The global distribution and anticipated increase of environmental microplastic (MP) pollution are concerning. However, while the impacts of macroplastic litter on wildlife are apparent, we know relatively little about the MP hazard potential. Moreover, the current ecotoxicological methodology is inadequate for solid waste particles and MP hazard assessment because it fails to distinguish particle and chemical effects. This thesis improves our understanding of the particle effects of MP relative to other microparticles.

First, a comparative analysis of effect studies on MP and mineral particulates across different biological organisation levels revealed high similarities in responses between these materials (Paper I). At the suborganismal levels, the similarity in the effect concentrations suggests shared particle effect mechanisms. At the higher levels, however, MP induced more severe impacts, possibly due to chemical leaching. Moreover, the highly variable MP effect concentrations motivated exploring the role of polymer properties and ageing status on MP effects; these aspects were addressed in Papers II-III.

In Paper II, the possibility of MP acting as a vector of contaminants was evaluated, showing enhanced transport of highly hydrophobic organic contaminants (HOC) at very high HOC and MP concentrations. However, observing it at environmentally relevant contaminant levels would be unlikely. 

Paper III compared behavioural and physiological responses in benthic amphipods to MP exposure using different polymers (polystyrene and polyethylene terephthalate) and clay as a non-plastic reference particle. The amphipods avoided sediments with high concentrations of the added material regardless of the material type, including aged and virgin MP and clay. 

Solid waste, including MP, co-occur with various suspended solids in aquatic environments; therefore, the natural solids can serve as reference material when evaluating the MP particle effect. In Paper IV, a novel method for testing MP effects in mixtures with reference particles was proposed. In the exposure experiment with daphnids, the method was used to derive hazard thresholds for the MP contribution to suspended matter conditional on the total suspended solid concentration in the water. 

Together, these studies add to our understanding of MP-biota interactions and suggest that similarly sized MP and natural particulates share similar particle effects. However, MP might have a higher potential as vectors of chemical contaminants, which needs to be further evaluated in environmentally relevant settings.