agricultural land into drainage systems and waterways. Atmospheric transport can carry pollutants over long distances before they are deposited directly onto the ocean or onto land from which they later reach rivers. Shipping contributes oil residues, sewage, waste, underwater noise, invasive species transported in ballast water and emissions that influence atmospheric chemistry. Offshore oil and gas activities, mining, aquaculture and fishing can also introduce pollutants or physical disturbances. The diversity of pathways makes marine pollution a complex problem requiring management across entire watersheds and coastal regions rather than only at the shoreline.
Nutrient pollution is one important category of marine pollution. Nitrogen and phosphorus are essential nutrients for aquatic life, but excessive inputs from fertilisers, sewage and animal waste can stimulate excessive growth of algae and other primary producers. When this organic material dies and decomposes, microorganisms consume dissolved oxygen. If oxygen consumption exceeds oxygen replenishment, the water can become hypoxic or anoxic, creating conditions known as dead zones. Nutrient pollution can therefore transform an ecosystem without requiring a conventional poison.
Other pollutants act through direct toxicity. Heavy metals such as mercury, lead and cadmium can interfere with biological processes, while some organic chemicals can damage organs, disrupt endocrine systems or affect reproduction. Persistent pollutants can remain in the environment for long periods and may accumulate in organisms. Some contaminants undergo biomagnification, in which concentrations increase at higher trophic levels because predators consume many contaminated organisms. The effects therefore extend from microscopic organisms to fish, marine mammals, birds and humans.
Plastic pollution is a major form of marine pollution because plastic is durable, widely used and capable of fragmenting into progressively smaller pieces. Large plastic objects can entangle marine animals, obstruct digestion when ingested, damage coral reefs and interfere with fishing and navigation. Smaller fragments include microplastics, generally defined as plastic particles smaller than five millimetres, although definitions and classifications vary. Nanoplastics are still smaller particles generally discussed at the nanoscale.
Plastics can also carry chemical additives and provide surfaces on which microorganisms and other organisms colonise. Floating plastic can be transported by currents and wind, while denser or altered particles can sink into sediments or the deep ocean. Plastic pollution therefore occurs throughout the water column and seabed rather than only at the visible surface. Removing plastic after it has dispersed is technically difficult and expensive, making prevention, improved waste management, product design and reduction of losses important parts of marine pollution control.
Marine pollution can also involve physical forms of environmental disturbance. Sediment released by deforestation, construction, mining and land erosion can increase turbidity, reduce light penetration and smother habitats such as coral reefs and seagrass beds. Thermal pollution occurs when heated water is discharged into rivers or coastal environments, potentially altering temperature sensitive ecosystems and oxygen solubility.
Underwater noise from shipping, construction, seismic surveys and sonar can interfere with communication, navigation and behaviour in marine mammals and other animals that rely on sound. Light pollution from coastal development and vessels can alter behaviour and biological rhythms of organisms. Introduced non native species can also be associated with human activities such as ballast water discharge and aquaculture, although biological invasion is often treated as a separate environmental pressure rather than simply a conventional pollutant. These examples demonstrate that marine environmental degradation includes more than visible rubbish or chemical contamination.
The consequences of marine pollution depend strongly on the pollutant, concentration, duration of exposure, environmental conditions and sensitivity of affected organisms. Acute pollution occurs when a large exposure causes rapid effects, whereas chronic pollution involves lower level exposure over extended periods. Some pollutants degrade quickly, while others persist for decades or longer. Temperature, salinity, acidity, sunlight, oxygen concentration and sediment characteristics can influence pollutant behaviour.
Oil, for example, can spread across the water surface, evaporate partly, dissolve in small amounts, form emulsions, attach to sediments and undergo microbial degradation. Heavy metals may bind to sediments and later become biologically available under changing chemical conditions. Organic contaminants can be transported through food webs. The same pollutant can therefore behave very differently in different environments. Ecological effects may include mortality, reduced growth, impaired reproduction, behavioural changes, altered community composition and loss of ecosystem functions. Human consequences can include contaminated seafood, reduced fisheries, damaged tourism, increased treatment costs and loss of coastal livelihoods.
Marine pollution has a long history, but its scale and composition changed dramatically with industrialisation, urbanisation, intensive agriculture, modern shipping and mass production of synthetic chemicals and plastics. Earlier societies discharged waste into water, but population growth and industrial production increased the quantity and diversity of contaminants entering aquatic systems. Major pollution events, scientific research and international environmental agreements increased public and governmental recognition of the issue during the twentieth century.
Modern marine pollution management includes wastewater treatment, chemical regulation, waste reduction, port controls, shipping standards, monitoring programmes, protected areas and international agreements. The United Nations Convention on the Law of the Sea establishes broad obligations concerning protection and preservation of the marine environment, while other agreements address particular sources such as oil pollution, dumping and pollution from ships. Effective management increasingly uses a source to sea approach because pollutants can travel through watersheds from inland activities to coastal and marine environments.
Marine pollution is therefore best understood as a broad category of human induced contamination and environmental disturbance affecting marine systems. It includes chemical contamination, nutrient enrichment, plastics, oil, sewage, heavy metals, sediment, physical debris, pathogens and several forms of energy or biological disruption. The most important feature is the relationship between the contaminant or disturbance and its environmental effects, rather than whether the material looks like conventional waste.
Marine pollution can damage organisms directly, alter food webs, reduce oxygen, change habitats, contaminate seafood and impose economic costs. Its causes are distributed across cities, farms, industries, rivers, coastlines, ships and atmospheric processes, which means that successful control requires prevention at sources as well as treatment and cleanup at sea.
