Plastic that enters the ocean doesn’t disappear. It breaks into smaller and smaller pieces, sinks, floats, leaches chemicals, absorbs pollutants, gets eaten by marine life, and moves through the food chain. An estimated 19 to 23 million tonnes of plastic waste leak into aquatic ecosystems every year, and virtually none of it fully decomposes.
How Plastic Breaks Down in Seawater
Plastic in the ocean doesn’t biodegrade the way food scraps or paper do. Instead, it fragments through a combination of sunlight, wave action, and chemical reactions. UV radiation is the primary driver. Though UV light makes up only about 5% of solar energy, it triggers oxidation reactions on the plastic surface, creating new chemical groups (hydroxyl, carbonyl, and carboxyl) that weaken the material’s structure. This process, called photooxidation, makes plastic brittle and prone to cracking.
Once weakened, physical forces take over. Waves, wind, sand, and collisions with rocks grind the brittle material into progressively smaller fragments. A plastic bottle doesn’t dissolve; it splinters into thousands, then millions, of tiny pieces. These fragments are classified by size: pieces smaller than 5 millimeters are called microplastics, and pieces smaller than 1 micrometer (roughly one-hundredth the width of a human hair) are nanoplastics. The complete mineralization of conventional plastic, meaning its full chemical breakdown into natural molecules like carbon dioxide and water, has never been confirmed in the ocean. One researcher estimated in 2005 that every piece of conventional plastic ever manufactured still exists on Earth in some form, unless it was incinerated.
Where It Ends Up
Most people picture plastic pollution as floating debris, but the ocean surface holds only a small fraction. A 2024 study using deep-sea remotely operated vehicles estimated that 3 to 11 million metric tonnes of plastic sat on the ocean floor as of 2020. That’s one to two orders of magnitude more than what floats on the surface, and roughly equal to the amount entering the ocean each year.
Plastic distributes itself throughout the entire water column. Lighter plastics like polyethylene (used in bags and packaging) tend to float initially, where currents concentrate them in subtropical gyres, the large circular current systems that form accumulation zones like the well-known Great Pacific Garbage Patch. Denser plastics like PVC sink relatively quickly. But even buoyant plastics eventually sink as they become waterlogged, colonized by organisms, or weighed down by attached sediment. The deep ocean floor, not the surface, is the largest reservoir of marine plastic pollution.
Chemicals Going In and Coming Out
Plastic in seawater is chemically active in two directions: it releases its own additives, and it absorbs pollutants from the surrounding water.
Plastics are manufactured with dozens of chemical additives, including phthalates (which add flexibility), bisphenols like BPA (used in hard plastics), and flame retardants. When plastic sits in seawater, these additives gradually leach out. Experiments published in Nature Communications measured the release of 25 different organic additives from polyethylene and PVC samples submerged in seawater. PVC released substantially more, with cumulative totals ranging from about 4 to 88 micrograms per gram of plastic depending on conditions. This makes weathered plastic debris a continuous, low-level source of chemical contamination in the marine environment.
At the same time, microplastics act like tiny sponges. Their large surface area relative to their volume, combined with changes in surface chemistry from weathering, gives them a high capacity to adsorb pollutants already present in seawater. The chemicals most frequently detected stuck to ocean microplastics include polychlorinated biphenyls (PCBs, industrial chemicals banned decades ago but still persistent), polycyclic aromatic hydrocarbons (PAHs, from fossil fuel combustion), and organochlorine pesticides. Even “forever chemicals” (PFAS) bind to plastic surfaces. This means a microplastic particle can carry a concentrated cocktail of toxins far more potent than the surrounding water.
Life on Plastic: The Plastisphere
Within hours of entering the ocean, plastic surfaces begin to accumulate a living community of bacteria, algae, fungi, and other microorganisms. Scientists have named this ecosystem the “Plastisphere.” Genetic surveys of these communities have identified a diverse mix: photosynthetic organisms, organisms that feed on organic matter, predators that eat other microbes, and even potential pathogens. Members of the bacterial genus Vibrio, which includes species that cause cholera and serious wound infections, have been found dominating individual plastic samples.
Some Plastisphere bacteria appear capable of breaking down hydrocarbons. Researchers have observed pits in plastic surfaces that match bacterial cell shapes, suggesting the microbes are slowly digesting the polymer. This biological degradation is real but extraordinarily slow compared to the rate at which new plastic enters the ocean. More consequentially, the Plastisphere changes how plastic behaves. Biological growth adds weight, potentially causing buoyant plastic to sink. It also creates a living coating that marine animals may mistake for food or that could transport invasive species across ocean basins on drifting debris.
What Happens When Animals Eat It
Over 690 marine species have been documented interacting with plastic debris, and ingestion is one of the most direct threats. The consequences depend on the size and shape of the plastic and the animal that swallows it.
Sea turtles commonly eat plastic bags and thin sheeting, likely mistaking them for jellyfish. This material can lodge in the digestive tract, creating a false sense of fullness. The turtle stops eating, loses nutrition, and can eventually starve. Sharp or rough plastic fragments create cuts inside the digestive system, leading to internal bleeding and infection. For seabirds, the effects are similar. Laysan albatross chicks that ingested large amounts of plastic weighed less than chicks that hadn’t, because plastic filling their stomachs left less room for actual food.
Beyond physical harm, ingested plastic delivers its chemical payload directly into an animal’s body. The pollutants adsorbed onto plastic surfaces can transfer to gut tissues, and the additives within the plastic itself continue leaching in the warm, acidic conditions of a digestive system.
How Plastic Moves Through the Food Chain
Microplastics don’t stay in one animal. Small particles ingested by zooplankton, filter-feeding shellfish, or small fish transfer to whatever eats them next, a process called trophic transfer. Plastic fragments have been found in the digestive tracts of organisms at every level of the marine food web, from tiny planktonic creatures to large predatory fish and marine mammals.
This means that contamination concentrates as it moves up the food chain. A mussel filters thousands of liters of water and accumulates microplastics. A fish eats hundreds of mussels. A seal eats hundreds of fish. At each step, the plastic particles and their associated chemical contaminants can accumulate in tissue. Humans sit at the top of many of these food chains, consuming seafood that has been filtering, eating, and concentrating microplastics throughout its life.
Why “Biodegradable” Plastic Isn’t a Fix
Plastics marketed as biodegradable don’t reliably break down in ocean conditions. In one widely cited experiment, bags labeled biodegradable could still hold a full load of groceries after three years submerged in seawater. Compostable plastic performed better in the marine environment, disappearing within three months, but remained intact in soil for years. The distinction matters: “biodegradable” typically means the material breaks down under specific industrial composting conditions (high heat, controlled moisture) that the ocean doesn’t provide. In cold, dark deep-sea conditions, degradation slows even further. Plastic that sinks to the seafloor enters a kind of cold storage, persisting for timescales no one has yet been able to measure.