Mussels are filter feeders and as such, are natural nutrient recyclers that connect the surface ocean with the seafloor. However, what happens when mussels ingest microplastics? Would an important benthic-pelagic coupling system be altered? Microplastics could change the density of mussel biodeposits because of their physical properties—namely, their small size and low or even positive buoyancy. Unlike natural particles like algae or silt, microplastics are often less dense than seawater, meaning they can float or suspend more easily. When mussels ingest these particles and incorporate them into their feces or pseudofeces, that the overall density of the resulting biodeposits could decrease, making them sink more slowly or remain suspended in the water column longer. Additionally, microplastics may stimulate extra mucus production in mussels, creating fluffier, looser biodeposits that further reduce density and increase buoyancy.

The project tested how microplastics affect four key attributes of mussel biodeposits: shape, particle content, sinking rate, and resuspension velocity. Mussels were fed two diets—one with only algae and one with algae plus fluorescent microplastic beads—under controlled lab conditions. The results were striking: biodeposits from mussels fed MP were up to 37% slower to sink and required up to 22% less force to become resuspended. Waste didn’t settle quickly into local sediment but instead stayed suspended or traveled further afield, altering where nutrients and plastic end up in the marine food web.

To reach these findings, the team conducted detailed lab trials at Friday Harbor Labs using mussels from the Pacific Northwest. Mussels were exposed to controlled diets in aerated seawater containers, and their waste was collected and analyzed over 24 hours. Biodeposits were measured for morphology with microscopy, particles quantified with hemocytometers, sinking rates assessed using a water column test, and resuspension velocity calculated in a custom seawater flume. Algae and microplastic concentrations were tracked in real time during feeding with a flow cytometer. Advanced statistical models confirmed that the presence of microplastics, not just the type of biodeposit, altered these properties.

If microplastic contaminated waste floats longer and travels farther, fewer nutrients may be retained in mussel beds—possibly reducing biodiversity there—while more microplastics could be delivered to organisms elsewhere, accelerating trophic transfer and ecosystem-level pollution. This study highlights how even tiny particles can have outsized effects when they interact with the natural behaviors of keystone species.

Full scientific paper can be found here