Retrospective determination of trophic relationships among pelagic fishes associated with Sargassum mats in the Gulf of Mexico

Stable isotopic composition of flora and fauna associated with floating Sargassum mats in the NW Gulf of Mexico was measured to identify the origin(s) of organic matter used by pelagic fishes and determine trophic relationships of associated taxa. Stable carbon (delta(13)C) and nitrogen (delta(15)N) isotopes were quantified from tissue samples of the primary autotrophs and several other members of the Sargassum complex (herbivores through apex predators). Distinct delta(13)C values were observed among autotrophs: Sargassum natans(-17.1 +/- 0.4%(0)),Sargassum fluitans (-16.3 +/- 0.2%(0)), epiflora (-18.3 +/- 0.9%(0)), and phytoplankton (particulate organic matter) (-21.0 +/- 0.5%(0)), suggesting the approach has promise for identifying source(s) of organic matter. In addition, nitrogen stable isotopes indicated that four distinct trophic levels were present in the Sargassum complex. Spatial and temporal patterns in delta(13)C and delta(15)N values were observed for selected autotrophs; and heterotrophs; however, signatures were relatively stable over scales investigated. Based upon expected modification due to fractionation, a simple mixing model predicted that the majority of organic matter reaching top-level consumers originates from phytoplankton production. Findings from this study challenge the theory that Sargassum is a direct source of energy to pelagic fishes, and therefore the value of this unique complex may be limited to its role as refuge for pelagic fauna.