Contrasting sediment carbon storage and turnover in vegetated littoral and open water habitats of boreal lakes

Kankainen, Max Oliver Aleksi; Lindroth Dauner, Ana Lúcia; Junna, Tuomas; Simojoki, Asko; Koho, Karoliina; Jilbert, Tom

doi:10.5194/egusphere-2026-3899

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https://doi.org/10.5194/egusphere-2026-3899

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20 Jul 2026

| 20 Jul 2026

Contrasting sediment carbon storage and turnover in vegetated littoral and open water habitats of boreal lakes

Max Oliver Aleksi Kankainen, Ana Lúcia Lindroth Dauner, Tuomas Junna, Asko Simojoki, Karoliina Koho, and Tom Jilbert

Abstract. In recent years, the research interest of aquatic systems as natural carbon sinks has grown rapidly. Diffuse organic carbon buried in lake sediments represents an important long-term carbon sink, with potential to mitigate climate change by removing carbon from active biogeochemical cycling on timescales of millennia. This sink may be particularly significant in boreal regions such as Finland, where lakes cover approximately 10 % of the land surface. However, major knowledge gaps limit our ability to assess the true sink potential of boreal lakes. Specifically, the spatial heterogeneity and balance between burial and remineralization of organic carbon, and the role of the vegetated littoral zone (“teal carbon habitat”) in carbon storage remain poorly constrained.

To address these gaps, we studied three boreal lakes along a latitudinal gradient across Finland, by sampling sediment, porewater, and surface water from three sites in the teal carbon habitat and three to four in the open water (limnetic) habitat. Our results reveal two contrasting carbon‑cycling regimes: teal carbon habitats might accumulate substantial organic carbon inventories in the top 10 cm (SOC₁₀) of the sediment, where turnover is negatively associated with inventory due to greater retention of fine-grained sediments, resulting in lower diffusive turnover efficiency. In open water habitats, total diffusive carbon flux is closely linked to organic matter quality, referring to its composition and reactivity, while SOC₁₀ remains consistent among sites. Turnover efficiency across all sites is low (< 5.5 %), implying only a small fraction of the near‑surface SOC pool is processed annually. These findings highlight the importance of protecting vegetated near‑shore belts from physical disturbance to safeguard SOC storage and managing catchment inputs to moderate open water carbon fluxes. Notably, not all teal carbon habitats are equal; sandy sites possess small SOC stocks below the top 10 cm and exhibit higher turnover rates, suggesting that interventions in these areas may carry lower risks.

Received: 01 Jul 2026 – Discussion started: 20 Jul 2026

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Max Oliver Aleksi Kankainen, Ana Lúcia Lindroth Dauner, Tuomas Junna, Asko Simojoki, Karoliina Koho, and Tom Jilbert

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-3899', Anonymous Referee #1, 23 Sep 2026

Paper summary
This paper presents a valuable assessment of the spatial heterogeneity of carbon-cycling processes across three boreal lakes. It demonstrates that littoral teal carbon zones play an important role and hold substantial SOC stocks, and it offers a useful comparison of carbon cycling between these littoral zones and open-water regions. The topic is timely, and the analysis is supported by an extensive and robust multiproxy, multisite dataset that could inform future targeted mitigation strategies in these systems. Given the strength of the dataset and the thoroughness of the interrogation of the results, I am happy to recommend the paper for publication subject to the minor revisions outlined below.
Major comments
The rationale for focusing on boreal lakes would benefit from expansion. A clearer statement of why boreal lakes are important to the global carbon cycle, and how they are disproportionately significant relative to their areal extent, would help situate the study's contribution for a broader readership.
The manuscript makes the important point that local characteristics, particularly sediment particle size within the teal carbon zones, play a key role in governing SOC stocks. Given how central this finding is, the paper would be strengthened by additional detail on how the teal carbon sites were selected in each lake. In particular, information on each site's proximity to established inflows and to the contemporary shoreline would allow readers to interpret the drivers of the observed variability and would reinforce the comparisons being drawn.
Additionally, in section 2.1, it is unclear why these specific lakes were selected for analyses aside from their distribution along a latitudinal gradient. Given the variable catchment sizes, hydrological setup and relative exposure to anthropogenic activity, the paper would be strengthened setting out a more clear rationale for their selection.
Specific comments
The key figure is overly complicated and tries to fit in too much information. The map and the histogram figure are repetitions of Figure 1 and should either be removed to make the figure entirely a schematic diagram illustrating key mechanisms, or refined to express these data more clearly.
Figure 1: Point sizes to show the location of sampling sites are too large, particularly for Kallavesi where some points overlap. Reduce size to make clearer distinction of each site and its proximity to the shore.
Line 194: Typo: ‘QGis’ replace with all capitals.
Line 536: ‘Proximity to land further enhances terrestrial inputs’. Unclear. Refine to reflect the increasing proximity

