A preliminary exploration of the physical properties of seagrass wrack that affect its offshore transport, deposition, and retention on a beach

Abstract

The transport, deposition, and decomposition of seagrass wrack facilitate significant marine subsidies of material, energy, and organisms to the terrestrial environment. Over the past decade we have improved our understanding of the on-beach decomposition of seagrass wrack and its impact on beach and island communities; however, there is a paucity of research on the transport processes that supply wrack to the beaches. The physical properties of wrack affect its buoyancy and therefore transport, but these properties vary with species, the condition of the wrack when it was formed, the time since the wrack was generated and its ambient environment in the sediment, the water column, at the water surface or on the beach. Understanding how wrack physical properties vary under a range of conditions is needed to predict wrack transport, yet these properties have not previously been reported. We modified classical parameterizations of particle transport to identify knowledge and data gaps for wrack transport processes. We present a preliminary exploration, for Posidonia sinuosa leaves and Amphibolis antarctica stems and leaves, of settling velocities of wrack fragments, critical shear stresses required for their resuspension, bulk physical characteristics of wrack accumulations on beaches (e.g., bulk density, porosity), and physical properties of key wrack components (e.g., tissue density, tensile strength). We also determined how these properties change with drying, aging, and subsequent rewetting.