the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Hydrological connectivity controls dissolved organic carbon exports in a peatland-dominated boreal catchment stream
Antonin Prijac
Laure Gandois
Pierre Taillardat
Marc-André Bourgault
Khawla Riahi
Alex Ponçot
Alain Tremblay
Michelle Garneau
Abstract. Peatland-derived dissolved organic carbon (DOC) exports from boreal peatlands are variable during the ice-free season, depending on the peatland water table and the alternation of low and high flow in peat-draining streams. However, calculation of the specific DOC exports from a peatland can be challenging considering the multiple potential DOC sources within the catchment. A calculation approach based on the hydrological connectivity between the peat and the stream could help to solve this issue, an approach used in the present study. This study took place from June 2018 to October 2019 in a boreal catchment in north-eastern Canada, with 76.7 % of the catchment covered by ombrotrophic peatland. The objectives were (1) to establish relationships between DOC exports from a headwater stream and the peatland hydrology; (2) to quantify, at the catchment scale, the amount of DOC laterally exported to the draining stream; and (3) to define the patterns of DOC mobilization during high river flow events. At the peatland headwater stream outlet, the DOC concentrations were monitored at a high frequency (hourly) using a fluorescent dissolved organic matter (fDOM) sensor, a proxy for DOC concentrations. Hydrological variables, such as stream outlet discharge and the peatland water table depth (WTD), were continuously monitored for 2 years. Our results highlight the direct and delayed control of subsurface flow from peat to the stream and associated DOC exports. Rain events raised the peatland WTD, which increased the hydrological connectivity between the peatland and the stream. This led to increased stream discharge (Q) and a delayed DOC concentration increase, typical of lateral subsurface flow. The magnitude of the WTD increase played a crucial role in influencing the quantity of exported DOC. Based on the assumption that the peatland is the major contributor to DOC exports and other DOC sources were negligible during high-flow periods, we propose a new approach to calculate the specific DOC exports attributable to the peatland by distinguishing the surface used to the calculation between high-flow and low-flow periods. In 2018–2019, 92.6 % of DOC was exported during flood events, despite accounting for 59.1 % of the period. In 2019–2020, 93.8 % of DOC was exported during flood events, which represented 44.1 % of the period. Our analysis of individual flood events revealed three types of events and DOC mobilization patterns. The first type is characterized by high rainfall leading to an important WTD increase favouring the connection between the peatland and the stream, leading to high DOC exports. The second is characterized by a large WTD increase succeeding a previous event that had depleted DOC available to be transferred to the stream, leading to lower DOC exports. The third type corresponds to low rainfall events with an insufficient WTD increase to reconnect the peatland and the stream, leading to low DOC exports. Hence, DOC exports are sensitive to hydroclimatic conditions. Moreover, flood events, changes in rainfall regimes, the ice-free season duration and porewater temperatures may affect the exported DOC and, consequently, partially offset the carbon sequestration capacity of peatlands.
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Antonin Prijac et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2022-426', Anonymous Referee #1, 30 Apr 2023
General comments
The authors collected a valuable data set on DOC export in small catchment, which is partly covered by peat. They highlighted the relationship between water table depths and DOC export, calculated area specific DOC export and analyzed DOC export mechanisms during individual flood events. Their data contributes to the ongoing discussion on mechanisms for DOC mobilization and its link to hydrological connectivity. I generally think that the paper is worthy for publication. However, there are a few points that need to be carefully addressed, especially regarding the calculation of area specific DOC export and peatland coverage.
- A large part of your paper focuses and the difference between peat-covered and not peat-covered area. In L128 you mention the surface, however, you do not explain how you assessed peat coverage in your catchment. I think this would be very important regarding how important these areas are for your argumentation.
- Also, your argumentation in section 5.2. is unclear to me. Of course, area specific DOC export increases with decreasing surface area. To switch between different surface areas according to flow, you need to be very sure of which area actually contributes to DOC export. In L424-430 you argue that peatlands are probably contributing less to DOC export during low flow periods because of a missing hydrological connectivity, so you use the total surface area to calculate area specific DOC export. You argue that during high flow the peatlands become more important for DOC export, therefore you use the smaller peatland-covered surface to calculate specific DOC export. But in my point of view, this leads to an overestimation of specific DOC export. How can you be sure that the rest of the area does not contribute to DOC during high flow? Would a higher hydrological connectivity not lead to a larger contributing area rather than a smaller one? This needs to be made clear. In this context, it would be useful to understand where the peats are located. Are they further away from the stream (this would be unusual) and therefore connected only during high-flow? Would it be possible to highlight peat-covered areas in Figure 1?
