The faecal microplastic pump: Zooplankton efficiently transport microplastics to the ocean depths

Microplastics (MPs) contaminate marine environments from surface waters to deep sediments. Approximately half of global plastic production consists of low-density polymers that should float in seawater. However, global estimates indicate that surface ocean MPs account for only a few percent of the total oceanic burden. The widespread occurrence of low-density MPs in deeper waters raises questions about their fate in marine environments. Specifically, microplastic interactions with biota may contribute to their transport throughout the water column. Experimental studies suggest that zooplankton consumption of MP and subsequent packaging in faecal pellets could facilitate their transfer to the deep ocean. Here, we built upon experimental and observational evidence to model the effectiveness of this biological transport mechanism. Our findings reveal that faecal pellets serve as an effective sink for floating and neutrally buoyant MPs, increasing global transport of MPs below 1000 m by 30% (range: 17%-160%). The simulations showed faecal pellets export of 0.7 to 1.6 Tg year^-1 of MP below 1000 m, modifying their global vertical distribution and budgets. These results further revealed that MP export is partly driven by polymer density in coastal regions, whereas grazing and sinking faecal pellets drive most of MP export in the open ocean. This faecal MP pump represents a critical mechanism for deep-sea MP sequestration and transforms our understanding of MP fate in oceanic systems.