Each day, hundreds of microplastic particles could be sent to down towards the seabed in each cubic metre of seawater thanks to the poop of tiny animals called copepods.
There are billions of these teeny planktonic creatures living in our seas, from the surface all the way down to the seafloor. There are so many of them that even basic functions can significantly impact the marine environment.
When copepods eat tiny pieces of ocean plastic, they can transport them up the food chain (when they are eaten) or send them down to the seafloor (through excrement).
“Copepod faecal pellets are negatively buoyant – meaning they sink down the water column,” Dr Matthew Cole, senior marine ecologist and ecotoxicologist at Plymouth Marine Laboratory (PML), says in a statement.
"When microplastics have been ingested by copepods, and then repackaged into the faecal pellets, the microplastics should drop down the water column with them.”
Now, researchers have measured how frequently copepods can consume microplastics and how long particles take to pass through their gut.
Based on these experiments, alongside estimates of how many copepods there are in the western English Channel, the scientists suggest copepod poop could be transporting 271 microplastic particles from the surface to the seafloor per cubic metre of seawater every day. The findings are published in Journal of Hazardous Materials.
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The researchers collected copepods off the coast of Plymouth in England to conduct lab experiments to see how long it took common types of microplastics to pass through their system. Fluorescent polystyrene beads, Nylon fibres and Nylon fragments all took around 40 minutes to move through the copepods’ guts.
“Zooplankton are constantly moving plastics through the water column, and into the food web,” says Dr Rachel Coppock, marine ecologist at PML. “Copepods don’t just encounter microplastics – they process and transport them, day in, day out.”
Ingesting microplastics isn’t just a problem for copepods because these little creatures are such an important part of the food chain. “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,” says Professor Penelope Lindeque, head of science for marine ecology and biodiversity at PML.
Understanding how plankton transport microplastics through the ocean is important for experts who are trying to identify microplastic hotspots and understand what this might mean for the ocean in the long-term.
“Our research has shown that zooplankton readily ingest microplastics 24/7,” says lead author Dr Valentina Fagiano, a researchers at the Oceanographic Centre of the Balearic Islands, COB-IEO-CSIC. “Having realistic numbers for ingestion and gut passage… 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.”
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