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Microplastic pollution: an emerging freshwater stressor

June 24, 2015

Microplastics found in the Magothy River, Chesapeake Bay, USA. Image: Chesapeake Bay Program | Flickr Creative Commons

In recent years, microplastic pollution has been identified as an increasingly pervasive and damaging environmental stressor in the world’s seas, found even in remote locations in the Arctic ocean and deep sea trenches, far from human settlements.

Microplastics are, as the name suggests, tiny particles of plastic (less than 5mm in size in this study) which enter aquatic environments either directly as manufactured pellets from industrial and farming processes and microbeads from cleaning and cosmetic products; or indirectly through the erosion and breakdown of larger plastic items such as fishing nets and household waste.  When ingested by fish and marine mammals, microplastics can obstruct or damage internal processes, cause bodily stress, and potentially lead to the uptake of harmful chemicals.

Global plastic production has increased exponentially since the 1960s, to around 299 million tonnes in 2013 alone, meaning there is a huge amount of plastic currently being used and thrown away around the world, a proportion of which is ending up in entering aquatic systems as pollution, and taking many years, if not centuries, to break down. So in short, microplastics are one of the most widespread and potentially damaging aquatic stressors emerging from the growth of consumer society in recent decades.

But as yet, most of the scientific and conservation work on microplastic pollution has focused on seas and oceans.  But what about microplastic pollution in freshwaters?


The microplastics found in the Magothy River, Chesapeake Bay exhibit a range of diverse shapes and sizes when viewed under a microscope. Image: Chesapeake Bay Program | Flickr Creative Commons

A paper published earlier this year in the journal Water Research, led by Dafne Eerkes-Medrano at the Aquatic Ecology Group, Department of Zoology at the University of Cambridge provides a timely overview of research on the impacts of microplastics on freshwater systems.

Whilst there is still only a small body of research on freshwater microplastics, the review work by Eerkes-Medrano and colleagues suggests that microplastic presence and impact may be equally as far-ranging in freshwaters as it is in marine habitats. Their study documented evidence of microplastics in freshwater environments as widely spread as Lake Hovsgol in Mongolia, Lake Geneva and the River Danube in Europe, and Lake Superior, Lake Huron and Lake Erie in Canada and the USA.

However, whilst there are a growing number of studies documenting microplastic pollution around the world, our knowledge of this emerging environmental stressor remains patchy.  As such, Eerkes-Medrano and colleagues suggest a number of key areas requiring further research to help develop and strengthen freshwater conservation and policy on microplastics.


Tiny microplastics that can be ingested by a range of freshwater life. Image: Chesapeake Bay Program | Flickr Creative Commons

First, we need to better understand how microplastics reach freshwater ecosystems, and how they disperse and degrade over long periods of time.  This is largely a question of understanding and mapping plastic pollution from household, industrial and agricultural sources, and studying how it is gradually broken down.  The movement and potential deposition of microplastics is shown to depend largely on river flow, wave action and releases of water from dams and hydropower plants.  As such, microplastic concentrations are likely to be high in areas where water velocity is low, and the sediment it carries is suspended and deposited.

A key question for the management of microplastic pollution is how to prevent it entering freshwaters at source.  A recent UN report on marine microplastics advocated more widespread and effective plastic recycling processes to reduce waste.  Here, it might be the case that creative and effective science communication work which engages the public with microscopic images of micoplastic pollution (as shown here from a study by the Chesapeake Bay Program in the USA) could help bring this otherwise largely invisible issue to life, and help foster more environmentally concious plastic use and disposal.

Second, we need better monitoring systems to detect microplastics in freshwaters.  This is a challenging process, because microplastics are so small and easily transported by the movement of water.  Similarly, when a sample of sediment is taken from the bed of a freshwater, it is difficult to separate the microplastics from other organic particles in the sediment.  Finally, without any historical data on microplastics in freshwaters, it is difficult to define any baselines for what might be ‘safe’ concentrations in an aquatic ecosystem, nor which particular plastics and sizes cause the most harm to aquatic life.

Third, and perhaps crucially, we need to better understand the impacts of microplastic pollution on freshwater biota.  Again, scientific studies of the effects of microplastics on freshwater life is relatively sparse, but tellingly in almost all surveyed studies, microplastics were ingested by freshwater species including catfish, freshwater snails, clitellate worms Japanese medaka and gudgeon.  Perhaps surprisingly, in one of these studies, tiny plastic particles between 20 and 1000nm were shown to accumulate in the tissues of the minuscule freshwater water flea Daphnia magna.


The tiny freshwater water flea, Daphnia magna, which was found to have taken up miniscule microplastics in one study. Image: Per Harald Olsen/NTNU | Flickr Creative Commons

Whilst there is variation in the ecological effects observed in the available studies, microplastics can have harmful effects on freshwater life by blocking internal digestion processes, causing physiological stress, and causing the uptake of potentially damaging chemicals.  Both the microplastics themselves, and the chemicals they leach, have the potential to bioaccumulate in larger animals such as predatory birds and fish, with harmful effects potentially cascading through an ecosystem’s trophic layers.  Here, microplastic concentrations and residence time (i.e. the amount of time they stay in the ecosystem without degrading) are suggested to be important in determining how much impact pollution is likely to have on freshwater life.

Fourth, there are many unanswered questions about the potential impacts of freshwater microplastic pollution on humans.  Eerkes-Medrano and colleagues ask, if microplastic pollution is increasingly recognised as a freshwater issue, what might its effects be on the freshwater resources used by humans, such as drinking and bathing water?  How might microplastics contaminate food production, both in freshwater, and potentially in the surrounding landscape where they might be deposited?

Whilst there are numerous unanswered questions about the presence, dynamics and effects of microplastic pollution, the key point of this new review by Eerkes-Medrano and colleagues is that microplastics are an important area for freshwater scientific research and policy development.  Greater knowledge and awareness of microplastics in marine environments mean that marine policy is slowly beginning to address their impacts, for example in the European Commissions Marine Strategy Framework Directive, which explicitly lists microplastics as an important and damaging source of ‘marine litter’ to be managed.

But forming new and effective environmental policy and conservation strategies requires a strong scientific evidence base, which in turn needs appropriate funding sources and monitoring systems.  This review shows that there is still much work to be done on surveying, monitoring and researching freshwater microplastic pollution, in order to support the formation of appropriate policies and strategies to manage this emergent and potentially widespread freshwater stressor.

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