Controlling harmful algal blooms in constructed wetlands: A hydrodynamic cascade driven by engineered microtopography

Constructed wetlands are widely used for eutrophication control, yet their efficacy in mitigating harmful algal blooms (HABs) is often limited by homogeneous flow paths that promote water stagnation and cyanobacterial dominance. This study introduces a novel, nature-based solution: engineered microtopography with a "deep-shallow-shoal" continuum, designed to initiate a hydrodynamic cascade that physically suppresses HABs. Through integrated computational fluid dynamics modeling and year-long field monitoring, we demonstrate that microtopography creates heterogeneous flow regimes-low-velocity vortices in deep zones enhance algal settling, while high turbulence in shallow zones disrupts cyanobacterial aggregation. These hydrodynamic patterns drive spatiotemporal gradients in key water quality parameters (TN, TP, COD_Mn, dissolved oxygen), which in turn promote a shift in phytoplankton functional groups from high-risk cyanobacteria to low-risk diatoms and cryptophytes. Structural equation modeling confirmed that flow field heterogeneity, mediated by water temperature (path coefficient = 0.868) and water depth (path coefficient = 0.17), is the primary driver of this transition. We propose a "Hydrodynamic-Template Cascade Model" to describe the causal pathway from physical habitat structure to algal risk reduction. This work provides a predictive, mechanism-based design strategy for sustainable wetland engineering, enabling proactive HAB control without chemical inputs.