the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Hydrology and riparian forests drive carbon and nitrogen supply and DOC:NO3− stoichiometry along a headwater Mediterranean stream
- 1Institute of Geography and Geoecology, Karlsruhe Institute of Technology (KIT), Karlsruhe, 76131, Germany
- 2Centre for Advanced Studies of Blanes, Spanish National Research Council (CEAB-CSIC), Blanes, 17300, Spain
- 1Institute of Geography and Geoecology, Karlsruhe Institute of Technology (KIT), Karlsruhe, 76131, Germany
- 2Centre for Advanced Studies of Blanes, Spanish National Research Council (CEAB-CSIC), Blanes, 17300, Spain
Abstract. In forest headwater streams, metabolic processes are predominately heterotrophic and depend on both the availability of carbon (C) and nitrogen (N) and a favourable C:N stoichiometry. In this context, hydrological conditions and the presence of riparian forests adjacent to streams can play an important, yet understudied role determining dissolved organic carbon (DOC) and nitrate (NO3−) concentrations and DOC:NO3− molar ratios. Here, we aimed to investigate how the interplay between hydrological conditions and riparian forest coverage drives DOC and NO3− supply and DOC:NO3− stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO3− concentrations, and DOC:NO3− molar ratios during both base flow and storm flow conditions at three stream locations along a longitudinal gradient of increased riparian forest coverage. Further, we performed an event analysis to examine the hydroclimatic conditions that favour the transfer of DOC and NO3− from riparian soils to the stream during large storms. Stream DOC and NO3− concentrations were generally low (overall average ± SD was 1.0 ± 0.6 mg C L−1 and 0.20 ± 0.09 mg N L−1), although significantly higher during storm flow compared to base flow conditions in all three stream sites. Optimal DOC:NO3− stoichiometry for stream heterotrophic microorganisms (corresponding to DOC:NO3− molar ratios between 4.8 and 11.7) was prevalent at the midstream and downstream sites under both flow conditions, whereas C-limited conditions were prevalent at the upstream site, which had no surrounding riparian forest. The hydroclimatic analysis of large storm events highlighted different patterns of DOC and NO3− mobilization depending on antecedent soil moisture conditions: drier antecedent conditions promoted rapid elevations of riparian groundwater tables, hydrologically activating a wider and shallower soil layer, and leading to relatively higher increases in stream DOC and NO3− concentrations compared to events preceded by wet conditions. These results suggest that (i) increased supply of limited resources during storms can promote in-stream heterotrophic activity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were gradually overcome downstream, likely due to higher C inputs from riparian forests present at lower elevations. The contrasting spatiotemporal patterns in DOC and NO3− availability and DOC:NO3− stoichiometry observed at the study stream suggests that groundwater inputs from riparian forests are essential for maintaining in-stream heterotrophic activity in oligotrophic, forest headwater catchments.
José L. J. Ledesma et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2021-401', Anonymous Referee #1, 03 Sep 2021
Review of “Hydrology and riparian forests drive carbon and nitrogen supply and DOC:NO3 - stoichiometry along a headwater Mediterranean Stream” by Ledesma et al.
General comments
This study analyses DOC, NO3- and the DOC:NO3- ratio along three stations in a Mediterranean headwater, and across flow conditions, during a 2-year period. Spatial variability is controlled by the presence of riparian forest and topography, while temporal variability is controlled by hydroclimatic conditions. The authors conclude that this spatiotemporal variability influences stream metabolic processes.
The manuscript is well written and the conclusions are clear. This work is within the scope of HESS but I find this is a modest contribution to the literature.
Here are three options to make a stronger paper:
- Relax the selection criteria for the storm events to include in the PLS regression (currently only 5 observations)
- Include data on DOC and N composition, not only in the discussion. The discussion suggests that such data is available. Is nitrate the only N form in this stream?
- Include more recent data to make a more complete synthesis of research in this catchment. The references indicate that other studies have taken place in this catchment since the monitoring period 2010-2012 considered here.
Specific comments
Figure 1: add location of the weather station
L115 “Rating curves obtained from the relationships between stream flow and stream water level measurements were used to construct daily time series of stream flow data at each site” can you provide the rating curves in SI?
L122 “the dynamics of this dataset capture well the dynamics of the groundwater table variation in the surrounding riparian area and therefore we are confident that the recorded pattern at the monitoring location was representative of the groundwater table variations in the riparian zone” please provide stronger evidence that this piezometer is representative of the whole downstream area.
L174 “hydroclimatic analysis of large storm events” I understand that the authors chose to analyze the largest storm events because they probably exhibit the clearest signal, but the selection criteria here are very strict and only 5 storm events were kept for analysis. This is a very small number, even though PLS regression can handle datasets with few observations and many variables. Wouldn’t it be more interesting to relax the selection criteria and include more storm events?
