Biological pumps: How zooplankton are transporting microplastics to the ocean depths
Renaud de RICHTER
Biological pumps: How zooplankton are transporting microplastics to the ocean depths
A new study has, for the first time, recorded and measured just how fast microplastics move through the gut passage of a key zooplankton species in real time—and used those measurements to estimate how much plastic these tiny animals might be transporting—and sinking down—through the ocean each day.
Zooplankton are already emerging as a major biological pathway for microplastics to transport through marine ecosystems. With over 125 trillion microplastic particles estimated to have accumulated in the ocean, understanding how these pollutants are moving through marine ecosystems and food webs is vital for predicting long-term consequences for ocean health.
Copepods are widely considered to be the most numerous zooplankton in our ocean, dominating zooplankton communities in nearly every ocean region, from surface waters to the deep sea. Their staggering numbers mean that even small actions by individual animals—like ingesting microplastics—can collectively drive substantial ecosystem-level changes.
Copepod microplastic transport
New research, authored by Dr. Valentina Fagiano (Oceanographic Center of the Balearic Islands, COB-IEO-CSIC) and PML's Dr. Matthew Cole, Dr. Rachel Coppock and Professor Penelope Lindeque, reveals that copepods may be transporting hundreds of microplastic particles per cubic meter of seawater down through the water column each and every day.
The paper, "Real-time visualization reveals copepod mediated microplastic flux," published in Journal of Hazardous Materials, provides one of the clearest quantitative pictures to date of how microplastics are cycled by zooplankton in the ocean.
Zooplankton, and copepods in particular, are central to the marine food web. They eat microalgae and are, in turn, eaten by fish, seabirds and marine mammals. They also drive the 'biological pump', packaging carbon into fecal pellets that sink into deeper waters.
In recent years, copepods have also been recognized as vectors for microplastics—ingesting tiny plastic particles suspended in seawater and potentially passing them on to predators, or exporting them to depth via their pellets and carcasses. But until now, there has been no precise way to gauge how much plastic an individual copepod processes and how fast.
Methods and experimental design
Through the study, researchers collected the copepods Calanus helgolandicus (a common North Atlantic copepod) through a fine-mesh plankton net, at the L4 Station of Western Channel Observatory—about six nautical miles south of Plymouth—aboard PML's Research Vessel Quest.
In the lab, the copepods were exposed to three common types of microplastics:
- fluorescent polystyrene beads
- polyamide (Nylon) fibers
- polyamide (Nylon) fragments
These were offered under different food conditions, allowing the scientists to test whether plastic shape or food availability changed how quickly particles moved through the gut.
Using real-time visualization, the researchers tracked individual microplastic particles as they were ingested and later expelled. This allowed them to measure two key metrics with high precision:
- Gut passage time—how long a microplastic particle stays inside the copepod
- Ingestion interval—how often a new plastic particle is consumed
Across all experiments, gut passage times clustered around a median of roughly 40 minutes, and were consistent across plastic shapes and food concentrations. In other words: beads, fibers and fragments all moved through the gut at similar speeds, and feeding conditions did not significantly slow or accelerate plastic throughput.
Findings and ecological implications
By combining these measurements with realistic estimates of copepod abundance in the western English Channel—one of the most highly studied bodies of water in the world—the team calculated that copepods could be driving microplastic fluxes on the order of about 271 particles per cubic meter of seawater per day, in that region.
Senior marine ecologist and ecotoxicologist at PML, Dr. Matthew Cole, explains how copepods sink microplastics after ingestion: "Copepod fecal pellets are negatively buoyant—meaning they sink down the water column—and so, when microplastics have been ingested by copepods, and then repackaged into the fecal pellets, the microplastics should drop down the water column with them."
Dr. Rachel Coppock, Marine Ecologist at PML, added, "Microplastic pollution is often framed as a surface ocean problem, but our study shows that zooplankton are constantly moving plastics through the water column, and into the food web. Copepods don't just encounter microplastics—they process and transport them, day in, day out."
Professor Penelope Lindeque outlines the wider implications: "Copepods are a primary food source for many fish larvae and small pelagic fish, and, if copepods routinely contain microplastics, then their predators will be chronically exposed to ingested plastics. This could influence energy budgets, behavior or health in subtle ways over time, especially when combined with other stressors. While our study focuses on flux rather than toxicity, it underscores how microplastic exposure is embedded into the foundations of the marine food web.
"I would liken this process to both a microplastic plumbing system, and a microplastic food delivery service. Zooplankton are both sinking microplastics down the water column, and passing them higher up in the marine food chain."
Modeling and future research directions
Up to now, many large-scale computer models of microplastic transport have lacked species-specific, process-based parameters for zooplankton ingestion and egestion. The quantitative framework developed here—based on gut passage times, ingestion intervals and realistic abundances—offers a way to:
- Integrate zooplankton behavior into ocean plastic transport models
- Reduce uncertainty around where microplastics accumulate over time
- Improve risk assessments for ecologically or economically important regions
Ultimately, that helps scientists and policymakers identify hotspots of microplastic exposure and potential intervention points.
Lead author Dr. Fagiano said, "By quantifying this flux, we can start to link what happens inside a single animal to how plastics are redistributed across entire ecosystems.
"Our research has shown that zooplankton readily ingest microplastics 24/7. Copepods don't just encounter microplastics—they are like mini biological pumps,—processing and repackaging the microplastics into their feces, which sink through the water column and accumulate in underlying sediment.
"Having realistic numbers for ingestion and gut passage is vital for computer models. It means we can better predict where microplastics end up, which species are most exposed, and how this pollution interacts with other pressures on marine ecosystems."
More information:Valentina Fagiano et al, Real-time visualization reveals copepod mediated microplastic flux, Journal of Hazardous Materials (2025). DOI: 10.1016/j.jhazmat.2025.140551
Greg Rau
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Institute of Marine Sciences
Univer. California, Santa Cruz
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Kevin Wolf
Evidence that zooplankton export microplastics to the deep ocean reinforces the long-standing understanding that their fecal pellets also export carbon. If models can accurately predict how much plastic reaches the seafloor through zooplankton ingestion and excretion, then those same models should be capable of estimating how much carbon is sequestered via the zooplankton component of the biological pump.
The Ocean Abundance Restoration Alliance advocates for experiments in which mineral dust is added to upwelling eddies once silica and iron concentrations decline to levels that cause diatom mortality. The addition of dust is expected to promote flocculation, aggregating dead and living diatoms together with particulate organic carbon (POC) into larger conglomerates. These aggregates sink far more rapidly than dead diatoms alone, as demonstrated in the work of Dr. Mukul Sharma.
Such experiments would quantify both dust-induced marine snow formation and resulting zooplankton biomass. Zooplankton populations are expected to increase because larger, energy-dense food particles become available. It is hypothesized that while marine snow is continuously sinking, zooplankton may no longer migrate nightly to the photic zone to graze on diatoms which can be harder to find that the bigger flocculated chunks of food. Reduced vertical migration would lower metabolic expenditure, potentially resulting in larger, healthier and more abundant krill and copepods.
One limitation of ocean iron fertilization (OIF) is that it does not explicitly aim to increase zooplankton biomass or strengthen higher trophic levels. We hope that studies such as the ones being proposed by Dr. Sharma and this one examining plastics and zooplankton will expand understanding of how supplementing dust and missing micronutrients can simultaneously support marine food webs and enhance carbon sequestration.
Rather than adding iron solely to sink diatoms, why not also ensure that this primary production feeds zooplankton and amplifies the biological pump throughout the ecosystem?
Kevin Wolf
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Greg Rau
Kevin Wolf
Greg Rau
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