10.1021/es902361j.s001 David B. Senn David B. Senn Edward J. Chesney Edward J. Chesney Joel D. Blum Joel D. Blum Michael S. Bank Michael S. Bank Amund Maage Amund Maage James P. Shine James P. Shine Stable Isotope (N, C, Hg) Study of Methylmercury Sources and Trophic Transfer in the Northern Gulf of Mexico American Chemical Society 2010 baseline δ15 N signatures food webs Δ201 influence MeHg accumulation MeHg sources GOM oceanic MeHg sources MR oceanic species 2010-03-01 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Stable_Isotope_N_C_Hg_Study_of_Methylmercury_Sources_and_Trophic_Transfer_in_the_Northern_Gulf_of_Mexico/2788570 We combined N, C, and Hg stable isotope measurements to identify the most important factors that influence MeHg accumulation in fish from the northern Gulf of Mexico (nGOM), and to determine if coastal species residing in the Mississippi River (MR) plume and migratory oceanic species derive their MeHg from the same, or different, sources. In six coastal species and two oceanic species (blackfin and yellowfin tuna), trophic position as measured by δ<sup>15</sup>N explained most of the variance in log[MeHg] (<i>r</i><sup>2</sup> ∼ 0.8), but coastal species and tuna fell along distinct, nearly parallel lines with significantly different intercepts. The tuna also had significantly higher δ<sup>202</sup>Hg (0.2−0.5‰) and Δ<sup>201</sup>Hg (∼1.5‰) than the coastal fish (δ<sup>202</sup>Hg = 0 to −1.0‰; Δ<sup>201</sup>Hg ∼0.4‰). The observations can be best explained by largely disconnected food webs rooted in different baseline δ<sup>15</sup>N signatures (MR-plume vs oceanic) and isotopically distinct MeHg sources, with oceanic MeHg having undergone substantial photodegradation (∼50%) before entering the base of the food web. Given the MR’s large, productive footprint in the nGOM and the potential for exporting prey and MeHg to the adjacent oligotrophic GOM, the disconnected food webs and different MeHg sources are consistent with recent evidence in other systems of important oceanic MeHg sources.