eXXpedition Then and Now: Women Sailing for Plastic Pollution Science and Solutions

Microplastics are changing the oceans, with millions of metric tons of plastic shedding trillions of pieces of plastic into our seas. They are harder to see than seabirds with stomachs full of plastic or sea turtles entangled in abandoned fishing nets, but just as concerning. Microplastics have infiltrated the entire marine ecosystem, from the sea surface to the seafloor, and these plastic particles are harming wildlife and disrupting critical ecological processes that regulate ocean health—and, in turn, the health of our planet.

In the oceans, sunlight, wind, waves, and other forms of weathering rapidly break plastic items up into microplastics and even smaller nanoplastics. Roughly 170 trillion plastic particles, weighing 2.3 million metric tons, now circulate the ocean’s surface waters alone. Scientists estimate an additional 11 million metric tons of plastic has been deposited on the seafloor, shedding an unknown amount of microplastic that mixes with ocean waters and sediments. An unknown amount of plastic and plastic particles permeate the middle ocean layers.

Scientists are just beginning to uncover the full extent of the damage caused by plastic pollution in the oceans, but it is already known to be harmful. Microplastics have been found to cause serious health problems and their consumption is linked to mortality in corals, fish, marine mammals, seabirds, sea turtles, zooplankton, and other ocean creatures. More broadly, microplastics are interfering with the ocean’s biogeochemistry—the web of biological, chemical, and geological interactions that drive vital Earth cycles, sustain marine ecosystems, and regulate the climate. These disruptions carry serious implications not only for marine life, but also for humans, as all life on Earth depends upon a healthy ocean.

Microplastics Spread Through the Ocean’s Depths

Microplastics travel through the entire water column, forming a “light smog” or “soup” just below the surface. Some plastics like polypropylene—commonly used in takeout containers—are buoyant and tend to float closer to the surface, while denser plastics such as polyethylene terephthalate (PET or PETE), often the material for water bottles, are more likely to sink lower toward the seafloor. Weathering and colonization by algaes, bacteria, and other microorganisms can also influence whether plastic particles are more likely to float or sink. 

Plastic particles accumulate in some of the highest concentrations within ocean gyres—large rotating systems of water currents driven by Earth’s rotations, differences in salinity, temperature, tides, and winds. Gyres help drive the “ocean conveyor belt,” which feeds other currents that circulate ocean water around the planet, and are critical to the ocean’s regulation of nutrient flows, temperature and salinity. 

Plastics have also been found at unprecedented depths, including in the deepest part of the ocean, nearly seven miles down, in the Mariana Trench. Scientists have found that turbidity currents (underwater “avalanches” of sediments) are at least partly responsible for transporting large volumes of microplastics down underwater canyons into the deep sea. Along the way, these currents can also pick up and carry harmful chemical pollutants linked to plastics, like heavy metals, PCBs, and PFAS. While turbidity currents play an essential role in moving oxygen and nutrients to deep-sea ecosystems—some of the most biodiverse and least understood on the planet—they are also inadvertently delivering waves of plastic pollution. 

The deep sea, Earth’s largest habitat, supports biological and chemical processes critical for Earth’s self-regulating functions, including oceanic heat absorption, release of nutrients from sediments, and long-term carbon cycling and storage. But plastic items and particles accumulating in these remote regions pose a growing threat to the delicate balance of the ocean’s regulatory systems, especially in the deep sea.

Microplastics are Disrupting the World’s Largest Carbon Sink

The ocean absorbs approximately 25% of human-generated carbon dioxide (CO2) emissions, and so is effectively the Earth’s largest carbon storage sink. The process of carbon sequestration in the oceans depends heavily on phytoplankton—microscopic plant life that, like land plants, photosynthesize using sunlight to draw carbon from the air—and zooplankton, tiny animals that feed on phytoplankton and help transfer that carbon to the deep sea through their feces, which rain down to the seafloor. 

These creatures, naked to the human eye, do much to protect us from experiencing the full impacts of our own greenhouse gas emissions. But emerging research suggests that microplastics are disrupting this delicate balance. Microplastics and even smaller nanoplastics may impair phytoplankton’s ability to photosynthesize, while chronic exposure is altering zooplankton behavior, reproduction, and even survival. These disruptions ripple through the food web and jeopardize the ocean’s ability to regulate atmospheric carbon.

One type of phytoplankton, Prochlorococcus, is responsible for generating 20% of the oxygen we breathe through photosynthesis. One single drop of ocean water may contain up to 20,000 of them, making them some of the most plentiful creatures on Earth. Researchers have found that exposure to microplastics is impairing prochlorococcus’ photosynthetic efficiency, potentially undermining one of the ocean’s vital processes for removing CO2  from the atmosphere and sustaining our planet’s oxygen levels. 

Even more concerning are the implications for the biological carbon pump—the process by which carbon is sequestered in the deep ocean for hundreds to thousands of years. When zooplankton ingest microplastics, their feces are made more buoyant, and this prematurely releases carbon higher in the water column instead of transporting it to the seafloor, where it would naturally fall. This interference could short-circuit one of the planet’s most effective climate-regulating mechanisms. Compounding the issue, increased die-off of microplastic-exposed plankton leads to more decaying organic matter near the surface, which depletes oxygen in seawater and threatens the survival of marine life.

