Modeling microplastic transport in open channel flows and ingestion dynamics by benthos

Understanding the dynamics of unintentional microplastic (MP) ingestion by benthos in aquatic environments is crucial for assessing the ecological impacts of MPs. Yet, this process remains poorly understood. To address this, we developed a high-fidelity, two-way coupled numerical model that integrates large-eddy simulation and Lagrangian point-particle tracking techniques. Three key parameters are examined: benthos predation types (filter-feeders, grazers, and burrowers), MP density, and benthos density, with benthos density emerging as the dominant factor. Specifically, an eightfold increase in benthos density results in a 5- to 22-fold rise in ingested MPs. Benthos types influence the final ingestion proportion (defined as the ratio of ingested to released MPs), with grazers showing the highest ingestion efficiency, followed closely by filter feeders-both approximately doubling the ingestion rate observed in burrowers at equivalent benthos density. MP density has minimal influence on ingestion across all benthic groups and densities, except under high-density filter-feeder conditions. Two distinct MP transport models during ingestion are identified: (i) a suspension mode observed in filter-feeders and (ii) a sliding mode prevalent in grazers and burrowers. The Rouse number (P) effectively differentiates these models, with the suspension mode dominating when P < 2.5 and the sliding mode dominating when P > 2.5. The Rouse number and spanwise turbulence intensity govern the number of MPs ingested by each benthic individual, while the cumulative predation width of all benthos accounts for the impact of benthos density and types. Consequently, the product of these two parameters serves as a robust predictor for the final MP ingestion proportion, where a strong linear relationship is observed across all simulations.