Preprints
https://doi.org/10.5194/hess-2021-401
https://doi.org/10.5194/hess-2021-401

  12 Aug 2021

12 Aug 2021

Review status: this preprint is currently under review for the journal HESS.

Hydrology and riparian forests drive carbon and nitrogen supply and DOC:NO3 stoichiometry along a headwater Mediterranean stream

José L. J. Ledesma1,2, Anna Lupon2, Eugènia Martí2, and Susana Bernal2 José L. J. Ledesma et al.
  • 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.

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José L. J. Ledesma et al.

José L. J. Ledesma et al.

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Short summary
We studied a small stream located in a Mediterranean forest. Our goal was to understand how stream flow and the presence of riparian forests, which grow in flat banks near the stream, influence the availability of food for aquatic microorganisms. High flows were associated with higher amounts of food because rainfall episodes transfer it from the surrounding sources, particularly riparian forests, to the stream. Understanding how ecosystems work is essential to better manage natural resources.