Comments:

Amptmeijer et al. “Bioconcentration as a key driver of Hg bioaccumulation in high

trophic level fish”. The authors clarified the Hg bioaccumulation, especially MMHg⁺, in high trophic level fish using models (GOTM-ECOSMO E2E-MERCY V2.0). The work addresses a critical knowledge gap by quantifying the cumulative impact of bioconcentration across trophic levels, highlighting the important role of MMHg+ bioconcentration. The hypotheses are rigorously tested under three hydrodynamic regimes, and the conclusion that Hg²⁺ bioaccumulation has negligible influence on MMHg⁺ dynamics is well-supported by statistical analyses. Overall, the study advances understanding of Hg dynamics in marine ecosystems. 

Line 40-50. The authors mentioned the “bioconcentration”, “bioaccumulation” and “biomagnification”. For instance, the statement “bioconcentration is the most important step in bioaccumulation” lacks a clear distinction from biomagnification, risking confusion for readers unfamiliar with the terminology. What’s the differences between bioaccumulation and biomagnification? In the subsequent manuscript, these two words were also used in confusion. The authors should explain and clarify them.

Line 79-85. The second hypothesis is confused. This hypothesis lacks evidence and references, making it appears speculative.

Line 140: As mentioned, “Quantifying the importance of the bioaccumulation of Hg2+ and bioconcentration of MMHg+ in consumers on MMHg+ bioaccumulation”. The authors should clarify whether prior models ignored multi-trophic bioconcentration, and then highlight the novelty of this work.

Line 226 Table 1. How to calculate the bioaccumulation and bioconcentration difference (%) ?

Line 257-301. Model limitations. I think model limitation is very important and necessary. “Overall the most important driver of our model is the fraction of MMHg+ that is bioaccumulated by bioconcentration for each trophic level, as this drives the relative importance of bioconcentration at the higher trophic levels.” I hope to get the quantification of model uncertainty caused by parameterization of bioconcentration and biomagnification.

Line 293-294: “Because the gills of fish are optimized to facilitate the exchange of gasses between water and fish blood, these gaseous Hg species can likely bioconcentrate into organisms.” Meanwhile, some recent studies have also approved that Hg0 could be absorbed by alga directly and such flux should not be overlooked. Such process seems not be considered in present model. I suggest the authors should add such discussion or model uncertainties.

Line 316 “15% per trophic level”. It is recommended to supplement sensitivity analyses.

The manuscript quantifies the significance of Hg²⁺ bioaccumulation and MMHg+ bioconcentration in high trophic level fish. Currently, the accumulation of Hg in the environment and its associated risk assessment are major research topics in the global mercury cycle. The bioaccumulation of mercury is crucial for both ecological safety and human health. Therefore, this study provides valuable theoretical data for mercury pollution risk assessment, with substantial environmental relevance. However, some sections of the manuscript are overly brief in their explanation and analysis, warranting further elaboration. Additionally, the references cited in the manuscript are somewhat outdated and should be updated to include more recent studies, ensuring the relevance and accuracy of the information presented. While the manuscript offers a detailed introduction to the foundational knowledge and prior research, the sections on model development and the interpretation of the model results are relatively concise and could benefit from further expansion. Overall, the manuscript could be reconsidered after major revisions. Detailed comments are given below：

1. The introduction mentions “MMHg⁺is a topic of serious concern because MMHg⁺is a dangerous neurotoxin that can bioaccumulate into levels. that are dangerous for human consumption in fish that are often consumed as seafood.” Could this phrase be a little more concise?
2. The introduction mentions “Since DMHg is susceptible to photodegradation, we can assume that it plays an important role in the coastal water investigated in this study,  until better observational studies confirm or correct this assumption.” There are problems with logic.
3. The introduction mentions “the bioconcentration process is complicated and recent studies show that the bioconcentration of MMHg⁺is influenced by cell-dependent factors, such as the thickness of the phytosphere, while this is not the case for Hg²⁺.” How does the thickness of the phytosphere affect the bioconcentration of MMHg⁺? Why is Hg²⁺ not affected by this?
4. The introduction mentions “While the MMHg⁺content of phytoplankton is the most important predictor of MMHg⁺in higher trophic levels, it “only” predicts 63% of the variability of the MMHg⁺ in fish.” Why is the remaining 37% unpredictable?
5. The introduction mentions “……based on Lavoie et al., 2013 we estimate that ……” Is there a problem with this expression?
6. The introduction mentions “we estimate that the biomagnification factor for MMHg⁺is 1.5 times higher than for Hg+.” How is this estimated, and what accounts for this difference?
7. The expressionof“phyto and zooplankton”, “Baltic Seas by (Bieser et al., 2023), which is expanded upon in (Amptmeijer et al., 2025) ” is not standard, please modify.
8. Mentioned in the introduction “While there is no change in the average tHg, the results of Amptmeijer et al., 2025 show seasonal variation. This means that even if the average concentrations of tHg are not altered,  there may still be an effect of Hg²⁺bioaccumulation on MMHg⁺bioaccumulation.” Why does seasonal variation indicate there may still be an effect of Hg²⁺ bioaccumulation on MMHg⁺ bioaccumulation?
9. The references cited in the manuscript are somewhat outdated and may benefit from incorporating more recent studies to ensure the relevance and accuracy of the presented information.
10. The paper is very detailed about the basic knowledge and the content of the preliminary research, but the description of the later model establishment and the data obtained from the model is relatively brief, which can be further supplemented. For example, “The contribution of bioconcentration in zooplankton of 3.97-10.07%……and the contribution of bioconcentration in fish between 8.14-21.82%……”

This study employed a fully coupled GOTM-ECOSMO-MERCY coupled system to quantify the effect of bioaccumulation of Hg2+ and the bioconcentration of MMHg+ on the bioaccumulation of MMHg+ into higher trophic levels. Quantifying the importance of the bioaccumulation of Hg2+ and bioconcentration of MMHg+ in consumers on MMHg+ bioaccumulation into higher trophic levels will provide a unique insight into the drivers of the bioaccumulation of MMHg+ bioaccumulation and increase our fundamental understanding of this process. However, both the description on the model and the discussion on the results are concise, resulting in the absence of in-depth discussion and thinking.

• Section 2.2: Are there any equations for the parameterization schemes in the bioaccumulation of Hg in the model? Model equations are important for understanding the critical processes of the substance. Meanwhile, what’s the critical parameters and coefficients for the critical processes in the model? The details of this model are not clarified in the method.
• Significantly, model performance should be evaluated against observations, which is deficient in this study.
• The literature Amptmeijer et al. (2025) is very important for this study. However, we cannot access to the paper because it is in preparation.
• The setup of the two scenarios is two sample. In my opinion, sensitivity analysis for critical parameters or uncertainty analysis of the results are needed.
• For the results and discussion, the illustrations are concise, where I cannot gain much in-depth discussion and thinking.

This study employed different models to identify the contributions of bioconcentration and biomagnification to Hg and MeHg bioaccumulation. The primary concern is that the description of the model applied and the data sources lack clarity.

1. Lines 6-9: This sentence is too long and not clear to me.

2. Line 110: Descriptions about the Modeled region in the Introduction section is weird. I suggest to move it to the M&M section.

3. Section 2.1: The model section is not clear to me. How to divide bioconcentration and biomagnification. Is there any data collected from in-lab measurement?

4. Table 1: What is the source of the data provided in this table?