Sources and Transfers of Methylmercury in Adjacent River and Forest Food Webs

Abstract

Nearly all ecosystems are contaminated with highly toxic methylmercury (MeHg), but the specific sources and pathways leading to the uptake of MeHg within and among food webs are not well understood. In this study, we report stable mercury (Hg) isotope compositions in food webs in a river and an adjacent forest in northern California and demonstrate the utility of Hg isotopes for studying MeHg sources and cross-habitat transfers. We observed large differences in both δ(202)Hg (mass-dependent fractionation) and Δ(199)Hg (mass-independent fractionation) within both food webs. The majority of isotopic variation within each food web could be accounted for by differing proportions of inorganic Hg [Hg(II)] and MeHg along food chains. We estimated mean isotope values of Hg(II) and MeHg in each habitat and found a large difference in δ(202)Hg between Hg(II) and MeHg (∼2.7‰) in the forest but not in the river (∼0.25‰). This is consistent with in situ Hg(II) methylation in the study river but suggests Hg(II) methylation may not be important in the forest. In fact, the similarity in δ(202)Hg between MeHg in forest food webs and Hg(II) in precipitation suggests that MeHg in forest food webs may be derived from atmospheric sources (e.g., rainfall, fog). Utilizing contrasting δ(202)Hg values between MeHg in river food webs (-1.0‰) and MeHg in forest food webs (+0.7‰), we estimate with a two-source mixing model that ∼55% of MeHg in two riparian spiders is derived from riverine sources while ∼45% of MeHg originates from terrestrial sources. Thus, stable Hg isotopes can provide new information on subtle differences in sources of MeHg and trace MeHg transfers within and among food webs in natural ecosystems.

Methylmercury degradation and exposure pathways in streams and wetlands impacted by historical mining

“…43,50 Studies in the relatively less-contaminated Eel River (CA Coast Range), and in the gold mining contaminated Yuba River, have allowed comparison of the estimated isotopic composition of MMHg in food webs with IHg in sediment and other environmental reservoirs to infer MDF between these Hg pools. 42,44 This study, combined with the results of our previous work in the Yuba River, 42 42 This result contrasted with lower δ 202 Hg in IHg compared to MMHg (+MDF from IHg to MMHg) that was previously observed in lakes, estuaries and the coastal ocean. [51][52][53][54] In Cache Creek and Yolo Bypass, sediment is thought to be an important source of IHg that can be methylated, leading to MMHg bioaccumulation in local food webs.…”

“…Figure 2A). We estimate the Δ 199 Hg of IHg and MMHg for each year by extrapolating these relationships to 0% MMHg (i.e., 100% IHg) and 100%MMHg ( Figure 2A; Table 1A Table 4) following [ 44 ]. Estimates for the δ 202 Hg and Δ 199 Hg of MMHg using each method were within error and therefore, for consistency with other studies (e.g., 42 ), we use linear estimates in the following discussion.…”

“…44,58 Briefly, up to 1 g of sample was placed into the first furnace of a two furnace combustion system. The temperature of the first furnace was increased to 750°C over the course of 6 hours while the second furnace was held at 1000°C.…”

“…The Hg isotopic composition of CRMs was consistent with previously reported values (SI Table 3). 32,44,46,53,[60][61][62][63][64][65] Long-term analytical uncertainty of Hg isotope ratio measurements was estimated from the standard deviation (2SD) of the mean Hg isotopic composition of the UM-Almáden standard solution during analytical sessions between Jan. 2013 and Dec. 2014 when run concentrations were between 3 and 5 ng/g (SI Table 3). We estimated external reproducibility using the 2SD of mean Hg isotope values from replicate processing and analysis of NIST 1944 (n=6) and NRC Tort-2 (n=11).…”

“…[42][43][44] Fish feeding studies show essentially no isotopic fractionation of MMHg during trophic transfer, [45][46][47][48][49] and therefore the estimated isotopic composition of MMHg provides insight into MMHg biogeochemical transformations in the environment prior to bioaccumulation. For example, changes in Δ 199 Hg values of MMHg have been used to identify spatial changes in the extent of MMHg photodegradation between different environments (streams, forests, etc.).…”