Once microplastics have made their way into rivers and the sea they become assimilated into the environment. Some will stay in the water column and continue their journey onwards and some will eventually sink to the bottom. Larger items that have escaped the water treament process often accumulate in specific areas based on their shape and size and the prevailing currents. A wet wipe reef in Battersea, a collection of the the plastic sticks from cotton buds in the corner of a bay or part of the general rubbish found on the river banks and strandlines of UK beaches and estuaries.
Clive Webster, author of The North Thames Estuary’s annual litter picking report said of the 4th quarter of 2020: "Over the course of 35 picks, they counted 589 plastic bottles, 908 drinks cans, 201 glass bottles and 150 disposable face masks proving that face masks haven't replaced plastic bottles in our estuary, they have simply added to it."
The impact of microplastic fragments is more subtle as it can't be seen directly, but river and coastal ecosystems are built around the invertebrates found in the mud and sand, many of whom are filter feeders who can't distinguish between food and plastic.
Filter feeders like cockles have difficulty distinguishing between plastics and food. This means they have to work harder to gather the energy and nutrients they need. In 2017 Dr Stephanie Wright (King’s College London) found that almost two thirds of mussels in the sea around Kent were contaminated with plastics. A 2018 study of microplastics along the river Tame in Birmingham notes evidence of microplastics in the casings built by caddisflies and in the carapace of water fleas (Daphnia). The implications of this are largely unknown - so many varied types of plastic are released into the water in combination with chemicals (for instance in detergents or body scrubs) that existing studies barely scrape the surface of how these combinations influence marine organisms.
Studies have found that exposure to microplastics affects mussels’ ability to grip rocks, and that while most particles eaten are excreted, some remain in the digestive systems of marine animals and can cause blockages. When predators eat smaller animals, plastics accumulate in their digestive systems too. A 2019 study found microplastics in the faeces of all human participants. While the study of microplastics' impact on animals is in its early days, the indications are that ingested plastics have a range of effects which, while initially moderate, will accumulate and intensify over time. It seems very likely that some of these effects will be unexpected and will only become apparent with the hindsight of decades or centuries.
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