Resuspension created by bedload transport of macroalgae: implications for ecosystem functioning

Previous studies suggest that current-driven plant transport in shallow lagoons and estuaries is associated with increased turbidity. Our hypothesis is therefore that macroalgae erode surface sediment while drifting as bedload. This ballistic effect of moving plants on surface sediment was tested in a series of controlled annular flume experiments, where simultaneous measurements of macrophytes transport and turbidity were conducted at increasing current velocities. Sediment erosion always started earlier in experiments with plants than in control experiments without plants. Turbidity increased immediately when plants started to move at current velocities of 2–4 cm s⁻¹. From a background concentration of 7–10 mg SPM l⁻¹, turbidity increased to 30–50 mg SPM l⁻¹ for Ceramium sp., Ulva lactuca and Chaetomorpha linum, while the more rigid Gracilaria sp., caused much higher turbidities (50–180 mg SPM l⁻¹). Such plant induced sediment erosion at low current velocity can explain the observed appearance of turbid waters in estuaries and lagoons in the absence of strong wind and wave action. Based on 3-D hydrodynamic modelling, it was determined that plant driven erosion occurs during most of the growth season in a shallow eutrophic estuary (Odense Fjord, Denmark).