Where do all the microplastics go?

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Scientists call for more targeted research into interactions between marine life and microplastics.

A recently published review, conducted by a collaborative team from PML and the University of Exeter, identifies the need for urgent research into the fate of microplastics in the marine environment and the impact of these tiny particles on marine life, as well as human health. The study also highlights the ways in which different organisms may control the distribution of microplastics and observes that interactions between marine organisms and plastics are most likely to occur in biologically productive coastal waters close to densely populated coastal towns and cities.

Marine plastic debris is considered a major environmental and economic concern due to the large quantities entering the global ocean. It is estimated that approximately 269,000 metric tons of plastic float at or near the surface of the ocean; this is the equivalent to approximately six billion large fizzy pop bottles that, if laid end-to-end, would stretch about 50 times around the Earth. Due to slow degradation rates in sea water, marine plastic can remain in the environment for years, decades or even centuries where it is a risk to marine life and food security.

Microplastics are generally defined as plastic particles smaller than 5 mm in size. They are either derived from the fragmentation of larger plastics or fibres from some man-made cloths, or are manufactured directly to be of a small size; often referred to as microbeads, such plastics are commonly used as exfoliates in cosmetics. Microplastics enter the ocean either directly, as a result of human activity (e.g. shipping, coastal tourism), or indirectly via rivers and sewage outflows.

Microplastics have been detected across the globe, including the open ocean, polar icecaps, deep-sea sediments and the coasts of remote mid-oceanic islands. Due to their tiny size and the vastness of the ocean, collecting data on these microscopic particles has proved challenging and we know little of their impact on the marine life and ultimately, society.

This interdisciplinary review, published in the journal Frontiers in Ecology and the Environment, examined existing literature alongside laboratory, field and modelling studies to provide an overview of the current understanding of what happens to microplastics after they enter the ocean and how marine life and microplastics can interact.

The review’s scientists conclude with three key research questions:

- How common are interactions between plastics and marine organisms in regions of high productivity?
- Do microplastics accumulate within the marine food web, including commercially exploited species of fish and shellfish destined for human consumption?
- Are microplastics actively consumed and cycled by marine organisms and do these organisms facilitate the transport of plastic to deeper waters and sediments?

Plastic feast

Research has confirmed that marine animals consume plastics. Sampling efforts have shown that over half of blue whiting and red gurnard from the English Channel and 83% of Norwegian lobsters from the Clyde estuary contain plastic particles in their digestive system. In addition to this direct ingestion, plastics can be transferred from one species to another by predators eating prey that have consumed microplastics.

Ingested microplastics can result in negative health impacts such as reduced stomach space for actual food and the build-up of plastic-derived toxic compounds. This is of concern as many of these species are consumed by humans and it is yet unclear to what extent this will impact upon human health.

Plastic in plankton poop

Laboratory studies have shown that copepods, a key organism in marine food webs, consume microplastics. Their faecal matter acts as a food source for other organisms but also helps transport carbon and nutrients down into the ocean’s depths. This can provide buoyant microplastics with a transport pathway away from the ocean’s surface to the sediments below. However, experiments have also demonstrated that faecal pellets containing microplastics can become more buoyant, potentially reducing the transport of carbon and nutrients to deeper waters. It is essential for us to gain a better understanding of these exchanges given the expectation that the number of microplastics in the oceans will increase in the future.

Dr Matt Cole, co-lead author and Natural Environment Research Council (NERC) Research Fellow at the University of Exeter, said: “This vital research highlights that plastics and marine animals are most likely to interact in coastal areas. Microscopic plastics are readily consumed by animals and this can have significant implications for the health of those organisms. Further work is needed to understand what implications this will have on environmental health and food security.”

Dr James Clark, co-lead author and Marine Ecosystem Modeller at PML, commented: “At present there are many unknowns regarding the impact of microplastics in the marine environment which hinders decision making. Within the UK and elsewhere, the technical expertise exists for a world leading, interdisciplinary consortium to be formed which would help fill these knowledge gaps. Such a research effort could provide meaningful advice to policy makers, businesses and members of the general public on how best to manage existing debris levels and to deal with plastic end of use moving into the future.”

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