A Global Model for Methylmercury Formation and Uptake at the Base of Marine Food Webs

Abstract

Monomethylmercury (CH3Hg) is the only form of mercury (Hg) known to biomagnify in food webs. Here we investigate factors driving methylated mercury [MeHg = CH3Hg + (CH3)2Hg)] production and degradation across the global ocean and uptake and trophic transfer at the base of marine food webs. We develop a new global 3‐D simulation of MeHg in seawater and phyto/zooplankton within the Massachusetts Institute of Technology general circulation model. We find that high modeled MeHg concentrations in polar regions are driven by reduced demethylation due to lower solar radiation and colder temperatures. In the eastern tropical subsurface waters of the Atlantic and Pacific Oceans, the model results suggest that high MeHg concentrations are associated with enhanced microbial activity and atmospheric inputs of inorganic Hg. Global budget analysis indicates that upward advection/diffusion from subsurface ocean provides 17% of MeHg in the surface ocean. Modeled open ocean phytoplankton concentrations are relatively uniform because lowest modeled seawater MeHg concentrations occur in oligotrophic regions with the smallest size classes of phytoplankton, with relatively high uptake of MeHg and vice versa. Diatoms and synechococcus are the two most important phytoplankton categories for transferring MeHg from seawater to herbivorous zooplankton, contributing 35% and 25%, respectively. Modeled ratios of MeHg concentrations between herbivorous zooplankton and phytoplankton are 0.74–0.78 for picoplankton (i.e., no biomagnification) and 2.6–4.5 for eukaryotic phytoplankton. The spatial distribution of the trophic magnification factor is largely determined by the zooplankton concentrations. Changing ocean biogeochemistry resulting from climate change is expected to have a significant impact on marine MeHg formation and bioaccumulation.

Methylmercury produced in upper oceans accumulates in deep Mariana Trench fauna

“…Our findings not only inform on the source of MMHg in the deep ocean, but also allow us to speculate about the production of DMHg in the deep ocean. Mean oceanic dissolved MMHg and DMHg profiles show similar concentration trends at depth throughout the world's oceans [10][11][12][13]24 . The limited global variation in MMHg and DMHg concentrations indicates slow transformations, supported by our finding that little IHg(II) methylation takes place in the deep NPO.…”

“…Mesopelagic MMHg can penetrate into deep oceans via downwelling or via complexation to or incorporation in sinking POM (i.e., biological pump). However, field data and models indicate that the downward transport of mesopelagic MMHg via downwelling and sinking POM is possibly limited, if not negligible 12,23,24 . Yet, available data, show abundant MMHg and DMHg in deep oceans 8,12,13 , implying that in situ Hg methylation and/or di-methylation in deep oceans is plausible.…”

Human Health Benefits of the Minamata Convention on Mercury

“…Over tropical and mid-latitudes, the effect of temperature on MeHg degradation becomes less important, but the change in ocean primary production and subsequent MeHg formation matters (Y. Zhang et al 2020;S. Dutkiewicz, Scott, and Follows 2013).…”

“…We simulate the chemistry, transport, and trophic transfer of mercury in the ocean using the MIT General Circulation Model (MITgcm) following Y. Zhang et al (2020). The model is driven by ocean physics data from the IGSM and biogeochemical parameters from the Darwin model.…”

“…The bioaccumulation and biomagnification of monomethylmercury in the marine plankton food web is simulated following Y. Zhang et al (2020).…”