Microplastics are an increasingly important contaminant in the marine environment. Depending on their composition and degree of biofouling, many common microplastics are less dense than seawater and so tend to float at or near the ocean surface. As such, they may exhibit high concentrations in the sea surface microlayer (SML – the upper 1–1000 μm of the ocean) relative to deeper water. This paper examines the accumulation of microplastics, in particular microfibres, in the SML in two contrasting estuarine systems – the Hamble estuary and the Beaulieu estuary, southern U.K., via a novel and rapid SML-selective sampling method using a dipped glass plate. Microplastic concentrations (for identified fibres, of 0.05 to 4.5 mm length) were highest in the SML-selective samples (with a mean concentration of 43 ± 36 fibres/L), compared to <5 fibres/L for surface and sub-surface bulk water samples. Data collected show the usefulness of the dipped glass plate method as a rapid and inexpensive tool for sampling SML-associated microplastics in estuaries, and indicate that microplastics preferentially accumulate at the SML in estuarine conditions (providing a potential transfer mechanism for incorporation into upper intertidal sinks). Fibres are present (and readily sampled) in both developed and more pristine estuarine systems.

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...

Saltmarshes are important natural ecosystems along many temperate (and other) coastlines. They stabilize sediments and act as biofilters for a range of industrial pollutants and, potentially, microplastics. Accumulation of microplastics along estuarine coastlines may be enhanced by the presence of saltmarsh species, as they offer better particle trapping efficiency than adjacent intertidal mudflats under prevailing flood and ebb tidal currents. However, the trapping efficiency of entire saltmarsh systems under varying flow conditions has not been widely assessed. While the effects of saltmarsh systems on water flow, and on sediment transport and trapping, have been relatively well studied, little is known about the contributions of saltmarsh halophytes, resident organisms and the associated saltmarsh sediments to the trapping of microplastics. To address this, a series of flume experiments were undertaken to examine transport and accumulation of Bakelite particles (~ 500 µm) and PVC nurdles (~ 5 mm) as model plastics in sub-sampled saltmarsh and intertidal mudflat monoliths. The results showed that saltmarsh systems influenced the hydrodynamics within and above the canopy, enhancing turbulence and shear stresses. With increasing flow velocities (≤ 0.51 m s −1 ), negligible quantities (2 × 10 −4 mg L −1 ) of sediments and Bakelite particles were eroded and resuspended. The algal biogenic roughness from the mudflat, and the vegetative roughness from the Spartina plants on the saltmarsh, inhibited the transportation of the microplastics within the tested systems. Resident burrowing crabs (Carcinus maenas) promoted the burial, release and transport of microplastics. The results of this study provide evidence of the contributory roles of saltmarsh systems in the sequestration of microplastics and sediment stabilization. Estuarine saltmarsh systems can act as sinks for microplastics with enhanced burial from burrowing crabs under favourable flow conditions. Vegetated intertidal habitats provide diverse support to natural, coastal and estuarine systems and valuable ecosystem services including protection against coastal hazards, regulation of contaminants and pathogens, and provision of habitats for macrobenthos such as bivalves, fishes, gastropods, crustaceans, and other invertebrates [1][2][3][4] . The positive contribution of coastal vegetated habitats to shoreline protection 5 is partly attributed to the combined inputs of the flora and fauna which occupy these habitats. The flora provide an enhanced biogeneic roughness which reduces the hydrodynamic shear stress 6-8 thus decreasing the erosion rate and prompting accumulation/ deposition of sediments. Meanwhile, the fauna offer biostabilization of the sediments through the secretion of biopolymers 9 , generation of armoured surfaces and other mechanisms that minimize the risk of erosion 10,11 . Mangroves, saltmarshes, seagrasses and reed beds occupy most estuarine coastlines that provide these biomorphodynamic and bioengineering functions 3,12,13 . Benthic organisms...

Many of the methods for microplastics quantification in the environment are criticised creating problems with data validity. Quantification of microplastics in the surface microlayer of aquatic environments using glass plate dipping holds promise as a simple field method, but its efficiency has yet to be validated. We tested a standard glass plate dipping method to assess recovery of four common polymer microfibres and two common natural fibres, under three different salinities (freshwater, brackish water, saltwater). Overall recovery rates were low (26.8 ± 1.54%) but higher recoveries were observed under saltwater treatments (36.5 ± 3.01%) than brackish water (24.5 ± 1.92%) or freshwater (19.3 ± 1.92%). The fibre types showed different recovery rates, with acrylic yielding significantly higher recovery rates (37.0 ± 2.71%) than other fibres across treatments. No clear relationship between the density of the fibres and the recovery efficiency was seen. We suggest that, where this method is used for monitoring microplastics, the results will typically underestimate the total amount present, but that recovery is sufficiently consistent to allow comparison of differences between sampling locations. When comparing data across river-estuarine or similar transects salinity should be monitored to account for salinity-induced differences in sampling recovery.