ABSTRACT: Bivalves influence both the ecology and the economy of coastal regions. By filter-feeding on particles in the water column, these organisms reduce turbidity and link benthic and pelagic production. In addition, production and sales of harvested bivalves are a source of income in coastal areas like the Eastern Shore of Virginia (USA). Phytoplankton are known to be a main food source to many bivalves; however, ocean-side lagoons off the coast of Virginia support extensive aquaculture of Mercenaria mercenaria (hard clams) in waters with relatively low chlorophyll concentrations. The ultimate energy sources supporting these clams are uncertain but significant because seagrass restoration, sea level rise, and climate change will potentially change the quality and quantity of primary production available to these populations. We measured the C, N, and H isotopic ratios of aquaculture clams and a variety of primary producers in a Virginia coastal lagoon over an annual cycle and conducted a Bayesian mixing model analysis to identify current energy sources for clams. By adding a third isotopic ratio (hydrogen), we were able to improve precision over a 2-isotope model based on C and N isotopes. Our analysis reveals that field-cultured clams in Virginia coastal lagoons are significantly supported by microalgae (23 to 44%) but gain most of their energy from macroalgae (55 to 66%), and only a small fraction from macrophytes (0 to 14%). While macroalgae are often an indicator of coastal eutrophication, these algae can be an important food source to bivalves when abundant in low nitrogen, oligotrophic systems. Our results also indicate hydrogen stable isotopes are useful in concert with other isotopes for tracing sources in coastal food webs.