CalcUlating the strength of the Plastic pump In counteracting the Deep export of Oceanic carbon (CUPIDO)

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The amount of plastic entering our oceans is increasing (8 million tonnes p.a.) with global implications for the health of our planet. As this plastic debris degrades in the ocean, fragmentation will shift particle size from large plastics to smaller microplastics, even in the absence of any new inputs. Thus, the problem of microplastic pollution will only increase in future years. However, the ways in which microplastics are transported to the deep ocean are still largely unknown. This limits our ability to determine the impacts of plastic debris on the ocean ecosystem and how these can be alleviated. Microplastic debris interact with dynamic communities of zooplankton. These small organisms, which are at the base of marine food chains, ingest microplastics and repackage them into faecal pellets which may become deposited deep in the ocean over the cycle of diel vertical migration. Microplastics may also become incorporated into zooplankton body tissue which, at the end of life, will sink as part of the carcass. I am introducing a new concept of the "Plastic Pump", to collectively describe the process of incorporation of plastics into biological processes and their subsequent movement to depth. The Plastic Pump may interacts with the biological capability of the ocean to export carbon from the surface to depth (a process known as Biological Carbon Pump, BCP). Further elucidation of the interaction between the Plastic Pump and the BCP is important since the BCP provides a critical ecosystem service in mitigating climate change through uptaking and storing anthropogenically-derived atmospheric CO2 in the deep ocean. Interference by the Plastic Pump may reduce the effectiveness of the BCP. This has yet to be determined.CUPIDO will undertake two cruise expeditions where a suite of cutting edge approaches, at the intersection of biogeochemistry, material science, and biology, will be used. These approaches include floating and moored platforms that will not only determine depth profiles of plastic concentrations over seasons, but also how plastics interact with the natural ecosystem over these depths. It will also deploy a unique device, built in-house to my own design and specifications, to evaluate how oceanic plastics alter over long time scales through incubating pre-selected meso- and microplastics in in situ conditions. CUPIDO will focus on two regions located in the Southern Ocean (SO) and the Mediterranean Sea (MS). The contrasting conditions of the two selected regions (relatively pristine vs. highly polluted) allow for a comparative analysis of the impact of the Plastic Pump on the ocean's ability to export and sequester C within a low (SO) and high (MS) plastic input regime. My CUPIDO team will measure how the characteristics of plastics alter as a function of exposure to the marine environment; the vertical distribution and export of plastic over daily and seasonal timescales, and the role of zooplankton as vectors of plastics through the water column. This wealth of novel data will be analysed and modelled to predict: (i) the accumulation of plastics in specific water layers through the water column and (ii) how the flux of plastics and C alters over an annual cycle in regions of high and low plastic debris input. Overall, CUPIDO will address the hypothesis that zooplankton and food web associated processes play a major role in promoting the sinking of plastic through the water column. These mechanisms will decrease the ability of the marine ecosystem to transfer C from the surface to the deep ocean (resulting in a slowing of the BCP). The service provided by the BCP in lowering atmospheric CO2 levels has an economic significance to the mitigation of climate change. Through parameterising the various components of the Plastic Pump, CUPIDO will assess the economic impact of microplastic debris on the BCP and the value of combatting marine plastic pollution to restore levels of climate change mitigation.