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Relationship between selected hydrodynamic indices and microplastic distributions across mesohabitats in urban rivers, Eastern Cape, South Africa
Summary
Researchers investigated how hydrodynamic indices — Froude number, Reynolds number, and shear velocity — shape the spatial and seasonal distribution of microplastics across different hydraulic zones in three subtropical urban rivers in South Africa's Eastern Cape Province. The study found that hydraulic conditions significantly governed microplastic retention in pools and backwaters versus riffles and runs, with seasonal variation between dry and wet periods.
Hydrodynamic conditions are critical drivers of microplastic (MP) transport and retention in riverine systems. However, their influence across different flow habitats remains poorly understood. This study investigated the role of three hydraulic indices, Froude number (Fr), Reynolds number (Re), and shear velocity (U*), in shaping the spatial and seasonal distribution of MPs across distinct hydraulic zones (pools/backwaters, runs/riffles) in three subtropical urban rivers in the Eastern Cape Province, South Africa. Microplastics were sampled in both suspended and settled fractions during dry/cold and wet/hot seasons. Samples were analysed via chemical digestion, density separation, stereomicroscopy, and Fourier Transformed Infrared spectroscopy (FTIR-ATR). Results revealed that settled MPs were strongly influenced by hydraulic dynamics, with Fr and U* showing significant zone- and season-dependent associations. The strongest effects occurred in run/riffle zones during the wet/hot season, where Fr had a significantly negative correlation with settled MP concentrations (p < 0.001). Notably, Fr was the only hydraulic predictor significantly associated with settled MP size class composition (multivariate p = 0.009), driven primarily by MPs in the 1-2 mm size range (p = 0.007), which appear most sensitive to hydraulic sorting. In contrast, suspended MPs exhibited weaker and more variable relationships with hydraulic indices, though elevated Re and U* values were occasionally associated with increased resuspension in deeper pool zones. Microplastic shape and size distributions did not vary significantly across zones and seasons in suspended samples, suggesting that turbulence and mixing override morphological sorting once entrained. These findings demonstrate that MP-hydraulic interactions are habitat- and season-specific, and underscore the importance of integrating hydraulic zoning, flow variability, and particle traits into MP monitoring and modelling frameworks. Improved understanding of these dynamics will support more accurate predictions of MP fate and inform targeted mitigation in fluvial systems.
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