A new study has examined how microplastics travel through the freshwater food web. It is well-known that animals encounter plastics in marine environments and on land, but little is known about how tiny pieces of plastic (measuring 5 mm or less) enter their bodies. In order to find out more, scientists from Cardiff University in Wales, UK, examined birds called dippers, which hunt for underwater insects and small fish by wading or diving in freshwater rivers.
Dippers have long been considered a key environmental indicator species on five continents. From the study's introduction: "The five Cinclus species are restricted to fast‐flowing piedmont or montane rivers, where they occupy a highly specialized niche feeding almost exclusively on aquatic invertebrate prey." It's known that microplastics occur in large quantities in the aquatic invertebrates that dippers rely on to eat, so they seemed "a suitable model for assessing plastic transfer across trophic levels."
"Because dippers provision nest‐bound young using loads of multiple, whole prey from well‐defined taxa, they also provide an opportunity to assess whether any plastic items are fed inadvertently to nest‐bound offspring through intergenerational transfer. This phenomenon has been illustrated in some seabirds but only in regurgitated catches or as whole plastic items."
In this case the researchers looked at regurgitated pellets and droppings, and found that roughly half of the 166 samples taken from adults and nestlings at 14 of 15 sites studied contained microplastic fragments. Concentrations were higher in urban areas and appeared to come from synthetic textiles (95 percent were fibers) and building waste. Based on this, the researchers estimate that dippers are consuming up to 200 microplastic fragments daily while foraging for their usual diet, and that these already present in the bodies of the organisms that the dippers are hunting.
One of the study authors, Joseph D'Souza, told the BBC, "The fact that so many river insects are contaminated makes it inevitable that fish, birds and other predators will pick up these polluted prey — but this is the first time that this type of transfer through food webs has been shown clearly in free-living river animals."
It appears that the fragments pass quickly through the birds, as the amounts found in faecal matter were similar to what the researchers thought were being ingested, but there is concern about the potential contaminants that could be introduced into the birds' bodies by these plastics, as well as an artificial sense of satiety.
Steve Ormerod, a professor at Cardiff University's Water Research Institute, expressed dismay at the findings. He is cited in EcoWatch:
"These iconic birds, the dippers, are ingesting hundreds of pieces of plastic every day. They're also feeding this material to their chicks... In almost 40 years of researching rivers and dippers, I never imagined that one day our work would reveal these spectacular birds to be at risk from the ingestion of plastics — a measure of how this pollution problem has crept upon us."
This will hopefully help people to think about plastic pollution in wildlife closer to home. So often the news coverage we see focuses on exotic, marine animals, such as a whale that's ingested too much plastic, a sea turtle with a straw in its nose, a sea horse clutching a Q-tip. This perpetuates the notion that rampant plastic contamination throughout the food chain is happening elsewhere, faraway, and yet it's in our own backyards.
This study joins the growing body of evidence that plastic is insidiously pervasive, that it does not stop at any level of the food chain but will continue to bio-accumulate, compromising the health of every species. The only solution is to halt superfluous plastic production at the source, to limit use of single-use plastics and opt for reusables whenever possible, and we need government policies to ensure this happens in a thorough, consistent manner.
