Beaches and coastal dunes act as natural barriers from sea level rise and extreme storms, providing significant protection services to coastal communities. These services are strengthened by sediment-stabilizing dune vegetation that facilitate dune building. However, sandy coastal environments are generally considered nutrient poor, potentially placing constraints on plant production. In the Pacific Northwest (PNW) ~45% of the coast is dune-backed sandy beach with the dominant vegetation consisting of two highly productive invasive grasses, Ammophila arenaria and A. breviligulata. We asked whether marine subsidies, in the form of macrophyte wrack, play a role in dune vegetation production, and thus the resultant geomorphology and associated coastal protection services of dunes more generally. To explore the role of marine wrack to dune vegetation, we surveyed 15 dune-backed beach sites from southern Washington to southern Oregon that vary in their exposure to ocean upwelling and their proximity to estuaries and headlands (which provide different sources of macrophytes). At each site, we measured the biomass and species composition of the wrack patches and dune plants along the beach-to-dune profile. We also collected wrack and dune plants to be analyzed for stable isotopes and nutrient content along the dune profile and at different sites.
Results/Conclusions
Wrack species composition and biomass varied with proximity to macrophyte source habitat and ocean upwelling and estuarine outwelling intensity. In the north, where upwelling is weaker and large estuaries are common, outwelling moves wrack from estuaries to the coast. At these sites there were higher proportions of eelgrass wrack. In the south, where stronger upwelling fuels macroalgal productivity and rocky habitats are more common, kelp contributions to wrack were greater. We found strong support for the incorporation of marine nutrients into dune plant tissue. Plant %N and δ15N signatures were enriched at the dune toe nearest the beach and at southern sites that had higher kelp wrack production. Furthermore, plant %C was greatest at the heel of the dune and at northern sites that had higher seagrass wrack production. Our findings suggest that marine subsidies delivered onshore could be a significant nutrient source for sand-stabilizing, invasive dune grasses, and thus be important to dune building and coastal protection services. We plan to expand on these results to explore the potential climate-driven shifts in beach grass production due to changing ocean and atmospheric conditions and to determine how such shifts might influence the protection services provided by this critical coastal ecosystem.