Citation: https://doi.org/10.5194/egusphere-2026-3899-RC1
- AC1: 'Reply on RC1', Max Oliver Aleksi Kankainen, 22 Oct 2026
  
  Thank you for the constructive review. We will attempt to incorporate the suggested revisions.
  Major comments
  The rationale for focusing on boreal lakes would benefit from expansion. A clearer statement of why boreal lakes are important to the global carbon cycle, and how they are disproportionately significant relative to their areal extent, would help situate the study's contribution for a broader readership.
  Reply: We will add this to the text as suggested.
  The manuscript makes the important point that local characteristics, particularly sediment particle size within the teal carbon zones, play a key role in governing SOC stocks. Given how central this finding is, the paper would be strengthened by additional detail on how the teal carbon sites were selected in each lake. In particular, information on each site's proximity to established inflows and to the contemporary shoreline would allow readers to interpret the drivers of the observed variability and would reinforce the comparisons being drawn.
  Reply: We will modify the text as suggested.
  Additionally, in section 2.1, it is unclear why these specific lakes were selected for analyses aside from their distribution along a latitudinal gradient. Given the variable catchment sizes, hydrological setup and relative exposure to anthropogenic activity, the paper would be strengthened setting out a more clear rationale for their selection.
  Reply: We will add this information to the text as suggested.
  Specific comments
  The key figure is overly complicated and tries to fit in too much information. The map and the histogram figure are repetitions of Figure 1 and should either be removed to make the figure entirely a schematic diagram illustrating key mechanisms, or refined to express these data more clearly.
  Reply: We will modify the key figure as suggested, by removing the map and the histogram.
  Figure 1: Point sizes to show the location of sampling sites are too large, particularly for Kallavesi where some points overlap. Reduce size to make clearer distinction of each site and its proximity to the shore.
  Reply: We will modify the figure as suggested.
  Line 194: Typo: ‘QGis’ replace with all capitals.
  Reply: We will modify the text as suggested.
  Line 536: ‘Proximity to land further enhances terrestrial inputs’. Unclear. Refine to reflect the increasing proximity
  Reply: We will modify the text as suggested.
  
  Citation: https://doi.org/10.5194/egusphere-2026-3899-AC1
RC2:
'Comment on egusphere-2026-3899', Anonymous Referee #2, 05 Oct 2026

