## 4 Discussion
Environmentally realistic experimental set ups are increasingly important to develop the knowledge of how tyre particles and leachates might be affecting biodiversity and ecosystem functioning in the natural environment. There is also the possibility that during this short-term experiment, the tyre particles used did not leach enough contaminants to significantly impact primary and secondary consumers. In tyre leachate studies, tyre particles are suspended in seawater at 20 \({}^{\circ}\)C and kept in constant motion [5; 23; 45]. In our experiment, tyre particles were mixed with sediments and maintained at 13 \({}^{\circ}\)C with minimal mixing (no tidal mimic), which may result in lower leaching potential. It must also be taken into consideration that our tyre particles were manufactured by Tyre Recycling Solutions using water pulseTM technology. This process could render the particles more chemically inert than other production methods. These results are indicative of short-term results over one season. During the experiment, the temperature reflected UK seasonality, which falls within the UK average summertime temperatures. Temperature can affect macro- and meiofauna and other biological processes. Therefore, it is important to look at the results of this work as a case study due to the short time frame of the exposure, as different seasons and temperatures may elicit different effects. As this was a multi-trophic level experiment; the indirect effects of tyre particle exposure must be taken into consideration. _S. plana_ has been shown to reduce the feeding rate in response to tyre particle exposure [16]. _S. plana_ are deposit feeders and use their siphons to ingest sediment, meiofauna, and microphytobenthos from the sediment surface. This reduced feeding may negate any impacts of tyre particle exposure on primary and secondary activity. A further consideration is that any negative impacts of tyre particle exposure on meiofauna may only show up at the population level after a longer period of exposure. Sublethal effects of exposure may not become apparent during this experimental time period. Individuals may experience sub-lethal effects from the toxicants, such as reduced growth, reproduction, and behavioural changes, and this could result in more energy being put into survival mechanisms rather than growth and reproduction, as these are metabolically costly [46]. Sub-lethal effects may not manifest as changes in the community composition within the first generations, as there is no significant mortality; however, there may be changes to biomass, as fewer resources may be put into growth. Additionally, these effects may take longer than five weeks to manifest, as the meiofauna may have compensatory mechanisms that can help mitigate the impacts of the stressor, allowing them to carry out basic mechanisms to survive for a short period [47]. The primary effect observed during this study was the increased abundance of Tanaidacea in mesocosms exposed to a high concentration of tyre particles. Tanaidacea have a high tolerance for environmental stressors [48; 49] and can also change their feeding strategy based on the environmental conditions they find themselves in [33]. This may give them an advantage in the mesocosms exposed to tyre particles. Tanaidacea were only present in the medium and high concentration treatments, and in the high treatment, they had a greater average biomass compared to the medium treatment. Whilst we cannot eliminate the possibility that this is a chance occurrence, this may also be due to an indirect effect of tyre particle exposure, leading to an alteration in food web dynamics. A prior experiment showed that the macrofauna present (_H. diversicolor_ and _S. plana_) consumed tyre particles within sediments, leading to a negative impact on their condition [16]. _H. diversicolor_ has a range of feeding modes, including being an active predator [50]. A lack of Tanaidacea in the control and low tyre particle exposures may be due to high levels of predation.