Review of

“Fate of legacy ammonium in the coastal Baltic Sea”

by Dana Hellemann et al.

Egusphere-2026-959

This is an interesting manuscript on legacy organic matter and ammonium in Baltic Sea sediments, and on benthic ammonium fluxes. The authors high-light and quantify (via modelling) the very long time the Baltic Sea needs to recover from decades of enhanced eutrophication and oxygen deficiency – even if external nutrient loads are drastically reduced. This quantification is welcome and highly needed.

Considering the very long time the Baltic Sea needs to recover (as mentioned above), it would be an improvement of the manuscript if the authors could include a section giving their view on the suitability of sea-based methods (often also called ecological engineering) to improve the environmental status of the Baltic Sea or at least the coastal part of the Baltic Sea. Would, according to the authors, for example oxygenation (in one way or the other) faster lead to improvements than only reduction of external nutrient loads? Are there any other sea-based methods that the others favour or recommend? Considering that the authors’ model results suggest so very strong persistence of the legacy OM and ammonium, one would  expect that the authors add this section and respond to the questions I have raised. I am sure that most future readers of this manuscript have the same questions.

I am not much involved in modelling any longer, so it essential that at least one of the other reviewers is a modeller and thoroughly scrutinizes the modelling part of the manuscript. I have not done that.

General: The model includes different levels of organic loading, but I did not notice whether you discussed variable C/N ratios of the deposited organic matter. Would doing that improve the model predictions?

Comments by line number

Line (L) 20: … accumulated via OM burial and mineralization…

Please instead write “…accumulated via OM deposition and mineralization…”

The term burial is often used to reflect the ultimate removal of a substance from further (re)cycling.

L 40 and elsewhere: …phosphate (PO₄^3-)….

PO₄^3- is not the main species of dissolved inorganic P in seawater. Please instead use Dissolved Inorganic Phosphorous (DIP) – here and everywhere in the manuscript.

L 45-46: …PO₄^3- due to its crucial role in the repeated occurrences of massive cyanobacteria blooms and metal recycling…

Do you mean the role of metal recycling in the dynamics (retention and release) of PO₄^3-(or DIP)? If so, this is not what you wrote.

Figure 1: There are more published papers on NH4 effluxes under varying bottom water oxygen conditions in the Baltic Sea than those cited in Figure 1. Since this manuscript focuses on the Baltic Sea, I suggest that the authors are more comprehensive in this regard and cite more of these papers.

L 77-78: This is not only a larger pool of bioavailable N than the pelagic pool of dissolved inorganic nitrogen (DIN) in the coastal Baltic Sea…

Please explain how you define the coastal Baltic Sea, preferably the first time you mention the coastal Baltic Sea.

L 111: … and thus available…

Should be: …and is thus available…

L 124-125: Only a small part of water column NO3- is assumed to diffuse back into the sediments, as coastal water turbulence is stronger than diffusion.

I do not think that this is correct. The nitrate formed during nitrification in the oxygenated bottom water may very well be taken up by the reduced sediment surface and consumed by either denitrification or DNRA – regardless of turbulence. Please rewrite or at least explain yourself in greater detail.

L 125-127: In the absence of hydrogen sulfide (H2S), this NO3- can be reduced to N2, while in the presence of H2S dissimilatory nitrate reduction to ammonium (DNRA) dominates, further enriching the NH4+ pool.

Please be more specific and correct: Denitrification can proceed with H2S as electron donor. Also, please provide references stating that DNRA dominates (over denitrification) in the presence of H2S.

L 281-287: In sediments with high organic loading, can an abundance of sulphide explain low rates of nitrification, and hence high ammonium effluxes, even under normoxic bottom water conditions?

L 335-336: “For instance, a higher salinity is often found in open coastal systems;…”

Is it? If so, why? And higher than where? I do not understand what you mean. A clarification is absolutely necessary.

Hellemann & Sun et al. – Fate of legacy ammonium in the coastal Baltic Sea (EGUsphere-2026-959)

Summary Statement

Hellemann & Sun et al. report results from a 1-dimensional reactive transport model (RTM) for coastal areas of the Baltic Sea aimed at understanding internal ammonium (NH4) loading from sediments under different organic matter loading scenarios. The topic of internal nitrogen loading in aquatic systems, especially in non-oceanic systems, is indeed very much under-studied and under-appreciated, so the study is timely and novel. Overall, the study represents an important contribution to our understanding of internal nitrogen dynamics and the magnitude of nutrient legacies accumulated in coastal sediments. Unfortunately, the study also shows that mitigating these legacies may take centuries of reducing external loading, even after a return to pre-industrial loading levels. I fear that the response of resource managers/politicians, etc., will be to throw up their hands and say that any economic costs of reducing external loads are not worth the benefits, since those potential benefits (which are not even certain to occur) likely won’t manifest until after the effects of climate change are more clear (e.g., AMOC collapse, melting polar ice caps, sea level rise, etc.), global geopolitics are likely much different, who knows what global populations might be, etc. However, humanity needs to realize that past economic growth and industrialization came at a huge cost, and perhaps there are lessons to be learned for future generations.

