Assessing the spatial and temporal variability of methylmercury biogeochemistry and bioaccumulation in the Mediterranean Sea with a coupled 3D model

Abstract

Abstract. Human exposure to mercury (Hg) is a cause of concern, due to the biomagnification of the neurotoxic species monomethylmercury (MMHg) in marine ecosystems. Previous research revealed that commercial fish species in the Mediterranean Sea ecosystems are particularly enriched in Hg, due to a combination of physical and ecological factors. Since the fate of Hg depends on the interactions among several biogeochemical and physical drivers, biogeochemical modeling is crucial to support the integration and interpretation of field data. Here, we develop and apply a coupled transport–biogeochemical–metal bioaccumulation numerical model (OGSTM–BFM–Hg) to simulate the biogeochemical cycling of the main Hg species (HgII, Hg0, MMHg, and DMHg) in seawater, organic detritus, and through the planktonic food web. The model is applied to a 3D domain of the Mediterranean Sea to investigate the spatial and temporal variability of methylmercury (MeHg) distribution and bioaccumulation and major uncertainties in Hg cycling. Model results reproduce the strong vertical and zonal gradients of MeHg concentrations related to primary production consistently with the observations and highlight the role of winter deep convection and summer water stratification in shaping the MeHg vertical distribution, including subsurface MeHg maximum. The modeled bioaccumulation dynamics in plankton food webs are characterized by a high spatial and temporal variability that is driven by plankton phenology and is consistent with available field data of Hg concentrations in plankton, as well as with other indicators, such as bioconcentration factors (BCFs) and trophic magnification factors (TMFs). Model results pointed out that the increment in water temperature linked to a decline of deep convection can cause an increase in water MeHg concentrations with cascading effects on plankton exposure and bioaccumulation.

The Multi-Compartment Hg Modeling and Analysis Project (MCHgMAP): Mercury modeling to support international environmental policy

“…In recent years, more comprehensive 3D ocean Hg models including MeHg chemistry have been developed. Currently, four marine Hg models based on numerical 3D hydrodynamic modeling are in active use Kawai et al, 2020;Rosati et al, 2022;Bieser et al, 2023 excluding models that are no longer actively being updated). Table F2 gives an overview of published 3D ocean Hg models.…”

“…Table F2 gives an overview of published 3D ocean Hg models. While all these models contain a complete marine Hg chemistry including MeHg, only , Rosati et al, (2022), and Bieser et al, (2023) consider uptake to and release from marine biota. This makes these three models the first hydrodynamic 3D Hg model to include the marine ecosystem.…”

“…There are few marine Hg model studies evaluating individual models against observations (Zhang et al, 2019c;Kawai et al, 2020;Rosati et al, 2022;Bieser et al, 2023), and so far, no ensemble studies have been performed. Here, we propose a framework for the first marine Hg model ensemble study.…”

“…Combined with projections of future anthropogenic Hg emission scenarios, Giang and Selin (2016) used Amos et al, (2013) and Schartup et al, (2022) used Soerensen et al, (2016a) to assess future changes in Hg levels. Threedimensional biogeochemical models including ecosystem interactions are increasingly applied to simulate the spatiotemporal dynamics of Hg species in the ocean including bioaccumulation in the marine food web Rosati et al, 2022;Bieser et al, 2023;Zhu et al, 2023).…”

A high-resolution marine mercury model MITgcm-ECCO2-Hg with online biogeochemistry

“…Later models were developed with improved ocean biogeochemistry. For example, Zhang et al (2020) and Rosati et al (2022) coupled Hg cycling with a biogeochemical and ecological model, considering the uptake to and release of Hg from marine biota. The lasted developed biogeochemical multi-compartment model for Hg cycling MERCY v2.0, developed by Bieser et al (2023), includes all currently known processes controlling marine Hg cycling.…”