The foodweb of El Tóbari Lagoon (central-east Gulf of California) was studied for four seasons through the carbon and nitrogen isotopic characterization of primary producers, invertebrates, fish, birds and potential food sources. The range of δ13C measured was much wider for potential food sources than for consumers. Many organisms presented enrichments of δ13C and δ15N values. There was a clear trend toward increasing δ13C and δ15N from base organisms to top-predators in the four seasons. The isotopic and percentage of contribution data confirmed that suspended particulate organic matter and phytoplankton are the main organic source supporting the foodweb. Our results also imply the occurrence of a nutrient transfer from zooplankton to some invertebrates and juvenile fishes. Consumers were composed in four trophic levels, with trophic level 2 occupied by zooplankton and filter-feeders and trophic level 4 occupied by carnivorous fish and most bird species. Carnivorous fish exhibited dietary similarities by a considerable sharing of resources, which could be related to the abundance of possible prey, between invertebrates and juvenile fishes. Crustaceans and fish represented the main food sources of birds, although some birds showed more dietary variation (marine and offshore prey).

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