## 1 Introduction  
Tyre wear particles (TWPs) may be the largest source of primary microplastics in the natural environment [1]. The increased use of electric vehicles is likely to exacerbate the situation, as they are heavier, causing increased friction between the tyres and the road [2]. TWPs have been relatively neglected in the literature compared with other microplastic types due to the difficulty in quantifying their abundance in the environment. Unlike other microplastics, they cannot be identified by conventional methods, such as FTIR or Raman Spectroscopy [1], as the carbon black (30-35% mass of the tyre) [3] creates interference by absorbing most of the light, making it difficult to determine polymer type [4, 5]. Pyrolysis GC-MS can be used to measure environmental concentrations of tyre chemical components,which can be extrapolated to give potential environmental concentrations of the particles themselves [6]. Using this method, [PERSON] et al. [7] found tyre wear pollution in 97% of samples from river and estuarine sediments across three continents, with loadings in some samples in Europe exceeding 1% of sediment dry mass. Approximately 40% of microplastics exported from rivers to the sea are estimated to be tyre and road wear particles [8; 9], with these particles estimated to make up 5-10% of microplastics in the marine environment [10]. Estuarine systems are likely to act as a reservoir due to the high density of the particles and the high levels of sedimentation [1; 6; 11; 12]. Tyre wear particles can enter rivers and estuaries directly, or through stormwater run-off [1; 6], and have been found in stormwater [8; 13], surface waters [6; 14], and sediments [4; 15]. There is evidence that tyre wear particles can negatively affect the health of marine and estuarine organisms. [PERSON] et al. [16] found that _Scrobicularia plana_ and _Hediste diversicolor_ consumed tyre wear particles, with sedimentary exposure leading to negative impacts on feeding, burrowing, and energetic content, whilst [PERSON] et al. [17] found that aquatic exposure of tyre particles affected growth and swimming behaviour in the silverside _Menidia beryllina_ and the mysid shrimp _Americamysis bahia_. Tyre particles have also been observed to have lethal effects; _Daphnia magna_ were observed to have an LC\({}_{50}\) value of 60 mg/L following exposure to pristine tyre tread particles [18]. These impacts of macrofauna may lead to changes in ecosystem functioning. Chemicals that leach from tyre wear particles (leachates) have been shown to contain significant toxic components [19] and negatively impact the health of marine organisms [20; 21; 22; 23; 24; 25; 26]. The toxic impacts of exposure to leachates are of concern as organisms do not have to interact directly with tyre particles to be affected. Instead, the chemicals leaching into the surrounding environment may impact a whole host of organisms, potentially negatively affecting biodiversity and ecosystem functioning. Whilst there is evidence of the impact of TWPs on marine organisms at the individual level, we know of no studies that address ecosystem-level effects of this pollutant. Estuarine ecosystems are some of the most highly productive ecosystems in the world, providing a wealth of ecosystem services, including carbon sequestration, remediation of waste, and lifecycle maintenance [27]. These ecosystem services are underpinned by a combination of complex ecosystem processes, such as nutrient cycling and primary and secondary productivity [28]. This study aimed to investigate the impact of different concentrations of sedimentary TWPs on estuarine ecosystem functioning in mesocosoms representing a natural estuarine ecosystem. Mesocosms contained estuarine mud with natural microhypotenthos and meiofaunal populations and two common bioturbating estuarine species: _Hediste diversicolor_ and _Scrobicularia plana_. Three key ecosystem functions were investigated: nutrient cycling, primary productivity, and secondary productivity.