Dune grass production is positively associated with marine nutrient subsidies on US Central Atlantic coast foredunes

Abstract

Resource subsidies, or the flow of energy across ecosystem boundaries, can influence community structure and function and act as important nutrient sources to less productive ecosystems, particularly in marine and coastal zones. Coastal dunes, connected to the nearshore through sandy beaches, likely depend on the delivery of marine subsidies to regulate the dune building biophysical feedback between vegetation and sand. Here we consider the role of marine subsidies to foliar nitrogen and dune grass production along the US Outer Banks coastline and ask: (1) Do macrophyte wrack biomass and composition, sand nitrate concentration, and dune grass production vary at local and regional scales? (2) Do dune grasses utilize marine-derived nitrogen (¹⁵N) and, if so, how does δ¹⁵N and %N vary across species, foredune profile locations, and islands? and (3) What factors, including macrophyte wrack biomass, sand nitrate, sand supply, and dune morphology, are important to foliar nitrogen (δ¹⁵N and %N) and dune grass production? We found that sand nitrate concentrations increased with latitude along the Outer Banks coastline but were unrelated to macrophyte wrack biomass. Dune grass δ¹⁵N was highest at the foredune toe and decreased landward, suggesting that grasses closer to the beach are using marine-derived nitrogen. Although dune grass %N content and production (density and biomass) did not vary across the foredune profile, they did increase with latitude. Moreover, sites with slightly negative shoreline change rates, steeper backshore slopes, and taller foredunes had greater sand nitrate concentration, foliar %N and δ¹⁵N, and dune grass production at the seaward side of the foredune. At the back dune, foliar δ¹⁵N and dune grass production were not correlated with sand supply or sand nitrates, indicating other factors at play on the landward side of the foredune. Given the important role of dune grasses in sand capture and dune building, any shifts in marine-derived nutrients from changes in ocean productivity or sand flux could have landscape-level consequences for dune building, coastal protection, and carbon storage services.