Small-scale topography explains patterns and dynamics of dissolved organic carbon exports from the riparian zone of a temperate, forested catchment

Benedikt J. Werner,Andreas Musolff,Gerrit H. de Rooij,Jie Yang,Oliver J. Lechtenfeld,Ulrike Werban,Jan H. Fleckenstein,Ralf Gründling

doi:10.5194/hess-25-6067-2021

Abstract

Abstract. Export of dissolved organic carbon (DOC) from riparian zones (RZs) is an important component of temperate catchment carbon budgets, but export mechanisms are still poorly understood. Here we show that DOC export is predominantly controlled by the microtopography of the RZ (lateral variability) and by riparian groundwater level dynamics (temporal variability). From February 2017 until July 2019 we studied topography, DOC quality and water fluxes and pathways in the RZ of a small forested catchment and the receiving stream in central Germany. The chemical classification of the riparian groundwater and surface water samples (n=66) by Fourier transform ion cyclotron resonance mass spectrometry revealed a cluster of plant-derived, aromatic and oxygen-rich DOC with high concentrations (DOCI) and a cluster of microbially processed, saturated and heteroatom-enriched DOC with lower concentrations (DOCII). The two DOC clusters were connected to locations with distinctly different values of the high-resolution topographic wetness index (TWIHR; at 1 m resolution) within the study area. Numerical water flow modeling using the integrated surface–subsurface model HydroGeoSphere revealed that surface runoff from high-TWIHR zones associated with the DOCI cluster (DOCI source zones) dominated overall discharge generation and therefore DOC export. Although corresponding to only 15 % of the area in the studied RZ, the DOCI source zones contributed 1.5 times the DOC export of the remaining 85 % of the area associated with DOCII source zones. Accordingly, DOC quality in stream water sampled under five event flow conditions (n=73) was closely reflecting the DOCI quality. Our results suggest that DOC export by surface runoff along dynamically evolving surface flow networks can play a dominant role for DOC exports from RZs with overall low topographic relief and should consequently be considered in catchment-scale DOC export models. We propose that proxies of spatial heterogeneity such as the TWIHR can help to delineate the most active source zones and provide a mechanistic basis for improved model conceptualization of DOC exports.

Highlights

Dissolved organic carbon (DOC) in streams and rivers is in itself of central ecological importance (Cole et al, 2007; Battin et al, 2008), but the amount and quality of dissolved organic carbon (DOC) shape water quality through interactions and co-export with other chemicals in terrestrial solute source areas
To elucidate spatial and temporal variability of DOC exports from riparian zones, this study combined a hydromorphological classification of a humid temperate riparian zone with a chemical characterization of DOC in source zones and the stream and detailed modeling of water fluxes and flow paths
The second cluster (DOCII) reflects microbially processed, nonpolar DOC with lower concentration and larger compositional variability across seasons that may be more persistent in the surface water

Summary

Introduction

Dissolved organic carbon (DOC) in streams and rivers is in itself of central ecological importance (Cole et al, 2007; Battin et al, 2008), but the amount and quality of DOC shape water quality through interactions and co-export with other chemicals in terrestrial solute source areas Climate and biogeochemical boundary conditions have increased DOC concentrations in surface waters and altered the quality of the exported DOC in the last decades (Larsen et al, 2011; Chantigny, 2003; Wilson and Xenopoulos, 2008). Routine management of DOC could help to comply with water quality directives and lower the cost of drinking water purification (Matilainen et al, 2011), but understanding how DOC changes and moves within and across ecosystem interfaces, linking aquatic and terrestrial carbon cycles, is still limited by large knowledge gaps (Butman et al, 2018; Drake et al, 2018; Vachon et al, 2021) that so far impede proper DOC management (Stanley et al, 2012)

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