MEPS 313:249-259 (2006)  -  doi:10.3354/meps313249

ABSTRACT: 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 (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‰ relative 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 tertiary 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 juvenile and 6 adult fish species were highly similar to POM and distinct from the other producers within the Sargassum complex.

KEY WORDS: Food web · Diet · Pelagic ecosystem · Trophic position · Large pelagic fishes