Overall, my comments are relatively minor and should be easily implemented, and I support publishing the manuscript after completing some minor revisions.

Specific Comments

Section 1 - Introduction

Line 41 --- Is ‘consumption’ the best word choice here for NH4, assuming that the authors are referring to nitrification? Perhaps ‘transformation’ would be better, since ‘consumption’ might imply assimilation into biomass, rather than transformation to NO2/NO3?

Line 47 --- “This is a critical oversight…” --- here and throughout the manuscript (e.g., Lines 77, 95, 138, 249, 257, 273, 281, 303, 351…I’ve likely missed others), I suggest specifying what ‘This’ refers to. In this case (line 77), perhaps something like “This omission is…”.

Lines 93-95 --- Consider changing ‘most’ to something like ‘a substantial proportion’ or ‘much’. I think that there are enough exceptions to this pattern to avoid using ‘most’ here.

Lines 96-97 --- Consider revising to something like: “Microphytobenthos, if present, can intercept and assimilate nutrients effluxing from sediments.”

Line 98 --- Consider a slight qualification to: “…leaving only residual fluxes originating from, for example, faunal activity”, since there are likely additional explanations for any ‘residual’ flux.

Section 2

Figure 3 caption --- I suggest including a citation for the statement about the effects of H2S on the balance between denitrification and DNRA. For example, Murphy et al. 2020 (Env Microbiol 22(6):2124-2139), but there are others.

Section 3

Line 242 --- “…regenerated from the in sediment…” --- needs editing.

Lines 281-283 --- Does nitrification lose its function, or is it functioning at its maximum (for whatever reason) under NH4-replete conditions? Denitrification efficiency (proportion of external N load removed via denitrification) often decreases with higher external N loads (e.g., Gardner & McCarthy 2009; Biogeochem 95:185-198), but denitrification is still occurring, presumably at or near its maximum under the conditions. So, it hasn’t lost its function, but its impact on the system might be less. Perhaps some clarification is needed to distinguish whether nitrification is shutting down (losing its function) or if it is functioning at a maximum level for the conditions, but perhaps unable to keep up with OM remineralization, as implied in the text.

Lines 302-304 --- If the modeled anoxic NH4 efflux is lower than it can be in nature, then would the timeline for a return to pre-industrial conditions potentially be over-estimated? The effects of climate change were not pursued in this study, but I wonder about the possible effects of sea level rise (e.g., Kapsi et al., 2023; J Mar Sci Eng 11(8):1514). Would sea level rise perhaps improve redox conditions in coastal areas of the Baltic Sea (or maybe even the opposite), and if so, would the timeline for return to pre-industrial conditions perhaps be over-estimated? (I’m fishing for hope here!).

Section 5

Line 345 --- Consider softening the language a bit here. For example, I’m not sure that you’ve ‘demonstrated’ anything here by using a model…demonstration probably requires empirical studies, direct flux measurements, etc. I suggest editing to: “Modeling results suggest that the eutrophication-derived OM pool in the sediments of the coastal Baltic Sea is likely the key factor that continuously maintains…”.

Line 349 --- “…continuously new deposited OM…” --- Consider editing to “…continuously-deposited, new OM…”

Line 372 --- I’m not sure that I fully agree with this statement. Sure, we anticipate that distance from OM/nutrient source (e.g., tributary discharge) is a factor in how severe the impacts might be (in terms of your modeling results), but without empirical data/direct measurements of fluxes across the sediment-water interface along some OM/nutrient gradient, then it seems too big of a leap to suggest only localized effects. The model also does not consider internal N loading from the water column, which may be much larger than from sediments, or even larger than external N loads (e.g., Lake Erie; Hoffman et al. 2022; L&O 67:2028-2041). A few sentences prior, the coastal current is mentioned as a conduit for nutrients and OM to be deposited in the open Baltic Sea, which might argue against the localized effects speculation? Basically, I don’t think that the speculation is incorrect, but I’m not convinced that the evidence from this study is comprehensive enough to fully support the speculation. I think the text here could be rewritten in a softer way, which still allows you to make the point about possible localization of effects, but doesn’t sound more definitive than can be justified (e.g., “…the impact of internal N loading, from sediments represented in our modeling study, on the coastal ecosystem may be localized, as despite…”).