Plastics are durable by design. In the sea, that durability turns into persistence: once lost or discarded, items resist breakdown and accumulate across shorelines, water columns and seabeds. Evidence shows that annual inputs to aquatic environments remain in the millions of tonnes, while removal is slow. This article explains the mechanisms of plastic weathering and fragmentation (from sunlight to abrasion and biofouling), why degradation is incomplete, and how persistence shapes risk for species and human systems. It also outlines how transparent recovery—both in the sea and along coastal areas—can reduce long-lived stocks.

What persists

Most commodity polymers (e.g., polyethylene, polypropylene, PET) are chemically stable, hydrophobic, and poor substrates for microbial attack. Additives (stabilizers, pigments, plasticizers) can increase resistance to oxidation and UV stress. Longevity grows once items sink, are buried by sediments (layers of particles that settle on the seabed), or become coated by biological films.

Weathering drivers

Along shorelines, intense sunlight triggers photo-oxidation (UV breaks polymer chains); waves and sand scrape surfaces, removing the chalky outer layer and exposing fresh material; daily heating–cooling and wet–dry cycles drive micro-expansion, while dissolved oxygen fuels further oxidation. Working together, these stresses lower tensile strength, create micro-cracks at flaws and edges, and turn rigid items brittle so they fracture under modest force.

Fragmentation pathway

Because complete mineralization is slow, the prevailing outcome is size reduction: macroplastics break into microplastics and, eventually, nanoplastics (<1 μm). Fibres detach from textiles during washing and enter wastewater; synthetic rubber abrades from tyres and runs off into rivers before reaching the sea. Once particles are coated by biofouling (rapid growth of biological and inorganic films), UV exposure drops and oxygen becomes limited, slowing further degradation. On the seabed, low temperatures and low light reduce rates even more.

Compartment dynamics

• Shorelines: strongest sunlight and constant rubbing by sand and waves. Plastic quickly becomes brittle; the surface cracks and flakes off.
• Surface waters: sunlight keeps breaking down exposed areas. Floating pieces gather together with algae and tiny organisms.
• Water column: there is little light here. Particles stick to "marine snow" (tiny natural flakes of organic matter), which makes them heavier and changes how fast they sink.
• Seabed: plastics can blanket seabed life and scrape it. With little light and low temperatures, further breakdown is very slow; long-lasting items (e.g., bottles, ropes) can remain visible for years.

Stocks accumulate

Inputs to aquatic systems have been estimated at 19–23 million tonnes in 2016, with business-as-usual trajectories projecting tens of millions of tonnes per year by the 2030s–2040s. Because removal is slow and pathways are cumulative, long-lived stocks persist in rivers, estuaries and marine compartments, where they can be periodically remobilized by storms and currents.

Degradation timelines (indicative)

How fast an item degrades depends on what it is made of, how much sunlight and oxygen it receives, the temperature of the water, and whether it becomes buried by sand or mud. The list below are orders of magnitude drawn from primary sources and refer to typical conditions. In shaded waters or once buried, the same items persist for longer, while continuous sunlight and abrasion can make them break up faster.

• Cigarette filter (cellulose acetate): around 5 years on exposed shorelines. Filters are compact and fibrous; under sunlight and abrasion the fibres clump, yellow, and slowly fragment rather than dissolving.
• Plastic bag (light-weight film): about 20 years on shorelines. Thin films photo-oxidize and tear into smaller pieces quickly, but those pieces remain in the environment as microplastics.
• Plastic cup: roughly 50 years with shoreline exposure. Rigid cups embrittle, crack along stress lines, and shed flakes that waves and wind can transport.
• Fishing line (nylon): up to 600 years in the sea or on the seabed. The polymer is strong and often UV-stabilized; once it sinks or is buried, low light and oxygen slow further change. These ranges are not “expiry dates”. They indicate when items lose structural integrity and fragment. The material does not disappear; it persists as smaller pieces that continue to interact with organisms and habitats.

Persistence shifts the priority from “disappear” to “document and remove”. Two complementary levers make sense at the waterline (where waste meets water) and in marine environments:

• Intercept earlier along coastal areas: capture ocean-bound waste before it reaches marine environments, when materials are cleaner and easier to sort and recycle.
• Recover what persists in the sea: support Fishing for Litter activities with local fishers, retrieve ghost gear and benthic debris, and ensure responsible end-of-life.

Ogyre is the first global platform based on the Fishing for Litter model, operating with local fishers in Italy, Brazil, Indonesia, and Senegal. Activities cover both offshore collection in the sea and interception along coastal areas to stop ocean-bound waste before it reaches marine environments. All recovered materials are delivered to certified cooperatives for sorting, recycling, or responsible disposal—aiming for the most sustainable outcome. Each batch is tracked through a blockchain-based registry, ensuring transparency, traceability, and data integrity across collection, allocation and credit retirement. This system prevents double counting and links recovery to verified end-of-life. The approach turns persistent stocks into shared value for communities and companies.

Persistence means plastics will not “go away” on human timescales once they enter aquatic systems. The practical response is to:

• Reduce inputs upstream (design for reuse; eliminate low-value items that are not collected at scale).
• Both intercept earlier along coastal areas and remove persistent stocks at sea through documented programmes with fishers.
• Route recovered materials to the best available end-of-life option locally, prioritizing recycling where feasible, energy recovery or, if unavoidable, engineered landfilling for contaminated marine debris.
• Maintain full traceability, external verification and social safeguards across the chain.

• FAO (2021), Seabed Sources of Marine Litter link
• OECD (2022), Global Plastics Outlook link
• Ogyre (2025), Ogyre Code of Conduct link
• Ogyre (2025), Ogyre Protocol link
• United Nations Environment Programme – UNEP (2021), From Pollution to Solution: A Global Assessment of Marine Litter and Plastic Pollution link
• United Nations Environment Programme – UNEP (2024), Global Waste Management Outlook 2024 link
• United Nations Environment Programme – UNEP (2016), Marine Plastic Debris and Microplastics: Global Lessons and Research to Inspire Action and Guide Policy Change link
• WWF (2022), Impacts of Plastic Pollution in the Oceans on Marine Species, Biodiversity and Ecosystems link
• WWF (2018), Mediterraneo in trappola. Come salvare il mare dalla plastica link

👉 How big is global plastic waste today? link

👉 How does mismanagement fuel marine pollution? link

👉 How does plastic enter the Ocean? link

👉 What is marine litter made of? link

👉 How to classify micro and macroplastics? link

👉 Floating vs. seabed plastic: which is worse? link

👉 Why is ghost gear choking the Ocean? link