1The 3D biogeochemical marine mercury cycling model MERCY &#8211; linking atmospheric Hg to methyl mercury in the marine food web.
Authors: Bieser Johannes , Amptmeijer David , Daewel Ute , Kuss Joachim , Schrum Corinna 
Published: March 2022

&lt;p&gt;Mercury (Hg) is a pollutant of global concern. Due to anthropogenic emissions, the global Hg burden has been ever&lt;br&gt;increasing since preindustrial times. Hg emitted into the atmosphere gets transported on a global scale and ultimately&lt;br&gt;reaches the oceans where it is transformed into highly toxic methylmercury (MeHg) that effectively accumulates along&lt;br&gt;the food chain. The international community has recognized this serious threat to human health and in 2017 regulated&lt;br&gt;Hg under the UN Minamata Convention.&lt;br&gt;Currently, the first effectiveness evaluation of the Minamata Convention on mercury is being prepared and besides&lt;br&gt;observations, models play a major role in understanding environmental Hg pathways and to predict the impact of policy&lt;br&gt;decisions and external drivers (e.g. climate, emission, and land-use change) on Hg pollution. Yet, the available model&lt;br&gt;capabilities are mostly focused on atmospheric models covering the Hg cycle from emission to deposition. With the&lt;br&gt;presented model for marine mercury cycling (MERCY) we want to contribute to the currently ongoing effort to further&lt;br&gt;our understanding of Hg and MeHg transport, transformation, and bioaccumulation in the marine environment with the&lt;br&gt;ultimate goal of linking atmospheric Hg emissions to MeHg in sea food. MERCY is the first fully resolved 3dbiogeochemical&lt;br&gt;model linking atmospheric Hg to MeHg in higher trophic levels. Most importantly, the MERCY model&lt;br&gt;is prgrammed in a way that allows for the coupling of the Hg chemistry, ecosystem, and bioaccumulation models with&lt;br&gt;most established hydrodynamic ocean models. This is achieved using the Framework for Aquatic Biogeochemical&lt;br&gt;Models (FABM).&lt;br&gt;In this talk we present the MERCY model and its application using different hydrodynamic drivers. Moreover, we&lt;br&gt;discuss its capabilities and shortcomings in reproducing the key Hg species Hg0, Hg2+, and MeHg as well as Hg loads&lt;br&gt;in biota. The presented model evaluation is a first step in establishing quality criteria for marine Hg modelling. We show&lt;br&gt;that the model can reproduce observed average concentrations of individual Hg species (normalized mean bias: HgT&lt;br&gt;(aq) -17%, Hg0 2%, MeHg -28%). Moreover, it is able to reproduce the observed seasonality and spatial patterns. We&lt;br&gt;find that the model error for HgT (aq) is mainly driven by the limitations of the physical model setup in the coastal zone&lt;br&gt;and the poor quality of data on Hg in rivers. Morover, the model error in calculating vertical mixing and stratification&lt;br&gt;contributes to the total Hg model error.&lt;br&gt;skill is in a range where further model improvements will be difficult to detect. Finally, for MeHg, we find that we are&lt;br&gt;lacking the basic understanding of the actual processes governing methylation and demethylation. Here, the model can&lt;br&gt;reproduce average concentrations but falls short in reproducing the observed value range. The results prove the&lt;br&gt;feasibility of developing marine Hg models with similar predictive capability as established atmospheric chemistry&lt;br&gt;transport models. Yet, there are still major knowledge gaps in the dynamics governing methylation and&lt;br&gt;bioaccumulation. Based on our findings we discuss these knowledge gaps and identify the major uncertainties in our&lt;br&gt;current understanding of marine Hg cycling from a modeller&amp;#8217;s perspective.&lt;/p&gt;