This manuscript investigates the impact of bioaccumulation in marine mercury (Hg) cycling, highlighting how ecosystem feedbacks can lead to higher concentrations of Hg in water (compared to an abiotic system, i.e. the status of many Hg cycling models), as well as seasonal and spatial differences in the magnitude of these effects across the North and Baltic Seas. These feedbacks are quantified through coupling a marine ecosystem model that incorporates mercury bioaccumulation, mercury cycling model, and hydrodynamic model in a fully online way for a 1D column model, and offline for a 3D model that allows for exploration of spatial variation. The study presents sophisticated model coupling, generally clear model evaluation, and well-designed model experiments. Overall, I found this manuscript to be an important and timely contribution, that makes a strong case for representing ecosystem effects on mercury cycling “even in cases where bioaccumulation is not of direct interest,” for both improved scientific understanding and environmental management. However, I believe the manuscript could benefit from revisions that focus on improved presentation, clarity, and contextualization of results, before publication, to maximize its impact and uptake.

MAJOR COMMENTS

1) While I recognize that this is a complex model and study design, the manuscript is on the long end, and there may be opportunities to streamline the text (particularly in 2.2-2.7) to avoid repetition and allow the core messages to come through more clearly. The authors could consider using a supplemental information section to present some details of model assumptions and parametrization, as well as for some supporting figures (e.g., Figure 12). In addition, the manuscript would benefit from additional review for typos and readability. Some have been flagged below in minor comments.

2) It could be helpful to provide a brief summary of the drivers of spatial and temporal variation in the results, as some of these details may be contained in the cited original model papers and therefore less clear to a reader. For example, for seasonality: to what extent is temperature dependence also considered in the bioaccumulation and toxicokinetic modeling, in addition to biomass modeling? For spatial variation: What determines the spatial distribution of higher trophic levels? Is migration relevant, and if so, how is it considered? If not, what additional implications could this have for the spatial dynamics?

3) See below for some places where clarification of some methodological details could be beneficial, potentially in supporting material (e.g., in model-obs comparison for 1D, initial conditions).

4) The authors may have the opportunity to deepen the reflection on next steps and future directions, given the importance of the call to better represent ecosystem effects in models. Some questions I am particularly curious to get their thoughts on are: a) is model coupling the only way to do this, is it reasonable to do a back-of-the-envelope adjustment factor that is regionally specific; b) how much trophodynamic complexity is needed — does capturing the base of the food web get most of the effect or do fish 1 and 2 shift the patterns; if so, what might be missing in this current simplified representation of the ecosystem

DETAILED COMMENTS

L22: Number of parties now exceeds the number of signatories (over 150), so could update the number https://minamataconvention.org/en/parties

L52: “In summary, there are three fractions… in our model.” Read as confusing as the model hasn’t been introduced yet.

L137: As defined in the first sentence, isn’t this bioconcentration only?

Fig. 1: Typos in title and Scenario C. Could consider overlaying the 1-D vs 3-D component too so that it captures that aspect of the design as well. Could incorporate map of locations as a side panel for global audience.

Section 2.4: Include grid resolution for the 3D models (may have missed this)

L299: pre-dated?

L312-316: A bit more detail on this model tuning/calibration process — what informed the choice of lowered value

L468-472: What are the observed values for biomass? Not sure if I missed their reporting somewhere. Could they also be put on Figure 2 for comparison?

L508: How is “high quality” defined?

This study examines, by coupling marine environment and Hg cycling models, how various ecosystem processes affect Hg cycling in coastal seas. The cycling of Hg in coastal seas is an important environmental and human health issue, largely due to the bioaccumulation of monomethylmercury in the food web. The study is interesting and quantifies the magnitude of how specific ecosystem processes affect Hg concentrations and fluxes.

The manuscript would benefit from further language editing, in particular for the Introduction. Overall, I have only minor remarks to the manuscript.

Line 5: I find this expression peculiar “Our results show that bioaccumulation can increase total methylmercury (tMeHg) in coastal pelagic waters from 0.059 to 0.092pM, a 44% increase.”. Bioaccumulation is a process enhancing concentrations in biota, not in water, which you also state in lines 138-139.

Line 31: Regarding “This can lead to insufficient data to understand the cycling and bioaccumulation of marine Hg at the base of the food web,”: I would argue that undoubtedly, measuring Hg only in biota is insufficient to understand Hg cycling. It may be useful to monitor the ultimate effectiveness of the Minamata treaty but certainly not to understand the observations. I suggest to reformulate this discussion.

Line 41-42: I suggest to replace the work equilibrium since the Hg2+ reduction and Hg0 oxidation are largely mediated by different, independent mechanisms including photochemical and biotic processes.

Line 43: replace the term “double methylated DMHg” with “dimethylmercury DMHg”

Line 70: The discussion on Hg uptake mechanisms here do not harmonize with he discussion of passive diffusion uptake in line 49.

Line 15: not only by “marine” microorganisms.

Line 34: I suggest to replace “are a perfect tool to” (which is hardly true) with “are an important tool to”.

Line 35: “Because MeHg formation and subsequent bioaccumulation in seafood are the dominant source of Hg exposure to humans,… ”. The sentence is grammatically incorrect, formation and bioaccumulation are not sources, seafood is the source.

Line 45: the following statement is grammatically incorrect: “Since only MMHg+ bioaccumulates, the term MeHg, in this paper, refers to the total methylated fraction of Hg in seawater.”.

Line 48: replace “inorganic chlorine complexes” with “inorganic chloride complexes”.

Line 54: avoid using the term “species” for microorganisms as it can be confounded with chemical species (which is discussed in the preceding lines).

Line 65: “Biomagnification can be estimated in nature by sampling stable carbon and nitrogen isotopes with Hg” – isotopes and Hg are not sampled, they are measured.

Line 80-81: avoid to use “reduce” both for chemical reduction and decrease , in particular in the same discussion.

Lines 138-139: an element cannot “undergo speciation”. Speciation is not a process it is the distribution of an element among different chemical forms. An element may undergo changes in speciation.