ABSTRACT: Organic matter from autochthonous and allochthonous sources provides energy and nutrients to nearshore food webs including filter-feeding bivalves. In Puget Sound, Washington, USA, the degree to which shellfish rely on these different organic matter subsidies may be important for their management and that of nearshore food webs in general. We explored patterns of terrestrial-marine connectivity in a large, temperate estuary using a combination of oceanographic modeling and isotopic mixing models. We first examined spatial connectivity by modeling freshwater contributions of the major river basins to Puget Sound (potential connectivity), then estimated the relative contribution of terrestrial, nearshore, and marine organic matter sources to nearshore particulate organic matter (POM) (actual connectivity) and to the diets of Pacific oysters Crassostrea gigas (realized connectivity). To estimate actual and realized connectivity, we analyzed the δ¹³C and δ¹⁵N values of oyster tissue, POM, and primary producers from intertidal, offshore, salt marsh, and upland habitats across the dry (summer) and wet (fall-winter) seasons. Mixing models indicated that both oyster bed POM and oyster diets were composed largely of intertidal macrophytes and salt marsh plants, with less important contributions of phytoplankton, benthic diatoms, and upland vegetation. Our findings suggest that oyster production may be driven more by coastal and marine primary production than by riverine sources, even in a fjord subject to strong freshwater influences.