Influence of Wrack on Coastal Foredune Development and Dune Habitat Complexity

Beach wrack (stranded algal and vegetation debris) is an important element of sandy coastlines. Wrack not only supports a biodiverse assemblage of organisms, it facilitates plant establishment on the backshore (via wrack-delivered seeds and rhizomes) and influences aeolian sediment transport. Yet, for recreating beach-goers, wrack is often viewed as unsightly, odorous, and it is commonly removed to enhance the recreational value of beaches. Although the ecological impacts of beach wrack and raking are well-described, only a handful of studies have examined how wrack influences the ecomorphodynamics of foredunes. Here, we examine how the frequency and quantity of wrack delivery alters the development of coastal dunes using a modified version of the Coastal Dune Model (CDM).

Wrack may alter backshore ecomorphodynamics via two mechanisms. First, detrital wrack (e.g., clumps of dead algae, seagrass) adds an immediate, ephemeral surface roughness element that decays over time as it desiccates, gets buried, or remobilizes during subsequent high tides. Second, vegetative wrack (e.g., individual living seeds and/or rhizomes capable of forming new plants) has the potential to create a persistent surface roughness element if it results in the growth of new vegetation. We examine these dual roles by modeling the dynamics of how detrital wrack and vegetative wrack influence beach and dune morphology using the CDM.

We find that increases in the rate of detrital and/or vegetative wrack lead to the formation of steeper beaches with shorter, wider, and more topographically complex dunes. We also find that beach-building by detrital wrack allows vegetation to colonize the backshore at more seaward-located positions because it elevates the backshore, thereby reducing mortality from wave overwash. These results suggest that both the surface roughness component and the vegetation colonization component of wrack influence backshore slope, the location of the vegetation line, and the topographic complexity of foredunes. This has management implications: though the removal of wrack (e.g., through beach raking) may improve recreational beach value and storm protection by increasing beach width and dune height, it also results in a less complex dune topography that will likely support less diverse ecological communities.