A coupled transport-biogeochemical-metal bioaccumulation model is developed in this study and is applied to simulate the biogeochemical cycling of Hg in the Mediterranean Sea. The model results reveal the spatial and temporal variation of methylmercury concentrations in this region and its pattern in the plankton food webs. I find the model scheme and parameters are up to date and has several novel features that have not been considered in previous modeling efforts, which I think are important progress in this field:

1. Online coupling with biogeochemical models facilitates the sensitivity analysis of biogeochemical parameters such as POC sinking velocity. The findings that this speed can influence the depth of maximum MeHg concentrations are interesting. This is a parameter that can be constrained by observations, pointing to important future research directions.
2. The inclusion of DOC-bound Hg pools, and a 4-pool partitioning scheme: POC, DOC, HgCl, and dissolved phase. Different Kd values are also used for different pools.

In addition, the authors have a detailed analysis of the seasonal cycle of Hg cycling at different locations, well constrained by observed MeHg profiles. This highlights the importance of both hydrodynamic and biogeochemical parameters.

Here are my specific comments:

Line 75: I suggest including a subsection to describe the general biogeoprovinces of the Mediterranean Sea. A very brief introduction to bathymetry and circulation patterns is also helpful. This will lay a basis for the discussion of different sites of the Sea in the later text.

Line 105-115: need a better layout for these equations.

Line 114-115: This sentence is quite misleading. If the KD values reflect a balance between adsorption and remineralization, then why KD is a constant value through the whole water column, given that the remineralization rate varies drastically?

Line 160: It's a first-order reaction approach, where does the 0.118 parameter come from? Any literature or a tunable parameter?

Line 198: Seems that only riverine load in the dissolved phase is considered? How about riverine discharge in the particulate phase, which is the dominant form of riverine Hg discharge?

Section 2.7: It is a very meaningful practice to test the sensitivity of Hg cycling to the POC sinking velocity. Changing this parameter will not only influence Hg but also C. Are there any sediment trap observations that help to constrain the POC sinking flux itself?

Dear Authors

please consider the following comments by an anonymous referee.

Best Regard

G Abril, asssociate editor

________________

Anonymous review

The paper describe a modelling study of Hg cycle in the Mediterranean Sea  using a high resolution regional model. It is a very interesting and important modelling effort since It is the first attempt to simulate this contaminant in this region with a coupled dynamical-biogeochemical model. The paper is well written and results are globally well illustrated and discussed. The modelling approach considers the main Hg species in the ocean and the related processes that control their exchanges and redistributions. The analysis focuses on MeHg species that represent the toxic species for ecosystems, and investigates the transfer and bioaccumulation in the low trophic level, the planktonic reservoir (phytoplankton and zooplankton).  It provides very new interesting and quantified informations on the spatial and temporal distribution of MeHg species in the different regions of the Mediterranean Sea. I then recommend this paper for publication, but suggest also some major revision on the description of the model. The paper focuses mainly on MeHg species, but the cycling of Hg in the Mediterranean sea is controlled by the distribution and exchange among all the different species that are not well document in this paper, while it is of importance to assess the consistency of the results.

Recently an assessment of Mercury in the Mediterranean Sea has been published (Cossa et al, 2022). It provides constraints on sources of hg in the Mediterranean Sea (atmospheric, riverine, sediments) , exchange fluxes at Gibraltar with the Atlantic Ocean, and budget of THg and MeHg in the western and Eastern basin. I suggest that before publication, , this paper compares in its supplementary material, its modelling results with the budget derived by this Mediterranean Hg assessments, in order to verify that the global modelling approach is coherent with observations.

Cossa et al.  Mediterranean Mercury Assessment 2022: An Updated Budget, Health Consequences, and Research Perspectives. Environ. Sci. Technol. 2022, 56, 3840−3862