- I think the clustering of events is very interesting and well done. However, I think that one important parameter is missing. What was the event size of these events? Was event size different between the clusters and did therefore influence DOC export in different ways? I think that you could elaborate much more on the reasons for the occurrence of the different event types (see specific comment).
- You often write C (in units) but I think you mean DOC. Either use DOC or make clear in the beginning that C refers to DOC in your case.
Specific comments
L25 – Could you state at which interval you monitored the WTD?
L45 – Please insert „it“ before „is crucial”.
L58 – Do you mean “Strong positive relationships”? Otherwise, one could think that there were strong relationships (which might be negative) and positive relationships (which might be weak).
L63 – Consider inserting “total” before “surface”.
L91-L92 I agree, but could you add references for this statement and give possible explanations? Also, some studies have shown that dry conditions could hinder DOC production (e.g. see references within Kalbitz et al. (2000))
L195 At which depth were the wells installed?
L122 – Do you have an idea about how this microtopography could influence the DOC dynamics at your sites? Recent studies have shown that microtopography can be important for chemical and hydrological processes (Blaurock et al., 2022; Diamond et al., 2021; Mazzola et al., 2021).
L133 This number doesn’t seem to be correct.
L137 This is not really the event size but rather the daily precipitation. But do you have event size data as well? This would be interesting as daily precipitation only gives us an average.
L175 How many samples did you use to calibrate? In Figure 3, it looks like you took 6 samples, which would be a very low number for a calibration. Do you have the calibration curves and R2 values? You could maybe add them to the supplementary material.
L192 Which uncertainties do you mean? Can you specify?
L206 Did you also measure snowfall? Was snowfall counted as precipitation?
L220 The 10th quantile of which period?
L290-293 Do you need all the decimals here? The error margin is probably much larger.
L315-L316 Maybe I missed it, but I think that you do not further elaborate on the importance on porewater temperature for DOC stream concentrations. Does the porewater temperature add information to stream temperature? What could be reasons for the negative correlation? I wonder why this is brought up here quite prominently but then not used in the discussion.
L323-324 I think it would be okay if you mention the rounded values again.
L401-402 With accretion you mean an increase of DOC concentrations in the stream? I am not sure if accretion is the right word here? Maybe accumulation? Or maybe add “in the stream”.
L432 These numbers refer to DOC only. If you use C, this would include DIC and POC in my point of view. As your write later, DOC only accounts for a small percentage of total C exports.
L467 ff It would be really interesting to know the different event sizes of the clusters. Do you have data on this? Event size could significantly influence DOC export.
L482 Again, better use DOC.
L483-L484 Maybe write “less negative”. At first, I thought “high” meant a large magnitude of the HI, which got me confused about your interpretation.
L497 “a single event”
L498-505 This is really interesting and I think you could elaborate much more on the different mechanisms which lead to the high DOC export. For example, the longer dry period could lead to an accumulation of DOC, which is being produced but not exported (Bb). And why is the Aa event so important? Is snowmelt the reason?
L504 “initiated”
Figure 2 Add to the caption that you mean the DOC flux in the stream.
Figure 3 In the caption b) is missing but d) is double.
Figure 4 You could add titles above the panels showing the corresponding year.
Table 2 Check the superscription of units in Table 2b).
Figure 5 I understand that you used normalized values here to better compare the hysteresis patterns. However, like this information on event characteristics gets lost. I wonder if you could prepare the same figure with unnormalized data for the supplementary material? Also, is the count always hourly? Add this information to the caption.
References
Blaurock, K., Garthen, P., Da Silva, M. P., Beudert, B., Gilfedder, B. S., & Fleckenstein, J. H., et al. (2022). Riparian Microtopography Affects Event‐Driven Stream DOC Concentrations and DOM Quality in a Forested Headwater Catchment. Journal of Geophysical Research: Biogeosciences, 127(12). https://doi.org/10.1029/2022JG006831
Diamond, J. S., Epstein, J. M., Cohen, M. J., McLaughlin, D. L., Hsueh, Y.-H., Keim, R. F., & Duberstein, J. A. (2021). A little relief: Ecological functions and autogenesis of wetland microtopography. WIREs Water, 8(1). https://doi.org/10.1002/wat2.1493
Kalbitz, K., Solinger, S., Park, J.-H., Michalzik, B., & Matzner, E. (2000). Controls on the dynamics of dissolved organic matter in soils: A review. Soil Science, 165(4), 277–304. https://doi.org/10.1097/00010694-200004000-00001
Mazzola, V., Perks, M. P., Smith, J., Yeluripati, J., & Xenakis, G. (2021). Seasonal patterns of greenhouse gas emissions from a forest‐to‐bog restored site in northern Scotland: Influence of microtopography and vegetation on carbon dioxide and methane dynamics. European Journal of Soil Science, 72(3), 1332–1353. https://doi.org/10.1111/ejss.13050
Citation: https://doi.org/10.5194/hess-2022-426-RC1 -
RC2: 'Comment on hess-2022-426', Anonymous Referee #2, 01 Jun 2023
Review of manuscript #hess-2022-426 by Prijac et al.