Plastics themselves also introduce additional greenhouse gases into the ocean as they break up, releasing climate-warming ethane and methane. And excessive amounts of CO2 cause acidification of ocean waters, which is deadly for many marine animals, particularly those with calcium carbonate bodies or shells such as corals and molluscs. Rising human greenhouse gas emissions, ocean warming, acidification, and plastic pollution are compounding issues that add further pressure to a system already stretched to its limits.

Plastic Pollutes From Sea to Shore

Plastics, plastic particles, and plastic chemicals easily escape or leach from landfills or dump sites, flow through rivers, or wash down drains to inevitably travel to the ocean. In this way, everyone is connected to the ocean, whether they live nearby or far from the coastline. Whether it’s microplastics intentionally added to cosmetics, plastic fibers shed from synthetic textiles, plastic pellets used in manufacturing, or larger plastic items breaking up at sea like single-use plastic packaging and ghost fishing nets, plastic particles don’t just float—they travel throughout the oceans, releasing a cocktail of toxic chemicals, including heavy metals and industrial pollutants, into marine ecosystems.

That serene sea breeze might not be so innocent either: Studies show that crashing ocean waves can emit PFAS into the air, transferring forever chemicals from sea back to land. While it is known that plastics contain any mix of over 16,000 chemicals, the effects of these chemicals on marine life are not yet fully understood, but also, existing evidence shows us, are certainly not benign. For example, one study found phthalates (a class of endocrine-disrupting chemicals (EDCs) commonly added to plastics) in the blood of dolphins were associated with altered thyroid hormone levels. Other EDCs like BPA are known to impair reproductive, metabolic, and developmental systems of marine species—raising risks of cancers, death, and infectious diseases.

More than 914 marine species are known to become entangled and/or ingest plastics—though this number likely underrepresents the full scope, as scientists estimate 91% of ocean species remain unclassified. Among those already affected: seabirds have been found to develop “plasticosis”—a disease marked by scarring of the stomach lining due to plastic ingestion—with pollutants even transferring from their bodies to their eggs; coral reefs, which support around 25% of marine biodiversity, have shown signs of microplastic-induced gut blockages and reduced food intake in coral polyps; and microplastics have been found in the bodies of over numerous and diverse marine animals, including crabs, whales, and zooplankton.

Toxic chemicals and plastic particles also bioaccumulate, or magnify, as they move up the food chain, concentrating in top predators like orcas—where they’ve been detected in fatty tissues and even breastmilk. These plastic particles and additive chemicals don’t just harm marine organisms—again, they also disrupt the delicate balance that sustains ocean life, climate regulation, and indeed the whole planet.

Ocean Balance in Flux

The ocean’s complex and dynamic equilibrium is maintained by interdependent processes like acidity (pH) buffering, oxygen regulation, nutrient (elements like nitrogen, phosphorus, and silicon) and carbon cycling, and self-regulating ecosystems—all processes that have evolved over time and are essential for a healthy ocean to support the diverse web of life that exists today. Plastics and the microplastics and nanoplastics they shed are now disrupting this balance, introducing toxic chemicals and greenhouse gases into seawater that interfere with nutrient cycling and contribute to oxygen loss, which is already worsening due to climate change.

Such disruptions occur throughout the entire water column. Microplastics harm the health of seafloor organisms and microbial communities that are vital to nutrient cycling, while also exacerbating harmful algal blooms. At the surface, microplastics can alter water currents and increase local water temperatures by influencing solar radiation absorption, further stressing marine ecosystems.

Plastic pollution amplifies the already serious effects climate change is having on the oceans. Melting glaciers and extreme weather events redistribute large volumes of water and plastic items and particles, while warmer waters accelerate plastic’s fragmentation. One estimate suggests that up to one trillion microplastic particles could be released into the Arctic Ocean within a decade of widespread, warming-fueled ice melt. On top of this, the ocean faces both the direct impacts of plastic pollution and its other forms of pollution including emissions from fossil fuel extraction, plastic production, shipping, landfilling, and incineration—further skewing the ocean imbalances and instability already set in motion.

How Microplastics Affect You and the Ocean

Around the world, many people rely on fish, seaweeds, and other seafood as key sources of nutrition. Unfortunately, as our oceans fill with plastic, so do the creatures living in them. And people who consume seafood may be ingesting elevated levels of microplastics and toxic chemicals that accumulate up the food chain. 

Industries such as fisheries and tourism, which are a part of today’s ocean economy, while often driving plastic pollution are also significantly affected by it as well. Plastic pollution and climate change are killing coral reefs, and this puts at risk the lives and livelihoods of millions of people who rely on coral reef ecosystems for food, income, and coastal protection. Finally, the ocean is our greatest ally against climate change, and ensuring a healthy ocean with ecological processes in balance is critical to a healthy planet.

Human health is deeply intertwined with the health of our oceans. Every second breath we take is made possible by the sea. Since it’s impossible to fully remove microplastics from the ocean, the critical step in restoring ocean health is to prevent more plastic pollution at its source, starting with significantly reducing plastic production. To learn more, view our recent webinar on how the United Nations Global Plastics Treaty can take this important step to protect the oceans.   

This World Ocean Month, let’s acknowledge the ocean’s vital role in maintaining Earth’s fragile balance—and act now to protect it, for the sake of our health and our future.