Shear redistribution in confined biofilm systems: decoupling structure and function in engineered water environments

Biofilms in engineered water systems often develop within confined, tortuous flow paths where shear stress is spatially heterogeneous. However, biofilm responses are commonly evaluated using bulk metrics that treat shear as uniform and externally imposed, overlooking how geometric confinement redistributes shear during biofilm growth. Here, we examined how confinement-driven shear redistribution governs biofilm-hydrodynamic interactions in labyrinth microchannels under constant pressure. By integrating time-resolved biofilm-altered geometries with flow simulations, we show that localized biofilm accumulation reorganizes preferential flow pathways, redistributes wall shear stress, and causes a disproportionate decline in discharge. Hydraulic deterioration was increasingly governed by constriction of the remaining effective flow paths rather than by biomass accumulation alone, revealing nonlinear hydraulic sensitivity to spatially localized growth. Controlled-shear experiments further showed that ATP-based physiological indicators remained comparatively stable and peaked at intermediate shear levels, whereas extracellular polymeric substance composition shifted toward higher protein-to-polysaccharide ratios with increasing shear. Together, these results demonstrate a partial decoupling between biofilm structure and activity-related or matrix-compositional indicators under confinement-generated shear redistribution, highlighting the limitations of bulk structural metrics for predicting biofilm behavior and hydraulic performance.