Divergent filtration mechanisms of fibrous and non-fibrous microplastics in towing-net sampling toward a harmonized...

Accurate assessment of microplastic pollution is hampered by the lack of standardized sampling protocols, particularly regarding microplastics lost in sampling. This study investigates the filtering mechanisms of microplastics during surface towing sampling (i.e., mesh-filtered methods) and develops a framework to correct abundance data. Paired-gear experiments using neuston nets (mesh sizes of 50-500 μm) were conducted in Xiangshan Bay, China. Probabilities of relative retention and retention of microplastics on the meshes were analyzed using SELECT models, and master selection curves were derived for fibrous and non-fibrous microplastics. Results show that larger mesh sizes (150, 330, 500 μm) significantly underestimate microplastic abundance by 1.4 to 2.9 times compared to a 50-μm net, with fibers being the primary contributor to losses. The filtration mechanisms differ fundamentally between shapes. For non-fibrous microplastics, retention is governed by physical interactions between the mesh opening and the particle's shortest diameter. In contrast, the fibrous microplastics sample could slip through the mesh vertically, but the ability of fibers to slip through the mesh is not determined by a simple size-exclusion rule. Their high flexibility allows bending and passage even at lengths far exceeding the mesh size. When the fiber length exceeds a certain ratio relative to the mesh size, further increasing the fiber length no longer significantly affects fiber retention. Based on the master selection curve, a harmonized assessment framework was established to re-evaluate data from 27 previous studies. The resulting mesh-specific correction factors highlight the severe underestimation of fibers by commonly used nets (e.g., 330 μm). This study clarifies shape-dependent filtration mechanisms in mesh-filtered sampling and provides a practical tool to enhance the comparability of historical and future microplastic monitoring data, advocating for standardized use of sampling net with smaller mesh sizes for accurate fiber assessment.