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

Abstract

The accumulation of seagrass wrack on beaches provides a link between the marine and terrestrial environments and has been shown to play an important role in beach ecology. However there are parts of the world where accumulations of wrack are so pronounced that the wrack piles affect beach shape, and erosion can block harbors and breakwaters. The production of poisonous gases caused by its decay can become a human health hazard. Ideally, numerical models should allow us to predict wrack accumulation under a variety of management scenarios; however, there are few data to use in such numerical modeling. Indeed our understanding of how wrack is deposited to and removed from beaches is limited. This paper provides an exploratory investigation of a range of physical properties of seagrass wrack and establishes a baseline understanding of how these physical properties might impact on its transport and accumulation onto beaches. 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.