Cascading processes linking bioturbation, sediment dynamics and geomorphic response

Biological processes play an important role in regulating the morphodynamic stability of tidal flats, and quantifying these biogeomorphic feedbacks is essential for coastal protection and sustainable management. However, compared to physical and chemical drivers, the mechanistic role of biological processes in hydrodynamic and geomorphic interactions remains inadequately understood. This study investigated a typical silty tidal flat in the Yangtze River Delta through integrated in-situ manipulative experiments and hydrodynamic monitoring. Here, we identified a dual pathway through which bioturbation may regulate tidal-flat stability: (1) Bioturbation increased bed roughness. Two transplantation experiments showed that the drag coefficient, used here as a proxy for effective bed roughness, increased to 2.61 and 2.77 times the values at the control station, respectively. This increase was accompanied by an approximately 20% increase in wave-induced shear stress, a 21∼73% increase in suspended sediment concentration, and 30∼40 mm of fluff-layer erosion under normal winds. (2) Bioturbation was associated with reduced sediment resistance to erosion. Specifically, the critical shear stress for sediment erosion decreased by 72.45%, accompanied by enhanced fluff layer erosion, resulting in mass erosion with a maximum depth of 110 mm under strong winds. In summary, the coupling of these two effects under strong winds drove the transition of tidal flats from centimeter-scale fluff layer erosion to decimeter-scale mass erosion.