Trophic Relationships within a Subtropical Estuarine Food Web from the Southeast Gulf of California through Analysis of Stable Isotopes of Carbon and Nitrogen

Abstract

We identified the sources of carbon supporting an estuarine food web in the Southeast Gulf of California. The trophic food web in the Estero de Urias Lagoon (EUL) was studied through the carbon and nitrogen isotopes in the potential food sources (plankton, macroalgae, plants) and organisms including filter-feeding mollusks, crustaceans, fishes and seabirds. The isotopic composition of sediment suspended organic matter (SSOM) and suspended particulate organic matter (SPOM) showed that there are diverse organic matter sources in EUL. The greater inputs of mangrove to detritus were reflected in their similar delta C-13 values with respect to SSOM and SPOM. The delta C-13 data suggest a direct transfer of C from SSOM and zooplankton to filter-feeders organisms and to a lesser degree from SPOM and phytoplankton. The isotopic composition of the different groups of organisms showed the complexity of the food web. However, there was a continuous gradient of N-15-enrichment from SSOM and SPOM to seabird with intermediate values for filter-feeders and crustaceans. The delta N-15 values in the EUL food web were consistent with 5 trophic levels. Fishes were strongly dependent on macrobenthos and pelagic derived nutrition. Cormorants occupied the highest trophic level and its major diet contributors were fishes. The studied food web was not segregated by time because isotopic trends were similar between dry and wet seasons.