Land-Ocean Coupling of Carbon and Nitrogen Fluxes on Sandy Beaches

Rivers link oceans with the land, creating global hot spots of carbon processing in coastal seas. Coastlines around the world are dominated by sandy beaches, but beaches are unusual in that they are thought to rely almost exclusively on marine imports for food. No significant connections to terrestrial production having been demonstrated. By contrast, we isotopically traced carbon and nitrogen pathways leading to clams (Donax deltoides) on beaches. Clams from areas influenced by river plumes had significantly different isotope signatures (delta C-13: -18.5 to -20.2aEuro degrees; delta N-15: 8.3-10.0aEuro degrees) compared with clams remote from plumes (delta C-13: -17.5 to -19.5aEuro degrees; delta N-15: 7.6-8.7aEuro degrees), showing that terrestrial carbon and sewage, both delivered in river plumes, penetrate beach food webs. This is a novel mechanism of trophic subsidy in marine intertidal systems, linking the world's largest shore ecosystem to continental watersheds. The same clams also carry pollution signatures of sewage discharged into rivers, demonstrating that coastal rivers connect ecosystems in unexpected ways and transfer contaminants across the land-ocean boundary. The links we demonstrate between terrigenous matter and the largest of all marine intertidal ecosystems are significant given the immense social, cultural, and economic values of beaches to humans and the predicted consequences of altered river discharge to coastal seas caused by global climate change.