Although the manuscript is well-written and offers valuable insights into carbon storage within boreal lake 'teal carbon' habitats versus open lake sediments, several major and minor concerns must be addressed. Therefore, I recommend a major revision.
General remarks
The authors chose an arbitrary depth, the top 10 cm, to quantify near-surface sediment organic carbon (SOC) inventories, fluxes, and turnover efficiencies across 'teal carbon' and open-water habitats. While using a standardised depth simplifies comparisons, doing so arbitrarily without accounting for potentially drastic differences in sedimentation, compaction, and accumulation rates between these two highly distinct habitats could raise some issues.
Typically, 'teal carbon' habitats experience higher organic matter inputs, more trapping of suspended solids, and different bioturbation/root-mixing dynamics compared to open lake sediments. Consequently, 10 cm of sediment in a littoral macrophyte patch may represent a significantly different chronological timescale (and mass of dry sediment) than 10 cm in the open lake. These issues and limitations should be addressed, as it requires further justification regarding why the authors chose this arbitrary depth.
Quite a few minor statements are over-supported by up to ten citations. In fact, the references take up more text than the actual assertion, for example: “Conversely, plant aerenchyma can transport CH4 directly to the atmosphere, bypassing oxidation (Van Der Nat and Middelburg, 1998a; Dorodnikov et al., 2011; Riutta et al., 2020; Villa et al., 2020; Yang et al., 2022; Ge et al., 2023, 2024; Chanton et al., 1989; Whiting and Chanton, 1993; Joabsson et al., 1999).” Such a high number of citations is redundant. Cutting down the number of references will make the manuscript much easier to read.
Abstract
Line no. 12–13: This is a minor point, as the abstract is otherwise clear. Since the manuscript places a strong focus on teal carbon habitats, the authors should consider explicitly mentioning the true extent of this land cover in the abstract. The current 10% figure only accounts for open inland waters (rivers and boreal lakes), thereby under-representing the substantial spatial footprint and carbon significance of the associated wetlands and peatlands.
Introduction
Line no. 48–49: Here, it is stated that lake carbon burial rates are comparable to marine carbon burial rates per unit area, yet this comparison overlooks critical variations in water depth across marine zones. For instance, while shallow coastal zones exhibit high burial efficiencies driven by rapid sedimentation, deep abyssal plains typically exhibit near-zero burial rates. This should be made clearer.
Line no. 91–110: This section of the introduction feels disconnected from the rest of the text and disrupts the narrative flow.
Line 136–144: This section would be better suited for the discussion, where it could be more seamlessly integrated into the broader context of the study.
Methods
Line 154: Consider replacing ‘existence’ with ‘availability’, and ‘accessibility’ can be removed to avoid redundancy.
Line 187: What is the justification for selecting four sites in Kallavesi? Two of them appear to be in very close proximity to each other.
Line 190: Remove the double dot.
Line 195: Does this imply that only the teal carbon areas where randomly selected?
Line no. 293–295: Was the temperature increase accounted for when calculating the maximum gas capacity in the headspace?
Line 321: This should be ‘phosphorus’ and not ‘phosphorous’.
Discussion
Line no. 530–535: During the sediment coring of the teal carbon sites, was there any significant biomass present (e.g., roots or other plant residues)? If so, it should be expected that the carbon content here is inherently much higher compared to samples taken from the open lake. Could the authors clarify if and how they accounted for, or separated, this living biomass from the sedimentary organic carbon pool?
Line no. 611: The acronym ACF has not been introduced yet.
Line no. 657–659: Are there any estimates available on how large the effects of plant-mediated transport and ebullition can be, based on similar environments? In other words, it is expected to be much larger than diffusion alone? Hence the ‘likely’ underestimate might be ‘heavily’ underestimated.

Citation: https://doi.org/10.5194/egusphere-2026-3899-RC2
- AC2: 'Reply on RC2', Max Oliver Aleksi Kankainen, 22 Oct 2026
  
