Comment on hess-2021-401

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.