L322 “given that this is a predominantly heterotrophic system (Lupon et al., 2016c).” please explain how this was determined (most readers won’t read the reference)
L340 “This result is in line with the idea that headwater streams can remove substantial amounts of NO3 - within relatively short distances (Peterson et al., 2001) […] providing groundwater inputs with low NO3 - concentrations driven by denitrification, as observed in temperate forest catchments (Cirmo and McDonnell, 1997).” Both instream removal and dilution from the middle part of the catchment can explain this decrease. Is it possible to estimate the share of each process?
L375 “The magnitude of change between flow conditions was different for DOC and NO3 - at the upstream site…” please specify which of DOC or NO3- increases more.
L395 “another study from Font del Regàs showed that DOM has a prominent protein-like character in both riparian groundwater and stream water (Bernal et al., 2018)” suggest to include this data in the analysis (not only just in the discussion) to make a more complete paper. The speciation of DOC and the N species other than nitrate should be analyzed further.
Technical corrections
L110 “All data and analyses were integrated and carried out for daily resolutions, which were determined by the availability of the stream chemical data.” This sentence is unclear
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AC1: 'Reply on RC1', José L. J. Ledesma, 01 Mar 2022
Dear Anonymous Referee #1,
We thank you for your evaluation of our manuscript. We appreciate the positive general assessment. Please, see our detailed point by point reply to all your comments in the attached document.
José L. J. Ledesma, Anna Lupon, Eugènia Martí and Susana Bernal
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RC2: 'Comment on hess-2021-401', Anonymous Referee #2, 09 Feb 2022
General comments:
This manuscript explores hydrological and riparian controls on DOC and NO3- stream chemistry in an oligotrophic Mediterranean stream. Their rationale was to understand how these dynamics may impact in-stream heterotrophic activity. To do this, the authors use data collected at three sites within the catchment collected over a period 2 years (2010-2012). Their results suggest that spatial (upstream to downstream) patterns are explained by riparian geomorphology and forest coverage and that temporal variability results from hydrological variability. The manuscript is well written, figures are clear, and conclusions are straightforward and supported by data. I particularly enjoyed the use of the hydrological metrics to understand controls on lateral exchanges of DOC and NO3- between the riparian zone. That said, the contributions to the literature are modest. I wonder if the authors could strengthen the manuscript by adding data beyond 2010-2012 and include more (smaller) storm events in their PLSR analysis. Large storms are relatively rare and DOC and NO3- transport during smaller, and likely more frequent, storms may influence stream metabolism more than the larger events. Also, I suspect antecedent conditions are even more important for DOC and NO3- transport dynamics during these smaller events. I also wonder if the authors have NH4+ and/or DON concentrations and might consider looking at the DOC:DIN or DOC:TDN ratios. Perhaps NO3- is the dominant form of N and these other N forms won’t impact the story much?
Minor concerns:
Lines 25-26: The authors state “These results suggest that (i) increased supply of limited resources during storms can promote in-stream heterotrophic activity during high flows . . .” They don’t actually measure this process, instead they infer it from DOC:NO3- ratios. Perhaps reword this result to be more accurate.
Lines 108-110: “All data and analyses were integrated and carried out for daily resolutions, which were determined by the availability of the stream chemical data.” This line is confusing, please reword.
Line 114: Please avoid use of “fortnightly” as most readers will not know what this means. Instead state the actual frequency (i.e., every two weeks).
Lines 122-125: Could you explain the key finding(s) from Ledesma et al. 2021 that convinces you that the groundwater level dynamics at the monitoring site are representative of the larger riparian network in the catchment?
Lines 197+: To differentiate between pre-event metrics and event metrics, consider including “event” in phrases such as “average event stream flow” and “average event groundwater table.”
Line 334: Please explicitly state which section you mean by “in this section” as it is not defined here.
Line 339: Typo. Should read “might not have been enough . . .”
Line 426: Consider changing “increment” to “increase”.
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AC2: 'Reply on RC2', José L. J. Ledesma, 01 Mar 2022
Dear Anonymous Referee #2,
We thank you for reviewing of our manuscript and appreciate the positive evaluation and that you enjoyed the use of hydrological metrics in the context of the study. Please, see our detailed point by point reply to all your comments in the attached document.
José L. J. Ledesma, Anna Lupon, Eugènia Martí and Susana Bernal
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AC2: 'Reply on RC2', José L. J. Ledesma, 01 Mar 2022
José L. J. Ledesma et al.
José L. J. Ledesma et al.
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