Microplastic sedimentation in the northern Gulf of Mexico

To effectively mitigate plastic pollution, it is imperative to understand the transport of microplastics in the water column. This research provides insights into the potential pathways of microplastic sedimentation in the northern Gulf of Mexico (nGoM), such as incorporation into fecal pellets and interactions with marine snow. The nGoM is a complex marine ecosystem, affected by naturally occurring forces, such as strong currents and anthropogenic activities, such as eutrophic river plumes and oil spills. It is a semi-enclosed sea that is regularly exposed to inputs of sediments and nutrients, transported by large water systems such as the Mississippi, Mobile and Atchafalaya rivers. For six consecutive years (2012 to 2018), time series samples were collected with a McLane sediment trap, positioned within the nGoM (28°40.8 N 88°21.7 W), in the plume of the Mississippi, at a depth of 1520 m. We hypothesize that microplastic sedimentation is driven by sinking marine particles. For the most final trap sampling year (September 2017 to August 2018), we compare the seasonality of the vertical flux of particulate organic carbon and nitrogen (POC/PON), calcium carbonate (CaCO₃) and biogenic silica (bSiO₂), to the sedimentation of marine microplastics larger than 20μm. To prepare microplastics for analysis, a gentle digestion step was used to remove excess organic material. Subsequently, microplastics were isolated via density separation using a sodium tungstate dihydrate solution (Na₂WO₄·2H₂O, density 1.6 g cm-₃). Potential microplastic particles are counted and characterized by size, colour and shape using a compound microscope and Raman spectrometry is used to determine the types of microplastics present. This research will strengthen our understanding of sedimentation pathways of microplastics in the nGoM and similar environments and ultimately contribute to the efforts to improve the health of anthropogenically impacted marine systems.