A History of Microplastic Pollution in UK Salt Marshes

The amount of plastic pollution in the environment has increased exponentially since its mass production began in the 1950s. As the vast majority of plastic cannot biodegrade, it instead slowly fragments through mechanical and chemical processes, producing microplastics (MPs; <5 mm) that are now found across the globe. Growing concern exists regarding the potential impacts of MPs on ecosystems. While considerable research has investigated MP pollution in aquatic and coastal environments, the quantity, distribution, and historical accumulation of MPs in UK salt marshes remain largely unknown.

Salt marshes are coastal wetlands with a unique range of flora and fauna, providing coastal protection and critical ecosystem services. However, they may also act as long-term sinks for MPs. Studies of MPs in salt marsh sediments are scarce, and very few have incorporated sediment dating using short-lived radioisotopes (210Pb, 137Cs, 241Am) to establish a history of plastic pollution.

In this study, sediment cores from four UK salt marshes (Welwick in the Humber Estuary, Lindisfarne on the Northumberland coast, Skinflats in the Firth of Forth, and Caerlaverock in the Solway Estuary) were collected and dated using 210Pb, 137Cs, and 241Am, with chronologies constructed using the Bayesian Plum model. MPs were extracted from the cores and identified, providing insights into the types, quantities, and temporal trends of plastics in UK salt marshes. At Welwick, an additional core was collected from an area with visible surface plastic to investigate the relationship between surface and subsurface MP accumulation. Welwick Saltmarsh was selected due to its location along the River Humber, a region known for high levels of plastic pollution.

To extract MPs, organic matter in the sediment cores was digested using 30% H₂O₂ in an ice bath, minimising polymer degradation compared to heated methods. MPs were extracted via density separation with 1.5 g/cm3 LST FastFloat and subsequently identified using Nile Red staining, fluorescence microscopy, and Raman spectroscopy.

The cores were found to contain sparse particles and fibres of polystyrene, polyethylene, polyethylene terephthalate and styrene-butadiene rubber. The core collected from Skinflats contained the greatest concentration of MPs (12 MPs), including a styrene-butadiene rubber particle likely derived from tyre wear associated with the nearby major road, whereas the remaining three cores contained only 2–3 MPs each. The oldest MPs found date from the 1950s. Overall, concentrations of MPs were surprisingly low.

This research is the first to examine historical microplastic pollution using dated sediment cores from multiple UK salt marshes across different estuarine systems. Although microplastics in soils can alter microbial communities and plant growth, the low concentrations observed here suggest minimal implications for salt marsh ecosystem functioning, including carbon sequestration processes and the success of habitat restoration or realignment efforts.