Zooplankton sink microplastics deep into ocean

Microscopic close-up of plankton, brown against orange background, with two fluorescent green beads visible within body.

Calculating how fast microplastics go through the gut passage of zooplankton means researchers can now estimate the sheer volume of polluting material these organisms transport down to ocean depths daily.

The new research reveals that tiny crustaceans called copepods may be transporting hundreds of microplastic particles per cubic metre of seawater down through the water column each and every day.

Widely considered the most numerous zooplankton in our ocean, copepods dominate communities in nearly every region, and from surface waters to deep sea. Their huge numbers mean even small actions by individual animals, like ingesting microplastics, can collectively drive substantial ecosystem-level changes.

Marine ecosystems and food web

With over 125 trillion microplastic particles accumulated in the ocean, learning how these pollutants move through marine ecosystems and food webs is vital for predicting long-term consequences for ocean health.

Already, zooplankton are emerging as a major biological pathway for transporting microplastics.

Importantly, this new study provides one of the clearest quantitative pictures to date of how microplastics are cycled by zooplankton in the ocean, contributing significantly to understanding of pollution impacts.

The findings are formally released in the research paper Real-time visualization reveals copepod mediated microplastic flux. Published in the Journal of Hazardous Materials, the study is authored by Dr Valentina Fagiano of the Oceanographic Centre of the Balearic Islands (COB-IEO-CSIC), with Dr Matthew Cole, Dr Rachel Coppock and Professor Penelope Lindeque, all from Plymouth Marine Laboratory (PML) in the UK.

Copepods in the English Channel

Zooplankton, and copepods in particular, are central to the marine food web. Not only do they consume microalgae, but they are then, in turn, eaten by fish, seabirds and marine mammals.

In recent years, copepods have also become recognised as vectors for microplastics, ingesting tiny particles suspended in seawater and potentially passing them on to predators, or exporting them to depth via their pellets and carcasses. However, there has been no precise way to gauge how much and how fast, until now.

For the purposes of this study, researchers collected examples of a common North Atlantic copepod, Calanus helgolandicus, through a fine-mesh plankton net. They collected the copepods 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 the exposed to three common types of microplastics:

fluorescent polystyrene beads;
polyamide (Nylon) fibres; and
polyamide (Nylon) fragments.

These various microplastics 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.

Tracked via real-time visualisation

Using real-time visualisation, 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; plus
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, fibres and fragments all moved through the gut at similar speeds, and feeding conditions did not significantly slow or accelerate plastic throughput.

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 copepods could be driving microplastic fluxes there in the order of about 271 particles per cubic metre of seawater per day.

Sinking down the water column

The study shows how zooplankton such as copepods effectively sink microplastics following ingestion.

It is all about buoyancy, explains senior marine ecologist and ecotoxicologist at PML, Dr Matthew Cole:

“Copepod faecal pellets are negatively buoyant — meaning they sink down the water column. So, when ingested by copepods and then repackaged into pellets, the microplastics drop down the column with them.”

The role of copepods changes the microplastics story, adds Dr Rachel Coppock, Marine Ecologist at PML:

“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.”

The implications of these findings could be considerable, suggests, Professor Penelope Lindeque of PML:

“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, behaviour 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 up the marine food chain.”

Hotspots and intervention points

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 behaviour into ocean plastic transport models;
Reduce uncertainty around where microplastics accumulate over time; and
Improve risk assessments for ecologically or economically important regions.

Ultimately, the framework and findings help identify hotspots of microplastic exposure, as well as potential intervention points — all of which could be useful not only to scientists, but policymakers, too.

Ecology, flux and collaboration

The study was a collaboration between visiting scientist Dr Valentina Fagiano, based at COB-IEO-CSIC in Spain, and PML’s Dr Matthew Cole, Rachel Coppock and Prof Penelope Lindeque.

The partnership combines PML’s expertise in zooplankton ecology and microplastic methods with Dr Fagiano’s emerging specialism in real-time visualisation and flux quantification.

Quantifying flux means scientists can start to link what happens inside a single animal to how plastics are redistributed across entire ecosystems, concludes lead study author Dr Fagiano, of COB-IEO-CSIC:

“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 faeces, which sink through the water column and accumulate in underlying sediment.”

“Having realistic numbers for ingestion and gut passage is vital for computer models. We can better predict where microplastics end up, which species are exposed, and how pollution interacts with pressures on marine ecosystems.”

A world leader in marine research, Plymouth Marine Laboratory (PML) is committed to delivery of impactful, cutting-edge environmental and social science in support of a healthy and sustainable ocean.

Under a banner of Science for Ocean Action, PML is focused on addressing the triple challenges of climate change, biodiversity loss and marine pollution. Its work harnesses the capabilities of marine observing systems, and involves use of AI, autonomy and advanced technologies for scientific research.

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