Evaluation of riverine macro- and mesoplastic monitoring approaches.

Globally, plastic pollution in aquatic environments has become an emerging concern. A multitude of monitoring techniques have been used to collect information on the presence of macroplastics (> 25 mm) and mesoplastics (> 5 mm ≤ 25 mm) in river systems. The differences between these methodological approaches, combined with limited knowledge of the fundamental processes that affect plastic presence and detection ability, lead to a lack of standardization between methods and limit comparisons of quantitative data on plastic pollution among studies. This study investigates how different monitoring approaches, e.g., larvae net, trawl net, and stow net, affect the observed abundance and composition of macro- and mesoplastics in the Rhine-Waal Rivers. Additionally, we performed a SWOT analysis highlighting the strengths, weaknesses, opportunities, and threats of the three methods. During trawl net and stow net monitoring, more unique macro- and mesoplastics categories were found in comparison with simultaneous larvae net monitoring. However, while mean macroplastic concentrations were higher in the stow net samples (1.22 × 10-3 items/m3) than in the larvae net (0.91 × 10-3 items/m3), mesoplastics concentrations were significantly higher in the larvae net samples (6.91 × 10-3 items/m3) compared to those recorded with the stow net (1.15 × 10-3 items/m3), indicating that the size plastic recovery rates depend on the sampling technique. The SWOT analysis pointed towards a better overall performance of the trawl net. The outcome of the current study can be used to support policymakers, industry, and the scientific community in developing successful monitoring strategies for macro- and mesoplastics pollution in rivers. The SWOT analysis may facilitate the choice of the approach that best aligns with the specific monitoring goals and the environmental conditions of the target area.