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Natural dietary markers (stable isotopes and fatty acids) were used to determine the trophic structure and characterize carbon source(s) of juvenile and adult fishes associated with float- ing Sargassum in mid-shelf waters of the Gulf of Mexico. Stable carbon isotope ratios (δ13C) of 4 autotrophs (Cladophora sp., phytoplankton (based on particulate organic matter, POM), S. fluitans, S. natans) were distinct (range -16.3 to -21.0‰), with S. fluitans and S. natans enriched by 2 to 5‰ rel- ative to Cladophora sp. and POM. Stable nitrogen isotope ratios (δ15N) of both S. fluitans and S. natans were depleted by 5 to 7‰ compared to Cladophora sp. and POM. The majority of δ13C values of consumers were between -16 and -18‰, and δ13C values were most depleted for juvenile shrimps, juvenile crabs and certain juvenile fishes (e.g. Aluterus heudeloti, Monacanthus hispidus, Abudefduf saxatilis, Histrio histrio, Seriola dumerili). Stable carbon isotope ratios of adult fishes varied from -16.1 to -17.5‰. Enrichment of δ15N occurred with increasing trophic position, and the lowest values were observed for juvenile crustaceans, which ranged from 6.0 to 8.7‰. The majority of juvenile fishes were secondary heterotrophs (δ15N values ca. 8.0 to 11.0‰), while most adult fishes were ter- tiary consumers with δ15N values ranging from 11.9 to 14.3‰. Carbon source estimates from a 2-source mixing model indicated that the 78% of organic matter supplied to consumers (pooled across taxa) in the Sargassum complex was derived from POM. Fatty acid signatures of the primary producers were significantly different, and were used to further evaluate organic matter contribution to Sargassum-associated consumers. C22 polyunsaturated fatty acids (PUFAs) (22:6n-3, 22:5n-3) were most abundant in POM, while high levels of C18 and C20 PUFAs were observed for Cladophora sp. and Sargassum spp. (18:2n-6 and 20:4n-6, respectively). Consumer signatures were dominated by 22:6n-3, and principal component analysis indicated that fatty acid signatures of each of the 6 juve- nile and 6 adult fish species were highly similar to POM and distinct from the other producers within the Sargassum complex.

Trophic ecology of Sargassum-associated fishes in the Gulf of Mexico determined from stable isotopes and fatty acids