| 19 Jun 2026
Structural Complexity and Preferential Flowpaths Govern Connectivity and DOC Export in a Remote Boreal Headwater Peatland Complex
Abstract. Northern peatlands store globally significant carbon stocks and are major sources of dissolved organic carbon (DOC) to downstream waters, yet the hydrological mechanisms controlling water and waterborne carbon export remain poorly resolved. This uncertainty is particularly important in structurally complex landscapes where wetlandtype, peat thickness, and mineral substrate properties shift over short distances, potentially creating localized but highly connected flowpaths that are difficult to capture in landscape-scale models and monitoring frameworks. Here, hydrological conditions, geochemistry, and DOC concentrations were monitored over two growing seasons across a peatland–lake transition in Canada’s Precambrian Shield and integrated with subsurface flowpath modelling to determine how landscape structure and flowpath activation regulate water and DOC export.
Flowpath geometry remained stable despite substantial variation in water availability, indicating strong structural control by landscape morphology and subsurface properties. In contrast, water table dynamics varied among landscape units, constraining near-surface flowpath activation. Limited near-surface connectivity in upgradient wetlands was insufficient to explain persistent groundwater discharge at the outlet. Instead, evidence supports preferential subsurface flowpaths linking upgradient peatland units to a groundwater spring, sustaining discharge and episodically mobilizing DOC-rich waters. Catchment-scale annual DOC export to the downgradient lake was estimated between 3.1 and 14.1 g C m⁻² yr⁻¹. More than 20% of this DOC export originated from the single groundwater spring, demonstrating that small, preferentially connected areas can exert disproportionate control on peatland carbon loss to downstream aquatic ecosystems. These findings suggest that predicting aquatic carbon export from northern peatlands under climate change and land-use disturbance requires greater attention to subsurface structure, threshold-mediated connectivity, and localized flowpaths that may be overlooked when peatlands are treated as spatially uniform source areas.
General comments:
In Balliston et al, authors conduct a detailed study of groundwater flow-path and hydrological connectivity through a complex wetland-dominated catchment and further link variation in site hydrology to DOC export over two growing seasons. Findings from what I imagine were quite labor intensive field campaigns are supported by hydrological modeling, resulting in an in-depth characterization of DOC movement through wetland units under variable hydrological connectivity. Overall, I found the scientific approach and methodology employed in this study to be well suited to addressing the authors' stated objectives and I consider this manuscript to be a valuable contribution to scientific understanding of the mechanisms underlying DOC export form peatlands.
specific comments:
My primary critique is as follows: Description of dynamic flow paths through peat and mineral soils as well as spatio-temporal variability in DOC is described at great detail. However, the connection between these results and the export of DOC from the peatland can only be found at the end of the discussion section. This seems a significant shortcoming given the title of the manuscript, and that – as written in the introduction - DOC export to surface waters is a primary motivation for this study.
I do believe this can be largely resolved by restructuring of the manuscript. Most significantly, section 560 to 575 and Table 3 should be moved from discussion to methods and results sections. These findings present an important synthesis of the paper, pulling together flowpaths, DOC and the groundwater spring. I also suggest that this analysis be elevated within the paper. Methods for developing table two could be explained in more detail (see comments below), but also, it would be very interesting to show how QDOC for all components in Table 3 changed over time – and in response to varying hydrological conditions (ie. water table height at 1A). Given the wide range presented for all contributors to DOC export, this could be quite informative.
In addition, the potential for peatland pipes as transporters of water and DOC should be introduced in the introduction. This was not an expected element of the study at conception, but it clearly had a significant impact on study results. Preferential flow paths are mentioned in the introduction but only briefly
Technical corrections
50 It’s not clear to me what “preservation” means in this context, can this word be removed?
Ex: “…slow transit times can result in accumulation of DOC…”
55 Quite a bit of the introduction is spent on the spill and fill concept, but it is minimally discussed further in the manuscript. I suggest authors increase discussion of how results either do or do not support the spill and fill concept – or remove some of this content from the introduction
120 I suggest rewording for clarity – “… in both regions, fine grained deposits can act as aquitards…”
125 Is “swamp” a synonym for wetland, or is it intended to mean something more nuanced? If it is not strictly a synonym, this could use some elaboration. If not, choose either wetland or swamp and use throughout the manuscript.
145 “at the inferred highest elevation part of the catchment” would be a bit more clear
290 “measured lateral hydraulic gradients remained consistently aligned with surface slope throughout the monitoring period, indicating stable downslope flow” It’s not clear to me how the table shows consistency through the monitoring period given that the table simple provides an average value for lateral gradient during the study period
305 extra “)” in table caption
Figure 2: I think reversing the y axis so that the deepest water table is also the lowest on the graph would be more intuitive.
310 suggest authors replace “boxes” with “bars” in figure caption
320 Based on Figure 3, it seems like the elevated Ksat at deeper peat levels extends to between 3A to 4A. Please amend text if that is correct.
335 I’m a bit confused by the wording of this sentence – “Under the shallowest water table conditions (Figure A6), minimum transit times to the outlet from Swamp 1, Bog 1 and Swamp 2 occurred the near surface acrotelm and were on the order of 10, 100 and 500 days, respectively.” Perhaps breaking it into two sentences would help
340 In the legend of fig 3, I find the either single of dual colors representing Kh a bit confusing. It would also be helpful if the color distinction were in the legend in addition to the figure caption. One way to solve this is to have two separate legend for kh, one representing peat layers and the other mineral layers
385 I suggest DOC be reported before other chemical variables as it is more central to the main message of the manuscript
Figure 6. include in the legend what the inset pipe with outflow represents.
415 There are only 2 sentences in this paragraph. I think breaking sentences up into 2 or more would help clarity.
425, remove “,” between complex and because
425 How does the fact that “similar landscape positions can support fundamentally different wetland morphologie” support the claim that “Flowpath geometry… is nonetheless complex”? Can you expand on that link more?
435-445 These are really interesting observations, but I think that these two paragraphs would be better placed later in the discussion section. Their placement now interrupts the flow from the first to the second section. I suggest giving this content its own section at the end of the discussion and more strongly linking the need for accurate characterization of surface structure with DOC export.
450 it would be good to bring back the fill and spill concept here as it was so thoroughly introduced in the introduction
480 What is the evidence for spill and fill inputs? Can you be more explicit about this link?
530 Do authors have recommendations for the further investigation of macropores – given their importance in this study? This would fit well here.
560 This is an important synthesis of this research – it should be moved to the methods and results section. However, a few things are not fully clear in Table 3: What is QDOC and how was it calculated, and why did authors normalize these values?
Can these flux estimates be corroborated with lake concentrations of DOC?
The figure following “water table position” is not specified