Freshwater Microplastics: Challenges for Regulation and Management
“Microplastics” refer to a large group of polymers with various chemical and physical properties, originating from different sources and entering the environment via different pathways [21,98]). Verschoor  identified five commonly applied criteria to define MP: (1) synthetic materials with high polymer content, (2) solid particles, (3) <5 mm, (4) insoluble in water and (5) not degradable. However, discussion goes on whether tire abrasion should be considered as “microplastics” and the same applies to the definition of a lower limit for particle size. The smaller the particles are, the more species might potentially ingest those. Furthermore, smaller particles can permeate through membranes and, hence, pose a higher risk for adverse effects in organisms. Thus, globally agreed standardization approaches covering the whole range from sampling to effect assessment are required in order to provide a basis for risk assessment and regulatory options.
As a first approach for MP assessment in freshwater environments, Miklos et al.  suggested a modular system initiating with the quantification of selected indicator polymers. When the concentration of these polymers exceeds a certain level, more specific analyses should be conducted. These subsequent analyses can take various criteria (such as polymer type, size, shape, additives, etc.) into consideration to further categorize the particles and support the selection of adequate mitigation measures. Here, approaches from chemical regulation might serve as examples where chemicals can be categorized based on molecular similarities (e.g., PAHs, PCBs, etc.), by field application (e.g., pesticides), or according to their mode of action (e.g., endocrine disruptors). Similarly, MPs can also be grouped together based on their physicochemical properties (e.g., polymer type, density), by their application (e.g., cosmetics, carrier bags, electrical devices) or by (eco)toxicological impacts.
The chemical effects could be the result of polymers themselves or their additives, or a combination of both, while mechanical effect could depend on particle size, shape or a combination of both. Since the (eco)toxicologists face the challenge to test the effects of myriads combination, there are efforts to prioritize and start with the presumed most harmful combinations. Ideally, these results will be transferable to a group of similar combinations.
The physicochemical properties of MP might lead to different behavior in test systems. High-density polymers will settle as sediments whilst the low-density polymers float on the surface. They are available only for surface-feeding organisms and are ar higher risk in environmental systems as they might feed selectively on floating materials and accumulate them from the water phase. Chemical testing is usually not focused on this feeding type. Above all, scientists should contemplate that organisms are adapted to natural particles of different materials (sand, clay or similar), but with similar properties as MP, in their natural habitats. It is critical to perform tests on MP particles in comparison with such natural particles. This implicates especially to tests regarding the “Trojan horse effect”— the transport of hydrophobic substances via MP into organisms . Studies need to address whether or not there are differences in the sorption of chemicals to MP versus natural particles.
To reduce the influx of MPs into the aquatic environment, there is a need to improve the management of wastewater and solid waste apart from affecting the production or application of plastics. Starting with product design, we can apply a range of possibilities that includes degradable polymers, polymers with high recycling quotas or a product design promoting a long and circular product life to reduce waste .
One of the most sustainable measures would be a social change, and to achieve such long-term objectives, policy can apply so-called persuasive instruments such as public information, environmental education and funding of research and development. Precisely because we currently know little about the consequences of MPs in aquatic systems, we should develop and implement measures to reduce further emissions.