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

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”.