Microplastics in the St. Lawrence

Photo credit: @grbender (iStock).

In October 2019, American researchers published a study that found larval fish nurseries in Hawaii rife with plastic pollution — so much so that trash bits outnumbered animals seven-to-one. As a result, baby fish were feeding on “prey-sized” plastic, a.k.a. microplastics — particles less than five millimetres in size.

Though the research covered but a tiny corner of the Earth’s vast oceans, the global ubiquity of this type of pollution is borne out daily on the ticker-scroll of social media: marine scientists find every water sample taken around the Galapagos Islands contains microplastics; an article in reports microplastics in the digestive system of Arctic beluga whales harvested as food by Indigenous communities; and a University of Victoria meta-analysis of 26 previous studies on microplastics in salt, beer, sugar, fish, shellfish, water and air delivers sobering results on our own unavoidable ingestion of these pollutants. 

Detailing the latter gives some idea of the scale of the problem. The study estimated annual rates of human consumption of microplastics as 40,000 particles for children and 50,000 for adults.

With inhalation factored in, the estimate for adults jumped to 74,000 to 121,000 particles. In addition, those who regularly drank bottled water, which delivered only an extra 4,000 particles. Shocking as these numbers are, they’re also gross underestimates: with the foods studied comprising only 15 per cent of a typical North American diet, the researchers suggested real numbers in the hundreds of thousands.

So, this much we know: our water and food contain microplastics. Though effects on our bodies are yet to be completely understood, studies show that while some plastics are expelled, others are absorbed. The tiniest particles enter the bloodstream and lymphatic system, carrying toxic chemicals that can affect immune response. In birds, microplastics have been found to restructure the surface of the small intestine, disrupting iron absorption and stressing the liver. 

With such clear connection between health and our consumer practices, microplastics are a serious concern that should dictate tolerance of plastic packaging of food and beverages. The caution extends to consuming foods harvested from, or affected by, microplastics-contaminated water sources. Which brings us to one ecosystem in particular: the St. Lawrence River, which drains the Great Lakes — the world’s largest freshwater ecosystem and home to 30 million people.

Back in 2014, Anthony Ricciardi, professor of invasion ecology and aquatic ecosystems at Montreal’s McGill University, raised the alarm over widespread presence of microplastics in St. Lawrence River sediments in the form of polyethylene microbeads used in toothpastes, makeup and body cleansers. Yet microbeads — since banned by the Canadian government — were only a part of the problem. As Ricciardi and his students broadened their studies, he cautioned that microbeads were getting all the attention, but were only one component of the broader category of microplastics. “As time goes on, people are going to realize the importance of the other pieces, too.”

Given high densities of microplastics in some river sediments, Ricciardi figured their inevitable ingestion by bottom-dwelling fishes and macroinvertebrates such as insects, worms and mollusks warranted attention. His lab now investigates microplastics in the freshwater food webs of the St. Lawrence. 

Meanwhile, similar ongoing studies in the Ashpole-Hill Lab at St. Lawrence College in upstate New York identified microbeads in sediment on that side of the river that also suggested a threat to food webs. Preliminary findings from a 2019 survey by graduate student Nathan Pollack confirm some expectations. First, abundance of microplastics significantly decreases with downstream distance from large cities/towns. Second, major physical forms of microplastic (beads, fibres, fragments) also change with downstream distance; at the head of the river, fibres slightly edge out fragments numerically, but far outnumber the latter further downstream, likely due to higher flotation ability. Finally, beads are significantly lower in frequency than either fibres or fragments at all sites. 

Pollack found the majority of sites contained a mix of polyethylene, polystyrene, polyester and nylon polymers. Furthermore, a significant relationship exists between the number of particles found in zebra mussels, and those extracted from their main predator, round goby (an invasive fish originating from the same Ponto-Caspian ecosystem as the invasive mussel), demonstrating transfer of microplastics between species. This mussel-goby relationship also has a direct quantitative tie to microplastics’ abundance in local sediments. 

This is worrisome given the numerous sport and commercial fisheries in the Great Lakes-St. Lawrence basin, adding to existing cautions against using plastic-toxin-containing water for crops or drinking. And it also harkens back to the open ocean. If larval fishes are eating microplastics, these are likely concentrating all the way up the food chain to top predators, including the fish we love to eat.