Flux of Total Mercury and Methylmercury to the Northern Gulf of Mexico from U.S. Estuaries

Abstract

To better understand the source of elevated methylmercury (MeHg) concentrations in Gulf of Mexico (GOM) fish, we quantified fluxes of total Hg and MeHg from 11 rivers in the southeastern United States, including the 10 largest rivers discharging to the GOM. Filtered water and suspended particles were collected across estuarine salinity gradients in Spring and Fall 2012 to estimate fluxes from rivers to estuaries and from estuaries to coastal waters. Fluxes of total Hg and MeHg from rivers to estuaries varied as much as 100-fold among rivers. The Mississippi River accounted for 59% of the total Hg flux and 49% of the fluvial MeHg flux into GOM estuaries. While some estuaries were sources of Hg, the combined estimated fluxes of total Hg (~5200 mol y(-1)) and MeHg (~120 mol y(-1)) from the estuaries to the GOM were less than those from rivers to estuaries, suggesting an overall estuarine sink. Fluxes of total Hg from the estuaries to coastal waters of the northern GOM are approximately an order of magnitude less than from atmospheric deposition. However, fluxes from rivers are significant sources of MeHg to estuaries and coastal regions of the northern GOM.

Mercury levels of yellowfin tuna (Thunnus albacares) are associated with capture location

“…As with previous studies we also detected relatively high levels of methylmercury in the fish caught from the GOM and NWAO, suggesting anthropogenic sources of mercury contamination in these fish. Indeed, this might be consistent with mercury contamination via the Atlantic Ocean Loop Current and/or mercury originating in the Mississippi River and being drained into the GOM (Buck et al, 2015; Harris et al, 2012a, 2012b).…”

Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet

“…2d) indicate substantial export of bioavailable Hg and potential Hg cycling pathways in subglacial ecosystems. Concentrations of dMeHg are relatively high from all three glacial catchments (>5 pM), exceeding values from most pristine freshwaters, and similar to those found in wetlands (below detection in rivers to 2.5 pM in the Everglades) 2,16,26 . Dissolved dMeHg accounts for 0.74 ± 0.17% (LG), 1.5 ± 0.97% (IS) and 2.1 ± 0.43% (RG) of the total dHg concentration, which is in the range of other freshwater systems (~1-10%) 2 , including Arctic rivers (~0.2-5 %) 10 , but lower than marine environments (4-22%) 27 .…”

“…4a; R 2 = 0.70 (P < 0.05; n = 9)) with lower concentrations at higher salinities implying seawater dilution, but this does not follow a two end-member mixing model using the data from the northern ice sheet catchments sampled. We hypothesize non-conservative behaviour at lower salinities due to removal of Hg by scavenging on flocculated particles, alongside biological and/ or photochemical reduction of Hg 2+ to Hg 0 with subsequent degassing 26 , which is implied by a more limited dataset from SS (Fig. 1) downstream of RG and LG inputs (see the dHg concentrations from fjord waters with salinity 0.4-8.3 in Extended Data Fig.…”

The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin)

“…Methylated Hg sources for coastal waters include (i) inputs from upwellings, rivers, groundwaters, atmospheric deposition and waste water point sources, and (ii) in situ Hg II i methylation in coastal waters and sediments (Cossa et al, 1996;Fitzgerald et al, 2007). The river-watershed contribution can be large due to direct inputs of MMHg to coastal waters (Coquery et al, 1997;Choe and Gill, 2003;Balcom et al, 2008 andMuresan et al, 2008;Buck et al, 2015) and continental groundwaters via submarine estuaries (Ganguli et al, 2012). Contribution of the open ocean to the methylated Hg load of oceanic margin waters has also been evidenced: DMHg is conveyed, via upwellings, from ocean interior to surface coastal waters (Conaway et al, 2009).…”