Zooplankton Act as ‘Biological Pumps’ Moving Microplastics to the Deep Ocean, Study Finds
Tiny ocean animals known as zooplankton are playing a far bigger role in transporting microplastics through the sea than previously understood, according to new research published in the Journal of Hazardous Materials.
The study, led by Dr Valentina Fagiano of the Oceanographic Centre of the Balearic Islands and scientists from Plymouth Marine Laboratory (PML), is the first to measure in real time how quickly microplastics pass through the gut of copepods – the most abundant zooplankton in the world’s oceans.
With more than 125 trillion microplastic particles estimated to be circulating in the marine environment, understanding how these pollutants move through ocean ecosystems is increasingly urgent.
Copepods are microscopic crustaceans found from surface waters to the deep sea. Although individual animals are tiny, their sheer numbers mean their collective impact is vast. The research shows that copepods ingest microplastics continuously and then expel them in faecal pellets that sink through the water column.
Using advanced real-time imaging, scientists tracked fluorescent plastic beads, nylon fibres and plastic fragments as they were eaten and excreted by the copepod Calanus helgolandicus, collected from the Western English Channel.
Across all experiments, microplastics spent a median of around 40 minutes in the animals’ digestive systems, regardless of plastic type or food availability.
By combining these measurements with realistic estimates of copepod abundance, the researchers calculated that copepods could transport around 270 microplastic particles per cubic metre of seawater each day to deeper waters in the region.
Dr Matthew Cole, senior marine ecologist at PML, explained that copepods act as an unexpected transport system. “Copepod faecal pellets are negatively buoyant, meaning they sink. When microplastics are repackaged into these pellets, they are carried down through the water column towards the seabed,” he said.
This process links surface pollution directly to deeper ocean layers and sediments, challenging the idea that microplastics are mainly a surface-ocean problem.
Dr Rachel Coppock, marine ecologist at PML, added that zooplankton are constantly redistributing plastics throughout marine ecosystems. “They don’t just encounter microplastics – they process and transport them every day, embedding plastic pollution into the marine food web,” she said.
Copepods are a primary food source for fish larvae and small pelagic fish, raising concerns about long-term exposure to microplastics higher up the food chain.
Professor Penelope Lindeque of PML warned that chronic ingestion could have subtle but significant effects. “If copepods routinely contain microplastics, their predators will also be exposed. This could influence behaviour, energy use or health over time, particularly when combined with other environmental stressors,” she said.
Until now, large-scale models of ocean plastic movement have lacked precise biological data. The new framework provides key parameters that can be used to improve predictions of where microplastics accumulate and which ecosystems are most at risk.
Lead author Dr Fagiano said the findings help connect individual animal behaviour to global-scale processes. “Zooplankton act like mini biological pumps, redistributing plastics throughout the ocean. Having realistic data allows us to better predict where microplastics end up and how they interact with marine life,” she said.
The research highlights how deeply microplastic pollution is woven into ocean systems, reinforcing the need for urgent action to reduce plastic entering the marine environment.
