Microplastics may reduce the efficiency of the biological carbon pump by decreasing the settling velocity and carbon content of marine snow

Plastics are pervasive in marine ecosystems and ubiquitous in both shallow and deep oceans. Microfibers, among other microplastics, accumulate in deep‐sea sediments at concentrations up to four orders of magnitude higher than in surface waters. This is at odds with the fact that most microfibers are positively buoyant; therefore, it is hypothesized that settling aggregates are vectors for the downward transport of microfibers in the ocean. However, little is known about the impact of microfibers on carbon export. We formed diatom aggregates with differing concentrations of microfibers using roller tanks and observed that microfiber addition stimulated aggregate formation, but decreased their structural cohesion and caused them to break apart more readily, resulting in smaller average sizes. The incorporation of positively buoyant microfibers into settling aggregates reduced their size‐specific sinking velocities proportional to the microfiber concentration. Slower sinking may extend aggregate retention time in the upper ocean, thereby increasing the time available for organic matter remineralization in the upper water column. Here, we show that at concentrations of 105 microfibers per cubic meter, microfiber incorporation into settling marine aggregates decreases potential export flux by 8–45%. Microfibers accumulating at such high concentrations, for example, in Arctic sea ice, may, therefore, be substantially reducing the efficiency of the biological carbon pump relative to the pre‐plastic era.