RESUMEN
Isotopic, biogeochemical and ecological structure can provide a new dimension for understanding material flows, and the simultaneous function and structure of an ecosystem. Distributions of δ13C and δ15N for biogenic substances in the Nanakita river estuary involving Gamo lagoon in Japan were investigated to construct isotope biogeochemical and ecological structure for assessing fate and transfer of organic matter, and food web structure. The isotopic framework of the ecosystem was successfully described in a δ15N–δ13C map. In this estuary the variations of isotope ratios of biogenic substances were clearly explained by the mixing of land-derived organic matter, and marine-derived organic matter. A trophic-level effect of 15N enrichment was clearly observed. Organisms were classified into three groups depending upon the contribution of land-derived organic matter in a food chain. Almost all biota except mollusca in the lagoon depend on organic matter of marine origin. The contributions of both land and marine organic matter were comparable for mollusca in the lagoon.