Biogeochemical Cycle of Mercury in the Northern Gulf of Mexico as Constrained by Carbon, Nitrogen, Sulfur, and Mercury Isotopic Ratios in Marine Fish

Mercury (Hg) in the environment has deleterious ecological and health affects for humans and wildlife and is primarily transferred to humans through the consumption of marine biota (USEPA, 2001). These ecological and health concerns are exacerbated by the production of methylmercury (CH3Hg+; MMHg) in aquatic ecosystems. Sulfur isotopes (δ34S) are an indicator of the strength of sulfate reduction associated with MMHg production. In chapter 1, we assessed the relationship between MMHg concentrations and reduced-sulfur stable isotope δ34S signals in four coastal consumer organisms (length = 26–75 mm) from Florida's Big Bend seagrass meadows: pinfish (Lagodon rhomboides); pigfish (Orthopristis chrysopter); black sea bass (Centropristis striata melana); and shrimp (Tozeuma carolinense and T. serratum). We found a significant correlation between depleted δ34S signatures in fauna tissue (R2 = 0.27; p < 0.001; total n = 179) with higher MMHg concentrations. A correlation was observed in lower δ34S isotopic signatures and higher MMHg concentrations of consumers from the southern region of the study area indicating a "hotspot" of net mercury methylation in the sediments near Tampa Bay, Florida. To better understand the sources of Hg to coastal pinfish and to assess the contribution of pinfish annual egress to offshore food webs, chapter 2 applies isotopic tracing (C, N, S) combined with mercury (Hg) concentrations and isotope ratios in sediments, juvenile pinfish, juvenile gag grouper (Mycteropera microlepis), and adult gag grouper and pinfish to assess exposure pathways of MMHg in the northeastern Gulf of Mexico. We found that pinfish from the northern and southern Big Bend regions had distinct Hg sources. Southern pinfish had enriched δ202Hg and, when combined with lower pinfish δ34S values and lower sediment δ202Hg values, suggested elevated microbial methylation/demethylation in the southern region. The southern, coastal pinfish exhibited similar Δ199Hg as offshore gag grouper suggesting pinfish from this region represent an important Hg source to offshore reef fish species and/or these two cohorts are exposed to the same Hg source. Results suggested that estuaries can be an important source of MMHg to adjacent, offshore commercially important reef species and confirmed the utility of Hg isotope analysis to identify multiple marine Hg sources and inform our understanding of the pathways of MMHg bioaccumulation in estuarine food webs. In the northern Gulf of Mexico (nGOM), the Hg cycle is further complicated by the influence of the Mississippi River (MR) and potentially confounded, since April 2010, by the Deepwater Horizon (DWH) oil spill. Approximately 3.0-4.9% of oil-derived carbon released during the Deepwater Horizon oil spill was deposited on the seafloor (Chanton et al., 2014; Valentine et al., 2014) causing local, but persistent, oxygen depletion (Kessler et al., 2011). In these low-oxygen environments, the formation of monomethylmercury (MMHg) is promoted and may have led to higher MMHg concentrations in commercially important reef fish species. Expanding our research to include the entire nGOM shelf, slope, and the far-reaches of the deep Gulf canyons, chapter 3 explores Hg cycling using benthic and pelagic fishes as indicator-samples. Fish in the western study region appeared to rely more on benthic feeding, as indicated by enriched δ13C values and depleted δ34S values. Fish species closer to the MR plume had enriched δ15N and depleted δ202Hg and Δ199Hg values suggesting the MR influenced the nutrient and Hg cycle in the northeastern GOM. Cutthroat eels (~1,000 m) had higher MIF signatures than slope snake eels (~250 M) and, when combined with decreasing δ13C values with depth, suggested the presence of near-surface derived Hg in the deep DeSoto Canyon. Our results highlighted the importance of the deposition of Hg associated upper water column OM such as the marine snow pelagic carbon. Chapter 4 investigates whether more reducing conditions cause by DWH led to higher levels of mercury in commercially important reef associated organisms. We compared pre-spill (collected 2007-09) and post-spill (collected 2012) fish species of similar length from hard-bottom reefs on the West Florida Shelf. Light stable isotope analyses of carbon, nitrogen, and sulfur was employed to assess feeding ecologies of those species exhibiting higher MMHg concentrations following the oil spill. Some species, like porgy, showed significant increase in Hg concentration. This increase may have been associated with a shift to a more pelagic, higher trophic feeding regime. The significance of this study is that it shows that the oil spill had an effect on the mercury cycling but that the effect is not uniform for each fish species.