Assessment of potential impact of invasive vegetation on cohesive sediment erodibility in intertidal flats.

In-situ erodibility experiments were conducted to reveal the effects of vegetation on sediment stability in an intertidal flat. Spartina alterniflora (Spartina), one of the most widespread types of vegetation, led to complexity in sediment erodibility. The long stems and leaves of Spartina, which grew to approximately 156 cm from May to November 2019, were effective in trapping suspended sediments in the water columns, eventually promoting the deposition of approximately 2.3 cm within its communities. Sediments eroded by increasing bed shear stress (τ) mainly originated from sediments that were adhered to the stems and leaves of Spartina (May: 76%; November: 54%). They protected subsequent bed erosion against τ. However, this was only an apparent effect because the Spartina caused the erosion rate (E) to stagnate by suppressing the outflow of eroded mass from the bed. As the protective effect of the stems was removed, the uppermost sediment layers in the Spartina communities became more vulnerable to erosion by τ, with the initial erosion thresholds lowered to 0.1 Pa (May) and 0.05 Pa (November). Despite continuous sedimentation by sediment trapping, the sediment bed in Spartina communities had not been consolidated under repeated tidal inundation, showing no distinct development of the critical shear stress for erosion. Thus, the differences in E between cases with or without Spartina's stems reached approximately 1.22 × 10 (May) and 1.83 × 10 kg m s (November) at τ = 0.6 Pa. Results suggest that the increase in thickness of erodible layers mainly contributed more than sediment volume fraction to the enhancement of erosion potential with τ. This study highlights the necessity to assess both positive and negative effects of Spartina on the stability of sediment beds in intertidal flats.