The 3D biogeochemical marine mercury cycling model MERCY v2.0 – linking atmospheric Hg to methylmercury in fish
Authors: Bieser Johannes , Amptmeijer David J. , Daewel Ute , Kuss Joachim , Soerensen Anne L. , Schrum Corinna 
Published: May 2023

Abstract. Mercury (Hg) is a pollutant of global concern. Due to anthropogenic
emissions, the atmospheric and surface ocean Hg burden has increased
substantially since preindustrial times. Hg emitted into the atmosphere gets
transported on a global scale and ultimately reaches the oceans. There it is
transformed into highly toxic methylmercury (MeHg) that effectively
accumulates in the food web. The international community has recognized this
serious threat to human health and in 2017 regulated Hg use and emissions
under the UN Minamata Convention on Mercury. Currently, the first effectiveness
evaluation of the Minamata Convention is being prepared, and, in addition to
observations, models play a major role in understanding environmental Hg
pathways and in predicting the impact of policy decisions and external
drivers (e.g., climate, emission, and land-use change) on Hg pollution. Yet,
the available model capabilities are mainly limited to atmospheric models
covering the Hg cycle from emission to deposition. With the presented model
MERCY v2.0 we want to contribute to the currently ongoing effort to improve
our understanding of Hg and MeHg transport, transformation, and
bioaccumulation in the marine environment with the ultimate goal of linking
anthropogenic Hg releases to MeHg in seafood. Here, we present the equations and parameters implemented in the MERCY model
and evaluate the model performance for two European shelf seas, the
North and Baltic seas. With the model evaluation, we want to establish a set
of general quality criteria that can be used for evaluation of marine Hg
models. The evaluation is based on statistical criteria developed for the
performance evaluation of atmospheric chemistry transport models. We show
that the MERCY model can reproduce observed average concentrations of
individual Hg species in water (normalized mean bias: HgT 17 %,
Hg0 2 %, MeHg −28 %) in the two regions mentioned above. Moreover,
it is able to reproduce the observed seasonality and spatial patterns. We
find that the model error for HgT(aq) is mainly driven by the
limitations of the physical model setup in the coastal zone and the
availability of data on Hg loads in major rivers. In addition, the model
error in calculating vertical mixing and stratification contributes to the
total HgT model error. For the vertical transport we find that the
widely used particle partitioning coefficient for organic matter of
log(kd)=5.4 is too low for the coastal systems. For Hg0 the
model performance is at a level where further model improvements will be
difficult to achieve. For MeHg, our understanding of the processes
controlling methylation and demethylation is still quite limited. While the
model can reproduce average MeHg concentrations, this lack of understanding
hampers our ability to reproduce the observed value range. Finally, we
evaluate Hg and MeHg concentrations in biota and show that modeled values
are within the range of observed levels of accumulation in phytoplankton,
zooplankton, and fish. The model performance demonstrates the feasibility of
developing marine Hg models with similar predictive capability to
established atmospheric chemistry transport models. Our findings also
highlight important knowledge gaps in the dynamics controlling methylation
and bioaccumulation that, if closed, could lead to important improvements of
the model performance.