End Plastics
Sinking Plastics: The Hidden Threat to Our Oceans
June 11, 2025
When most people think of ocean plastic, they think of floating bottles, grocery bags, or maybe the Great Pacific Garbage Patch. But a large fraction of ocean plastic does not float on the surface but instead sinks to the ocean floor or is suspended in the ocean’s water column. This is because some plastics are denser than seawater, and even buoyant plastic can and will eventually sink.
Plastic pollution is an urgent global issue that impacts all living things, yet remains complex due to the scale and hidden nature of the problem. Scientists struggle to estimate how much plastic is in the ocean and continue to debate whether the ocean’s surface, water column, sea floor, or coastlines serve as the largest ocean plastic reservoir. Currently, there is not enough reliable data to know, and existing models are often inaccurate or carry significant uncertainty.
Yet despite the gaps in data, the magnitude and severity of the problem remain clear. In some of the world’s most ecologically significant marine regions, sinking plastic presents a growing concern. From the Mariana Trench to the Tyrrhenian Sea to the Coral Triangle, plastics are posing a serious threat to underwater ecosystems.
How and Why Plastics Sink
Plastics that are less dense than seawater, such as polyethylene (PE) and polypropylene (PP), tend to float, whereas polyethylene terephthalate (PET) and polyvinyl chloride (PVC), which are denser, will sink. That being said, buoyant plastics can still sink due to processes such as biofouling and plastic breakdown.
Biofouling is a process where microorganisms, plants, and animals attach themselves to underwater surfaces— in this case, plastics. This causes the overall density of the plastic debris to increase, leading to its sinking.
Plastics also break down into microplastics over time and may eventually sink. Microplastics, small plastic particles between 1 nanometer and 5 millimeters, may enter oceans directly, but are also produced from the gradual breakdown of larger bits of plastic. Degradation is driven by a combination of sunlight, wind, heat, and waves, with sunlight being the most crucial and leading to a process known as photodegradation.
These smaller plastic particles, created from the degradation of larger pieces, disrupt the bottom of the food chain, being consumed by plankton and thus creating a cascade of harm. Small, plastic-contaminated marine organisms are consumed by larger species like fish and shellfish and may eventually harm humans through contaminated seafood. Plastics, large and small, are everywhere in the ocean. Understanding how and why plastics sink helps explain their omin-presence, even in some of the ocean’s most extreme environments— one of which being the Mariana Trench.
The Mariana Trench
The Mariana Trench is the deepest part of the world’s ocean, located in the western Pacific Ocean and reaching almost 7 miles deep. In 2019, a diver set out to reach the deepest point in the ocean, Challenger Deep, and found multiple pieces of plastic debris down there. In fact, in a database of 5,010 submersible dives, 3,425 man-made debris items were detected. Over 33% of these items were macro-plastics, or larger pieces of sunken plastic, 89% of which being single-use products. But it’s not just these large plastic objects that were found— small plastic particles are there in abundance too.
The worrying thing is that these plastics are essentially permanent; with little sunlight and cold conditions, plastic degradation rates slow down further and biological forces weaken. It’s not just the deep ocean that’s contaminated with plastics. The Tyrrhenian Sea, located off the west coast of Italy in the Mediterranean Sea, has been identified as a microplastic hotspot– an area of the ocean with an especially high concentration of microplastic. With 1.9 million pieces of microplastic per square meter, the Tyrrhenian Sea has the highest concentration ever recorded from the deep seafloor.
A Plastic Ocean Hot Spot
In the deep sea, bottom currents carry oxygen and nutrients across the sea floor. However, new research suggests that these currents also transport microplastics, which, as a result, become more available to deep sea organisms and allow them to enter the food chain.
Microplastics also have the ability to absorb toxic chemicals from the surrounding water, and these pollutants are then passed up the food chain and will accumulate in living organisms over time. Research suggests that plastic ingestion by fish is rising by 2.4% annually and plastic debris has been detected in 386 fish species, including 210 species used commercially. This build up can impact animal behavior in fish and sea birds, such as navigation and their ability to detect chemical cues. It is estimated that each year, plastic pollution leads to the deaths of 100,000 marine animals, such as whales, turtles, and dolphins, and 1 million seabirds.
The Coral Triangle
The Coral Triangle, another biodiversity hotspot home, is also impacted by plastics: 75% of the world’s coral species and 2,200 fish species, are under increasing threat from plastic pollution. The Coral Triangle is a marine region in the Western Pacific Ocean and includes the seas of Indonesia, Malaysia, Papua New Guinea, the Philippines, the Solomon Islands and Timor-Leste. The region is considered the global center of marine biodiversity, but is heavily polluted with plastic.
Corals are vital to these marine ecosystems. Tens of thousands of species and one billion people rely on coral reefs for storm protection, livelihoods, food, and even medicine. In the Coral Triangle, over 50 million people are dependent on them, yet more than 90% face serious threats from human activity.
Reefs impacted by plastic are also significantly more likely to develop disease. While unaffected reefs show only a 4% disease rate, this rises to 89% when plastic is present. This occurs because plastics cause abrasions in the coral, allowing pathogens and bacteria to enter. Additionally, plastic blocks light and can create low oxygen environments that encourage the formation of harmful microbes. A reduction in coral reefs has significant knock-on effects for marine ecosystems that rely on them for habitat, food sources, and protection from predators.
Looking Forward
Plastic pollution is not just an issue impacting the ocean’s surface. The sinking of plastics into our coral reefs and the deepest parts of the ocean reveals an urgent environmental crisis.
This World Ocean Day, check out EARTHDAY.ORG’s End Plastics Initiative page to learn more about plastic pollution and how you can take action today. Then, sign the Global Plastics Treaty petition to add your voice to the demand for action to address the plastics problem.
Together, we can push for systemic change and protect our oceans.
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