Isotope baseline shifts in pelagic food webs of the Gulf of Mexico

Diazotrophic inputs by Trichodesmium have been shown to support production in tropical and subtropical marine environments, but the importance of Trichodesmium in the Gulf of Mexico has not been widely investigated. Here, we use stable carbon (δ13C) and nitrogen (δ15N) isotopes to determine whether diazotrophy impacts the isotope baselines of pelagic food webs in the northern Gulf of Mexico. Phytoplankton (measured as particulate organic matter), Tricho desmium, and zooplankton were collected from neritic and oceanic environments (salinity <32 and ≥ 32, respectively). Our results show that zooplankton in both regions exhibited δ13C values that were constrained between −20 and −18‰, which reflects the use of carbon from both Trichodesmium and other phytoplankton. Despite the utilization of similar carbon sources, zooplankton δ15N values were significantly higher in neritic (mean ± SD = 5.4 ± 1.1‰) compared to oceanic samples (2.8 ± 1.4‰). We attribute the difference in nitrogen isotope values to the use of isotopically light nitrogen derived from diazotrophy in oceanic environments compared to enriched nitrogen from terrestrial sources transported by rivers in neritic areas. A comparison with published food web data from the Gulf of Mexico shows that disparate δ15N values which parallel our data suggest differential nitrogen cycling throughout the basin. The results of the present study indicate that diazotrophy impacts the estimation of consumer trophic positions by altering the δ15N values of organisms at the base of the food web. N2 fixation and freshwater inputs should be considered in future efforts to describe pelagic food webs in the Gulf of Mexico.