Microplastics are contaminants of increasing global environmental concern. Estuaries are a major transport pathway for land-derived plastics to the open ocean but are relatively understudied compared to coastal and open marine environments. The role of the "estuarine filter", by which the supply of sediments and contaminants to the sea is moderated by processes including vegetative trapping and particle flocculation, remains poorly defined for microplastics land to sea transfer. Here, we focus on the sea surface microlayer (SML) as a vector for microplastics, and use SML sampling to assess microplastic trapping in a temperate marsh system in Southampton Water, UK. The SML is known to concentrate microplastics relative to the underlying water and is the first part of rising tidal waters to traverse intertidal and upper tidal surfaces. Sampling a salt marsh creek at high temporal resolution allowed assessment of microplastics in-wash and outflow from the salt marsh, and its relationship with tidal state and bulk suspended sediment concentrations (SSC), over spring and neap tides. A statistically significant decrease in microplastics abundance from the flood tide to the ebb tide was found, and a weak positive relationship with SSC observed. Microplastics are defined as "any synthetic solid particles or polymeric matrix, with regular or irregular shape and size, and with size ranging from 1 µm to 5 mm, of either primary or secondary manufacturing origin, which are insoluble in water" 1. They are of increasing environmental concern due to their ubiquitous presence in oceans 2 , rivers 3,4 , the atmosphere 5 and on land 6. Microplastics are also more abundant by quantity compared to larger meso-or macroplastic debris 2. It is currently estimated that 80% of marine plastic debris is derived from land-based anthropogenic sources 7 , although these estimates are highly uncertain 7. Between 1.15 and 2.47 million tonnes of plastic debris of any size larger than 300 µm is estimated to be transported by rivers 3. Due to this large plastic throughflow, estuaries are recognised as an important transport pathway from land to sea for microplastics 8. As well as these riverine inputs, estuaries are frequently sites of intense urbanisation and industrial development, and receive plastic inputs from these sources directly, including through discharges from storm drains and waste water treatment works 9. Estuaries are, however, relatively understudied compared to beach and open marine environments with respect to both macro-and micro-plastics 10,11 , despite their likely importance for microplastic land-sea transfer and their ecological importance. Estuarine habitats such as salt marshes and mudflats are also potentially more favourable for the deposition of microplastics over high-energy environments such as sandy beaches 12. While several estuaries worldwide have been sampled to determine the abundance of microplastics (e.g. 13-15), it is only recently that detailed studies of microplastics cycling and trapping in estuaries have be...