Microplastics could enter marine food webs bottom-up

Microplastics — tiny pieces of plastic less than five millimeters — have been found in marine and freshwater animals ranging from tiny zooplankton to large whales.

However, researchers are still struggling to understand the impact of microplastics on aquatic species.

Scientists have found that microplastics have the potential to harm animals through pathways such as replacing food and leaching added chemicals into their bodies. However, the extent to which these effects are currently occurring in the environment is unknown.

Our recently published study explores how microplastics move through coastal marine food webs. We found that small animals feeding lower in the food web may be at greater risk of exposure to microplastics than large predatory animals.

Pollutants and food webs

Food webs are tangled webs of organisms feeding on each other. Where an animal feeds in this tangled web is called its trophic position and can determine its exposure to pollutants.

For example, mercury pollution accumulates in the muscles of animals and passes from prey to predators, reaching higher levels of concentration through the food web.

Read more: Killer whales: Why more than half of the world’s killer whales are threatened by leftover industrial chemicals

This process is called biomagnification, and it’s why animals like tuna and salmon end up with potentially dangerous levels of pollutants.

Do microplastics biomagnify?

During the summer of 2018, we collected individuals – including clams, mussels, sea cucumbers, crabs, starfish and fish – across a food web from multiple sites around the southern Vancouver Island.

A beach seine being hauled to collect fish for survey. We found that most individuals had up to two microplastic particles in each of their guts, and the particles were mostly fiber.
(Kieran Cox), Author provided

We then determined the concentrations of microplastics found in the intestines of animals and the liver of fish and linked these concentrations to each animal’s place in the food web.

A graphical representation of fish and other marine animals analyzed for their microplastic content.
The species of aquatic animals we analyzed for their microplastic content and their positions in the food web.
(Garth Coverton), Author provided

Animals higher in the food web did not contain greater concentrations of microplastics than animals lower in the food web, suggesting that there was no biomagnification.

A part of our past works also showed a lack of evidence for the biomagnification of microplastics. In this work, we compared the concentrations of microplastics in the intestines of fish, reported in the scientific literature, with estimates of their place in food webs.

Some species may be more at risk

Although we found no evidence of biomagnification, we found that microplastic concentrations were higher for some smaller species relative to their body weight.

An illustration of the authors' findings after comparing the concentrations of microplastics reported in the intestines of fish with the trophic level of the fish.
Microplastics did not increase at higher trophic levels – higher positions in the food web – according to a review of the literature.
(Garth Coverton), Author provided

This included filter-feeding animals such as clams, mussels and some sea cucumbers, as well as one type of fish, the minnow. These fish could ingest more microplastics because the particles are similar in size and shape to their favorite food — small aquatic micro-organisms such as zooplankton and other small invertebrates.

However, the number of microplastics we found in all animals was less than two particles per individual on average. While this may mean the health risks to these animals are low, we still need to understand how long-term exposure to low levels of microplastics could affect their health.

In our research, we have limited ourselves to studying particles larger than 100 microns – about the width of a human hair – because particles smaller than this are very difficult to study using an ordinary microscope. However, emerging methods may make them easier to study in the future. These small particles are potentially more toxic and we cannot rule out biomagnification on this scale, although it does not occur for larger particles.

How do microplastics affect aquatic food webs?

As environmental pollution from microplastics increases, we need to understand its possible effects to avoid potential ecosystem disasters in the future.

An aerial view of a lake with experimental enclosures.
An aerial view of a lake with experimental enclosures where microplastics research is being conducted at IISD-ELA in northwestern Ontario. Studying microplastics in natural freshwater laboratories will advance our understanding of how they might affect aquatic food webs.
(Garth Coverton), Author provided

Freshwater ecosystems, for example, are often more directly exposed to microplastics and may contain higher concentrations.

Researchers, including a member of our team, are currently carrying out work at International Institute for Sustainable Development Experimental Lakes Area to help understand how exposure to microplastics could affect freshwater ecosystems and food webs.

This work, along with that of other researchers, should advance our understanding of how microplastics can affect aquatic ecosystems, particularly the effects on small animals at the base of food webs that may ingest more of these particles.

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