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Microplastic pollution could inhibit stream ecosystem functioning

January 17, 2020
Microplastic fragments. Image: Wolfram Burner | Flickr Creative Commons

Could plastic pollution affect how a stream ecosystem functions? According to a newly published study, the answer is yes.

Plastic pollution is rapidly growing in visibility as one of the key environmental concerns of this ‘Anthropocene’ age. Researchers around the world are increasingly focusing their efforts on understanding the effects that plastics – and particularly microplastics – might have on aquatic ecosystems.

As yet, however, this work has been largely focused on seas and oceans. Whilst there is a growing body of research on the effects of plastic pollution on freshwater ecosystems, there are still many unanswered questions.

The new study, led by Naiara López-Rojo, shows that microplastic pollution may cause significant effects on how stream ecosystems function. The research team used microcosm experiments (essentially glass jars filled with stream water) to study how different concentrations of microplastic pollution affected the growth of caddisflies, and the rates at which they decomposed leaf litter.

Leaf litter decomposition is a vital component of many stream ecosystems. Leaf litter – the leaves that fall into a stream from surrounding vegetation – is a key energy source for many invertebrates at the heart of stream food webs. Its decomposition – accelerated by invertebrates such as caddisflies – helps release carbon and nutrients to the wider ecosystem.

Recent studies show that microplastics are being found even in remote sites, carried on atmospheric currents. How might their presence affect stream ecosystems?

Caddisfly survival and leaf litter decomposition decreased with increasing microplastic concentrations in the study. Image: Naiara López-Rojo et al (2020)

López-Rojo, from the University of the Basque Country, Spain, and colleagues, exposed caddisflies and leaf litter to different concentrations of microplastics in water. They found that microplastics were rapidly ingested into the bodies of the caddisflies – most likely through the ingestion of particles attached to leaf litter – and then excreted.

This finding is consistent with recent studies, such as that by Fred Windsor and colleagues in rivers in South Wales, UK, which show evidence of microplastic uptake by invertebrates.

López-Rojo and colleagues found that higher concentrations of microplastics caused increased caddisfly mortality – which increased 9-fold at the highest concentration. However, they found that altering microplastic concentrations did not significantly affect caddisfly growth.

The researchers observed that increasing the concentration of microplastics in the microcosms caused leaf litter decomposition rates by the caddisflies to decline.

A Sericostoma sp. caddisfly larvae. Image: Wlodzimierz | Wiki Creative Commons

The study, published in the journal Environmental Pollution, is short in length and based on a relatively small sample size (32 microcosms, observed over a number of weeks). So why are its findings significant?

First, it provides more evidence to show that microplastics can be rapidly ingested into the bodies of freshwater organisms, and thus potentially accumulate and move through the food web into larger animals. Second, it suggests that the functioning of stream ecosystems – in this case through the key process of leaf litter decomposition – could be inhibited by the presence of high concentrations of microplastics.

The authors highlight the need for better monitoring of microplastic pollution in stream ecosystems to understand the extent of the pressures it might exert. In particular, they suggest that more research is needed to understand how microplastic pollution might affect ecosystems already influenced by multiple contaminants and stressors.

What is clear is that microplastic pollution is a growing issue for freshwater conservation and policy. López-Rojo and colleagues’ study is likely to be only the latest advance in the ongoing scientific effort to document and unravel its effects on freshwater ecosystems.

López-Rojo, N. et al (2020, “Microplastics have lethal and sublethal effects on stream invertebrates and affect stream ecosystem functioning,” Environmental Pollution, Volume 259, April 2020, 113898

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