How does vegetation affect sedimentation on tidal marshes? Investigating particle capture and hydrodynamic controls on biologically mediated sedimentation

摘要

Plants are known to enhance sedimentation on intertidal marshes. It is unclear,however, if the dominant mechanism of enhanced sedimentation is direct organicsedimentation, particle capture by plant stems, or enhanced settling due to a reduction inturbulent kinetic energy within flows through the plant canopy. Here we combine severalpreviously reported laboratory studies with an 18 year record of salt marsh macrophytecharacteristics to quantify these mechanisms. In dense stands of Spartina alterniflora(with projected plant areas per unit volume of >10 m−1) and rapid flows (>0.4 m s−1),we find that the fraction of sedimentation from particle capture can instantaneouslyexceed 70%. In most marshes dominated by Spartina alterniflora, however, we findparticle settling, rather than capture, will account for the majority of inorganicsedimentation. We examine a previously reported 2 mm yr−1 increase in accretion ratefollowing a fertilization experiment in South Carolina. Prior studies at the site have ruled outorganic sedimentation as the cause of this increased accretion. We apply our newlydeveloped models of particle capture and effective settling velocity to the fertilized andcontrol sites and find that virtually all (>99%) of the increase in accretion rates can beattributed to enhanced settling brought about by reduced turbulent kinetic energy in thefertilized canopy. Our newly developed models of biologically mediated sedimentation arebroadly applicable and can be applied to marshes where data relating biomass to stemdiameter and projected plant area are available.