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