The authors conducted a study to examine the relationship between hydrological connectivity and carbon exports in a peatland-dominated watershed. They aimed to achieve several objectives: a) establish the connection between dissolved organic carbon (DOC) exports and peatland hydrology, b) quantify the lateral export of DOC into the stream at the catchment scale, and c) identify patterns of DOC mobilization during high-flow events. They propose a method to estimate carbon exports based on the relative contribution of the peatland in relation to the whole watershed.
The authors deployed a multiparameter probe at the watershed outlet to measure fDOM, turbidity, DO, SpC, water temperature, and pH hourly from June 2018 to May 2020. The relationship between fDOM and DOC was assessed by analyzing temperature-corrected fDOM signals and DOC concentrations obtained from grab samples collected during 5 and 4 sampling events in 2018 and 2019, respectively. Streamflow was estimated from year-round water level measurements, with the relationship calibrated using field streamflow measurements, except for the spring thaw period, where a PHIM model was used. Hidden Markov chains were used to classify the streamflow data into high and low flow periods. Water table elevations were recorded hourly at six wells during the growing season from June 2018 to October 2020.
The design of the study is sound. The paper is well-written, the figures are clear, and the methods section includes good detail. Altogether, the study is a good contribution to the field and improves our understanding of the role of boreal headwater streams in the carbon cycle. I have some recommendations I think will improve the flow of the manuscript and its impact. My major recommendation is to tone down some of the statements regarding WTD and Q relationships since the study does not technically prove lateral flow directionality (see comments below regarding Lines 399 and 515) and to slightly expand the discussion to explain better the results on the context of other studies done in the same site and other boreal streams (see comment below regarding L457).
Specific comments:
L190 “The calculation method was…” replace by “The calculation method for…. was calculated….”
L191 Please explain how you measured streamflow used in your calibration.
L192-193 “daily water discharge was modeled using PHIM” – please clarify if you modeled streamflow only during spring thaws or for all year round.
L195 - How deep were the wells?
L196 – “water-level data logger… from June 2018 to October 2020” – is that right? Or were they only deployed during the growing season?
L203 – please include at how many sites.
L220 – “Gap filling (..) could not be performed during (…) the non-growing season due to the bad quality of the model (i.e., low linear relationships between the predicted and measured values” however, in L146-147 you indicate that grab samples were only collected during the growing season. Which one is correct?
L222 – “the 10th quantile of the DOC concentration was used to fill the gaps”: Please explain the rationale behind using the 10th quantile.
L232 – “HMM was used to classify the time series”: Specify which time series – flow with PHIM outputs? Or original water level?
L244 “For each flow event” – Replace by “For each of the 12 flood events”
Figure 3 – add units to discharge
L373 – “Although the events of cluster 1 had the highest ΔDOC” but in L369 “but the events in cluster 1 presented the lowest ΔDOC”.
L399 – “The increase in WTD led to an increase in Q” – here and in general, I would tone it down, maybe “the increase in WTD coincided with an increase in Q”. I don’t believe you are strictly proving causation.
L457-459 – “DOC only accounts for 13.6%-18.8% of the total aquatic carbon” – it would be worth expanding this paragraph since it can provide a comprehensive context for your research, for example, how did you account for total carbon – is that including particulate exports? Can you use the data from Taillardat et al 2022, your data, and bibliography to provide evidence that the lower-than-expected DOC exports are due to higher rates of transformation to GHG?
L515 – “given the lack of a direct link between peat porewater discharge and DOC exports from the stream” – Maybe I missed that detail, but my understanding is that you did not measure WTD during the non-growing season, when the majority of low flow occurs. If that is the case, I would tone down the statement and acknowledge the limitations of the study in this regard.
Citation: https://doi.org/10.5194/hess-2022-426-RC2
Antonin Prijac et al.
Antonin Prijac et al.
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