  Thank you for the constructive review. We will attempt to incorporate the suggested revisions.
  General remarks
  The authors chose an arbitrary depth, the top 10 cm, to quantify near-surface sediment organic carbon (SOC) inventories, fluxes, and turnover efficiencies across 'teal carbon' and open-water habitats. While using a standardised depth simplifies comparisons, doing so arbitrarily without accounting for potentially drastic differences in sedimentation, compaction, and accumulation rates between these two highly distinct habitats could raise some issues.
  Typically, 'teal carbon' habitats experience higher organic matter inputs, more trapping of suspended solids, and different bioturbation/root-mixing dynamics compared to open lake sediments. Consequently, 10 cm of sediment in a littoral macrophyte patch may represent a significantly different chronological timescale (and mass of dry sediment) than 10 cm in the open lake. These issues and limitations should be addressed, as it requires further justification regarding why the authors chose this arbitrary depth.
  Reply: We made an attempt to date the teal carbon sediments using ¹³⁷Cs, this exercise showed that the sediment accumulation at these sites is not continuous as evidenced by a lack of a clear 1986 horizon and generally low ¹³⁷Cs enrichments (Lindroth Dauner et al., 2026). Therefore, we interpret these settings as temporary accumulation zones for organic matter rich sediments, which may later be eroded and transported to deeper areas. We acknowledge the difficulties of comparing carbon dynamics of such environments with those of the continuous accumulation zones of open waters, but we still consider it to be a useful exercise. Moreover, we recognize the potential of continuous accumulation zones to store more carbon in the long term. We will modify the text to emphasize these points.
  Quite a few minor statements are over-supported by up to ten citations. In fact, the references take up more text than the actual assertion, for example: “Conversely, plant aerenchyma can transport CH4 directly to the atmosphere, bypassing oxidation (Van Der Nat and Middelburg, 1998a; Dorodnikov et al., 2011; Riutta et al., 2020; Villa et al., 2020; Yang et al., 2022; Ge et al., 2023, 2024; Chanton et al., 1989; Whiting and Chanton, 1993; Joabsson et al., 1999).” Such a high number of citations is redundant. Cutting down the number of references will make the manuscript much easier to read.
  Reply: We will modify the text as suggested.
  Abstract
  Line no. 12–13: This is a minor point, as the abstract is otherwise clear. Since the manuscript places a strong focus on teal carbon habitats, the authors should consider explicitly mentioning the true extent of this land cover in the abstract. The current 10% figure only accounts for open inland waters (rivers and boreal lakes), thereby under-representing the substantial spatial footprint and carbon significance of the associated wetlands and peatlands.
  Reply: We will add this information to the text as suggested.
  Introduction
  Line no. 48–49: Here, it is stated that lake carbon burial rates are comparable to marine carbon burial rates per unit area, yet this comparison overlooks critical variations in water depth across marine zones. For instance, while shallow coastal zones exhibit high burial efficiencies driven by rapid sedimentation, deep abyssal plains typically exhibit near-zero burial rates. This should be made clearer.
  Reply: We will clarify this in the text as suggested.
  Line no. 91–110: This section of the introduction feels disconnected from the rest of the text and disrupts the narrative flow.
  Reply: We will add some clarification to explain the paragraph.
  Line 136–144: This section would be better suited for the discussion, where it could be more seamlessly integrated into the broader context of the study.
  Reply: We believe that this paragraph provides useful context for the motivations and implications of this study. Therefore, we respectfully disagree and intend to keep the text in its current place.
  Methods
  Line 154: Consider replacing ‘existence’ with ‘availability’, and ‘accessibility’ can be removed to avoid redundancy.
  Reply: We will modify the text as suggested.
  Line 187: What is the justification for selecting four sites in Kallavesi? Two of them appear to be in very close proximity to each other.
  Reply: The design included one site with a nearby replicate located a few tens of meters away to assess reproducibility (sites S04 and S05). Carbon stocks at these sites were similar, indicating that sedimentation patterns are relatively homogeneous at this spatial scale. By contrast, sediment diffusive fluxes differed markedly between the two sites. This divergence, despite similar carbon stocks, highlights that flux dynamics can vary at fine spatial scales and warrants discussion of potential drivers.
