Preprints
https://doi.org/10.5194/hess-2021-358
https://doi.org/10.5194/hess-2021-358

  28 Jul 2021

28 Jul 2021

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

A parsimonious model of longitudinal stream DOC patterns based on groundwater inputs and in-stream uptake

Stefan Willem Ploum1, Anna Lupon4, Jason A. Leach2,3, Lenka Kuglerová1, and Hjalmar Laudon1 Stefan Willem Ploum et al.
  • 1Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
  • 2Natural Resources Canada, Canadian Forest Service, Sault Ste. Marie, ON, Canada
  • 3Environment and Life Sciences Graduate Program, Trent University, Peterborough, ON, Canada
  • 4Integrative Freshwater Ecology Group, Centre for Advanced Studies of Blanes (CEAB-CSIC), Blanes, Spain

Abstract. The supply of terrestrial dissolved organic carbon (DOC) to aquatic ecosystems affects local in-stream processes and downstream transport of DOC in the fluvial network. However, we have an incomplete understanding on how terrestrial DOC inputs alter longitudinal variations of DOC concentration along headwater stream reaches because groundwater discharge, groundwater DOC concentration and in-stream DOC uptake vary at relatively short spatial and temporal scales. In the riparian zone, the convergence of subsurface flow paths can facilitate the inflow of terrestrial DOC from large upslope contributing areas to narrow sections of the stream. We refer to these areas of flow path convergence as discrete riparian inflow points (DRIPs). In this study, we ask how longitudinal patterns of stream DOC concentrations are affected by DRIPs, as they are major inputs of terrestrial DOC and important locations for in-stream processes. We used a mixing model to simulate stream DOC concentrations along a 1.5 km headwater reach for fifteen sampling campaigns with flow conditions ranging from droughts to floods. Four sets of model scenarios were used to compare different representations of hydrology (distributed inputs of DRIPs vs diffuse groundwater inflow), and in-stream processes (passive transport vs in-stream biological uptake). Results showed that under medium (10–50 l/s) and high flow conditions (> 50 l/s), accounting for lateral groundwater inputs from DRIPs improved simulations of stream DOC concentrations along the reach. Moreover, in-stream biological uptake improved simulations across low to medium flow conditions (< 50 l/s). Only during an experimental drought, longitudinal patterns of stream DOC concentration were simulated best using diffuse groundwater inflow and passive transport scenarios. These results show that the role of hydrology and in-stream processes on modulating downstream DOC exports varies over time. Importantly, we demonstrate that accounting for preferential groundwater inputs to the stream is needed to capture longitudinal dynamics in mobilization and in-stream uptake of terrestrial DOC. The dominant role of DRIPs in these transport and reaction mechanisms suggests that consideration of DRIPs can improve stream biogeochemistry frameworks and help inform management of near-stream areas that exert a large influence on stream conditions.

Stefan Willem Ploum et al.

Status: open (until 16 Oct 2021)

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Stefan Willem Ploum et al.

Stefan Willem Ploum et al.

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Short summary
Groundwater conveys carbon from the landscape to streams where some of the carbon is used by microbes in the stream. This process is very dynamic, and is not very well represented in carbon budgets. We combined groundwater inputs and microbial uptake to calculate carbon concentrations in streams. We found that at groundwater inputs there are large changes in stream carbon concentrations. Depending on the flow conditions (e.g. droughts or floods) this is attributed to groundwater or microbes.