Sandy beach ecosystems are generally characterized by low nutrient conditions, potentially constraining production in dune-building coastal vegetation. However, these systems are also frequently subsidized with macrophyte wrack (i.e., macroalgae, seagrass, and other estuary plants) from the coastal ocean. While wrack deposits are known to play a significant role in coastal food web structure and diversity as a base resource for sandy beach communities, the relationships between wrack deposition and dune vegetation production remain relatively uncharacterized. In this study, we explored the application of natural abundance stable isotope analyses to evaluate the contribution of marine nitrogen to common dune plant species in the U.S. Pacific Northwest coastal dune ecosystem. We surveyed the beaches and coastal dunes at 15 sites spanning 700km of coastline, comparing plants and wrack deposition patterns across sites and dune profile locations.
Results/Conclusions
We found not only that dune plants are using marine nitrogen, but also that there is a spatial signature of marine nitrogen use along the dune profile. Plant nitrogen isotope signatures were enriched at the foredune toe (nearest to the wrack-nutrient source) with enrichment decreasing over the foredune profile (as distance from the beach and nutrient subsidy increased). Our findings suggest that wrack deposits are a significant nutrient source for coastal dune vegetation, with higher production of beachgrasses at sites with more wrack deposition. Understanding this subsidy-production relationship could prove essential to dune formation and coastal protection services given the key role of coastal vegetation in sediment stabilization and dune building. Furthermore, as ocean and atmospheric conditions increase in unpredictability and nitrogen subsidies change with climate, these plant production dynamics gain in importance and relevance. Such variability in growth environment could shift dune plant production and thus the protection services provided by coastal dunes.