  Line 190: Remove the double dot.
  Reply: We will modify the text as suggested.
  Line 195: Does this imply that only the teal carbon areas where randomly selected?
  Reply: The lakes and overall site locations were selected purposively, based on prior knowledge of water quality, the availability of long‑term environmental monitoring data, and aerial imagery showing extensive macrophyte coverage. Within the chosen teal carbon patches, the starting point of the coring transects were selected at random. For open water sampling we prioritized basins and areas that had less prior coverage in existing datasets and sampled distinct sub‑basins in each lake. The specific sampling point within each selected sub‑basin was chosen at random. We will modify the text to emphasize these points.
  Line no. 293–295: Was the temperature increase accounted for when calculating the maximum gas capacity in the headspace?
  Reply: Yes. The maximum gas capacity in the headspace was calculated using the temperature at equilibrium, e.g. the temperature of the lab air.
  Line 321: This should be ‘phosphorus’ and not ‘phosphorous’.
  Reply: We will modify the text as suggested.
  Discussion
  Line no. 530–535: During the sediment coring of the teal carbon sites, was there any significant biomass present (e.g., roots or other plant residues)? If so, it should be expected that the carbon content here is inherently much higher compared to samples taken from the open lake. Could the authors clarify if and how they accounted for, or separated, this living biomass from the sedimentary organic carbon pool?
  Reply: We made an attempt at separating and quantifying this biogenic biomass from the organic sediment (see. Lindroth Dauner et al., 2026). The measurements discussed in this paper were performed on the organic sediment fraction. Other recent studies in coastal reed beds have found that even when you have lots of roots the sediment organic matter is still larger than the biomass carbon (Williamson et al., 2026a, 2026b).
  Line no. 611: The acronym ACF has not been introduced yet.
  Reply: We will modify the text by introducing the acronym at its first conception in the discussion. Also, we will change the terminology by using ‘turnover’, instead of ‘flux’, as we believe that this is more descriptive.
  Line no. 657–659: Are there any estimates available on how large the effects of plant-mediated transport and ebullition can be, based on similar environments? In other words, it is expected to be much larger than diffusion alone? Hence the ‘likely’ underestimate might be ‘heavily’ underestimated.
  Reply: That is correct, plant-mediated transport and ebullition are expected to be much larger than diffusion alone for methane flux. We will modify the text as suggested by adding the word ‘heavily’ after the word ‘likely’ on line 658.
  References
  Lindroth Dauner, A. L., Kankainen, M. O. A., Väkevä, S., Asmala, E., Järvinen, M., Koho, K., and Jilbert, T.: Shoreline exposure controls teal carbon accumulation in boreal lakes, Biogeosciences, 23, 3637–3653, https://doi.org/10.5194/bg-23-3637-2026, 2026.
  Williamson, M. F., Jilbert, T., Norkko, A., and Gustafsson, C.: Carbon storage in coastal reed ecosystems, Biogeosciences, 23, 1327–1340, https://doi.org/10.5194/bg-23-1327-2026, 2026a.
  Williamson, M. F., Jilbert, T., Gustafsson, C., and Norkko, A.: Seasonal Changes and Environmental Drivers in Coastal Reed Bed Carbon Storage, https://doi.org/10.21203/rs.3.rs-9814137/v1, 2026b.
  
  Citation: https://doi.org/10.5194/egusphere-2026-3899-AC2

Max Oliver Aleksi Kankainen, Ana Lúcia Lindroth Dauner, Tuomas Junna, Asko Simojoki, Karoliina Koho, and Tom Jilbert

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https://doi.org/10.5194/egusphere-2026-3899-supplement

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Contrasting sediment carbon storage and turnover in vegetated littoral and open water habitats of boreal lakes Max O. A. Kankainen et al. https://doi.org/10.5281/zenodo.21061456

Max Oliver Aleksi Kankainen, Ana Lúcia Lindroth Dauner, Tuomas Junna, Asko Simojoki, Karoliina Koho, and Tom Jilbert

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Short summary

Lakes are important for the global carbon cycle, yet the variability in carbon burial and degradation, especially in vegetated shorelines, is not well understood. We found that vegetated shorelines can accumulate sediment carbon over long periods with low degradation or over short periods with high degradation, depending on hydrodynamic energy. In contrast, open water sediments accumulate less carbon but do so more reliably over time, with degradation influenced by the source of organic matter.


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