Miniscule polymer pieces the size of a sesame seed or tinier, microplastics pose a growing pollution threat to marine environments worldwide. To understand how microplastics accumulate in nearshore, urbanized environments, researchers quantified the prevalence of microplastics in and around the Zostera marina meadows of Deerness Sound, in the Orkney Islands of Scotland. Mark Hartl and colleagues at Heriot-Watt University found that microplastic flakes, fibers, and fragments were twice as concentrated in the water above eelgrass meadows as in adjacent control areas of sandy sediments. Sediments within the meadows contained 40% more microplastics than in the sandy areas. The scientists also found plastics attached to every one of the 60 blades of eelgrass they examined; in fact, microplastics were 20% more abundant atop eelgrass than in control areas. Eelgrass meadows are prized for their ability to absorb wave energy and increase rates of sedimentation; the researchers suspect this talent for slowing the travel of water helps floating microplastics settle out of the water column onto meadow areas. Meanwhile, biofilms such as algae and the microscopically rough surface of seagrass blades help microplastics adhere to the surface of the plants. The prevalence of these contaminants on eelgrass itself—a source of food for marine grazers, as well as detritivores such as amphipods that eat dead plant matter—indicates eelgrass may be a route for microplastics to enter the marine food web. Indeed, the Scottish scientists found microplastics in more than 80% of the snails and other eelgrass grazers they sampled. Whether the ability of eelgrass to trap microplastics is a boon or bane to the Bay remains an open question in the burgeoning field of marine microplastics research.

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Results of the Habitat Risk Assessment Cumulative risk scores ranged from 0 to 10.7, out of a possible maximum cumulative risk score of 21. Credit: Rudebusch et al. 2020
 

The increasing flow of microplastics entering San Francisco Bay from trash, fleece clothing, car tires, and myriad other sources is likely being trapped by a surprising filter: native eelgrass (Zostera marina).

Miniscule polymer pieces the size of a sesame seed or tinier, microplastics pose a growing pollution threat to marine environments worldwide. To understand how microplastics accumulate in nearshore, urbanized environments, researchers quantified the prevalence of microplastics in and around the Zostera marina meadows of Deerness Sound, in the Orkney Islands of Scotland. Mark Hartl and colleagues at Heriot-Watt University found that microplastic flakes, fibers, and fragments were twice as concentrated in the water above eelgrass meadows as in adjacent control areas of sandy sediments. Sediments within the meadows contained 40% more microplastics than in the sandy areas. The scientists also found plastics attached to every one of the 60 blades of eelgrass they examined; in fact, microplastics were 20% more abundant atop eelgrass than in control areas. Eelgrass meadows are prized for their ability to absorb wave energy and increase rates of sedimentation; the researchers suspect this talent for slowing the travel of water helps floating microplastics settle out of the water column onto meadow areas. Meanwhile, biofilms such as algae and the microscopically rough surface of seagrass blades help microplastics adhere to the surface of the plants. The prevalence of these contaminants on eelgrass itself—a source of food for marine grazers, as well as detritivores such as amphipods that eat dead plant matter—indicates eelgrass may be a route for microplastics to enter the marine food web. Indeed, the Scottish scientists found microplastics in more than 80% of the snails and other eelgrass grazers they sampled. Whether the ability of eelgrass to trap microplastics is a boon or bane to the Bay remains an open question in the burgeoning field of marine microplastics research.

About the author

Bay Area native Kathleen M. Wong is a science writer specializing in the natural history and environment of California and the West. With Ariel Rubissow Okamoto, she coauthored Natural History of San Francisco Bay (UC Press, 2011), for which she shared the 2013 Harold Gilliam Award for Excellence in Environmental Reporting. She reports on native species, climate change, and environmental conditions for Estuary, and is the science writer of the University of California Natural Reserve System.

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