the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jul 2026
Contrasting sediment carbon storage and turnover in vegetated littoral and open water habitats of boreal lakes
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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3899', Anonymous Referee #1, 23 Sep 2026
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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
Data sets
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
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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