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The faecal microplastic pump: zooplankton efficiently transport microplastics to the ocean depths
Summary
Tiny ocean creatures called zooplankton are eating floating plastic bits and pooping them out in pellets that sink to the deep sea, acting like a natural elevator that carries roughly a third of the ocean's "missing" microplastics down to depths below 1,000 meters. This matters because it helps explain where all the plastic we've dumped into the ocean actually ends up, and it means deep-sea ecosystems (and the food chains tied to them, including seafood we eat) are absorbing more plastic pollution than previously realized.
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 neutral 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 primarily driven by polymer density in coastal regions, whereas grazing and sinking faecal pellets facilitate 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.