## 2 Methods  
### Experimental Design and Set Up  
The sediment for the mesocosoms was collected from the top 2 cm of sediments from the Yealm Estuary, Devon, UK (4\({}^{\circ}\)03\({}^{\prime}\)28.36\({}^{\prime\prime}\) W, 50\({}^{\circ}\)32\({}^{\prime}\)24.51\({}^{\prime\prime}\) N), in September 2021 and sieved through a 2 mm mesh to remove gravel and macrofauna. The surface layer was collected, as anything lower than 2 cm was generally anoxic. On the same day _H. diversicolor_ and _S. plana_ were collected from the same location and placed in buckets of estuarine water with a thin layer of mud and maintained at 13 \({}^{\circ}\)C in temperature-controlled 12 L holding tanks supplied with continuously aerated 23 ppt salinity water. This temperature and salinity mimicked the conditions of the sampling site. Animals were acclimatised for two days prior to the experiment. Sediment grain size and organic matter content for the Yealm site have been described in [PERSON] et al. (2016), with a mean particle size of 45 (\(\pm\)0.2) \(\upmu\)m and an organic matter content of 9.0 \(\pm\) 0.2%.  
Tyre particles were purchased from Tyre Recycling Solutions (www.trs-ch.com), Prevenerges, Switzerland. Particles were produced from end-of-life passenger tyre tracks using water pulse\({}^{\text{TM}}\) technology for micronsisation. Tyre particle size was measured using a Malvern Mastersizer 2000 (Malvern, UK). The mean particle size was 106 \(\upmu\)m (range 30-350 \(\upmu\)m).  
Experimental exposure took place at the University of Plymouth Marine Station in 3.2 L semi-opaque mesocosms with a diameter of 18.4 cm (\(n=20\); 4 treatments \(\times\) 5 replicates). They were established containing 1.8 kg of natural sediment at a depth of approximately 8 cm, with exposure mesocosms dosed with tyre particles (0.2 (low), 1 (medium), and 5% (high) by dry weight of sediment [7]). Given the propensity of tyre particles to clump, they were added to solutions of filtered seawater with added surfactant (0.1% \(v/v\) Tween-20, Sigma Aldrich, St Louis, MO, USA) and vortexed to ensure the tyre particles were well dispersed in the sediment [29]. Sediment and tyre particle solutions were mixed vigorously with a fork in a glass 5 L beaker until homogeneous, before addition to the mesocosm. Controls (0% tyre particles by dry weight of sediment) were prepared the same way without tyre particles.  
To provide a fresh seed population from which microphytobenthos and meiofauna could be established, extra sediment was collected from the upper 1 cm of the Yealm Estuary at the start of the experiment. Sediment was sieved and dosed with the appropriate tyre particle concentration, and 200 g was added to the surface of each mesocosm.  
A total of 30 g of _S. plana_ (5 individuals) and 7 g of _H. diversicolor_ (approximately 20 individuals) were added to each mesocosm to provide a model estuarine macrofaunal community. In this experiment, the effects of tyre particles were not assessed for the macrofauna; for effects, please see [PERSON] et al. (2022). Mesocosms were maintained in the aquarium under natural light, and their positions were rotated three times a week to ensure that each mesocosm received similar levels of photosynthetically active radiation (PAR). PAR was measured weekly throughout the experiment using an SKP200 PAR meter (Skye Instruments Ltd., Llandrinodd Wells, UK). Filtered seawater, adjusted to 23 ppt salinity, was maintained in an 800 L tank and chilled to 13 \({}^{\circ}\)C using a Boyu water chiller (L-075, Huanggangzen, China) before being supplied using a closed partial recirculation system at a rate of 600 L/H. After 31 days, the experiment was terminated, and overlaying water was carefully removed from the mesocosms. This is the final time point that destructive sampling took place for meiofauna, organic matter, chlorophyll content, and the condition index.