Dissecting the word biogeochemistry harks at the root of the field. Biogeochemists study how chemical elements and compounds cycle through biological and geological processes in order to build a global conceptual model. Some of the most well-known biogeochemical cycles first introduced to students include the water, carbon and nitrogen cycles. These cycles have existed on Earth of millennia, and I spent my PhD researching how humans modified these cycles. However, after graduate school, a new emerging cycle caught my eye: the plastic cycle.
I first encountered the idea of the plastic cycle when Law and colleagues reported their efforts to understand plastic content and transport in the Northwest Atlantic Ocean (Science 329, 1185–1188; 2010). Although the cycle was not explicitly named, throughout the text they described how plastic entered, was transported within, and exported from the ocean — hallmark terms and processes of a biogeochemical cycle.
The oceanic plastic cycle begins with plastic entering the ocean, which can occur through different means. Plastic litter being blown into the ocean from land or dumped off sea-going vessels are two canonical pathways. However, as Law et al. pointed out, another substantial source of plastics comes from shipping containers filled with preproduction plastic pellets, often called virgin plastic. These containers fall off cargo ships while en route to a factory; however, the flux of plastic via this source is decreasing due to programs instituted to prevent pellet pollution.
Once in the ocean, plastic is transported by currents. Due to the buoyant nature and long deterioration time of plastic, slow currents can move plastic long distances. How plastic was transported and where it ended up in the Atlantic was at the heart of Law and colleagues’ experiments. They towed nets behind boats for 20 years to determine the oceanic abundance of plastic pieces, and tracked drifters meant to mimic plastic transport. Law et al. found that currents induced an accumulation of floating plastic in the Northwestern Atlantic Gyre convergence zone off the coast of the Mid-Atlantic Bight.
Once in the convergence zone, plastic is removed from the surface mixed layer through sinks and export processes. A substantial sink within the mixed layer is ingestion by animals, such as sea turtles that live in the algal mats of the Gulf Stream (Mar. Poll. Bull. 28, 154–158; 1994). Plastic is also exported to the deep ocean when its density increases due to fouling organisms that grow on the plastic. Once in the deep ocean, plastic can be either ingested by animals or buried (Annu. Rev. Mar. Sci. 9, 205–229; 2017).
I am currently focusing on these export processes, namely the flux of plastic into coastal sediments, and how this flux is affected by oceanic plastic abundance. My lab is comparing the concentration of small plastic pieces in coastal sediment adjacent to and removed from areas of plastic convergence. Our preliminary results indicate significantly higher concentrations of plastics in sediment along the strandline of beaches adjacent to areas of high oceanic plastic concentrations. It will take many more individual experiments to understand each part of the plastic cycle as well as we do other biogeochemical cycles. Law et al. provided the foundation to build these experiments.
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Lecher, A.L. Piecing together the plastic cycle. Nature Geosci 11, 153 (2018). https://doi.org/10.1038/s41561-018-0077-9
Science of The Total Environment (2019)