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

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 floating Sargassum in mid-shelf waters of the Gulf of Mexico. Stable carbon isotope ratios (delta C-13) of 4 autotrophs (Cladophora sp., phytoplankton [based on particulate organic matter, POM], S. fluitans, S. natans) were distinct (range -16.3 to -21.0 parts per thousand), with S. fluitans and S. natans enriched by 2 to 5 parts per thousand relative to Cladophora sp. and POM. Stable nitrogen isotope ratios (delta N-15) of both S. fluitans and S. natans were depleted by 5 to 7 parts per thousand compared to Cladophora sp. and POM. The majority of VC values of consumers were between-16 and-18 parts per thousand, and VC values were most depleted for juvenile shrimps, juvenile crabs and certain juvenile fishes (e.g. Aluterus heudeloti, Monacanthus hispidus, Abudefduf saxatilis, Histrio histrio, Seriola dumerih). Stable carbon isotope ratios of adult fishes varied from -16.1 to -17.5 parts per thousand. Enrichment of delta N-15 occurred with increasing trophic position, and the lowest values were observed for juvenile crustaceans, which ranged from 6.0 to 8.7 parts per thousand. The majority of juvenile fishes were secondary heterotrophs (delta N-15 values ca. 8.0 to 11.0 parts per thousand), while most adult fishes were tertiary consumers with delta N-15 values ranging from 11.9 to 14.3 parts per thousand. 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. C-22 polyunsaturated fatty acids (PUFAs) (22:6n-3, 22:5n-3) were most abundant in POM, while high levels of C-18 and C-20 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 juvenile and 6 adult fish species were highly similar to POM and distinct from the other producers within the Sargassum complex.