Groundwater flow paths drive longitudinal patterns of stream dissolved organic carbon (DOC) concentrations in boreal landscapes

Abstract

Preferential groundwater flow paths can influence dissolved organic carbon (DOC) concentration and export in the fluvial network because they facilitate the inflow of terrestrial DOC from large upslope contributing areas to discrete sections of the stream, referred to as discrete riparian inflow points (DRIPs). However, the mechanisms by which DRIPs influence longitudinal patterns of stream DOC concentrations are still poorly understood. In this study, we ask how DRIPs affect longitudinal patterns of stream DOC concentrations under different hydrologic conditions, as they can simultaneously act as major sources of terrestrial DOC and important locations for in-stream processes. To answer this question, we tested four model structures that account for different representations of hydrology (distributed inflows of DRIPs vs. diffuse groundwater inflow) and in-stream processes (no DOC uptake vs. in-stream DOC uptake downstream of DRIPs) to simulate stream DOC concentrations along a 1.5 km headwater reach for 14 sampling campaigns with flow conditions ranging from droughts to floods. Despite the magnitude and longitudinal patterns of stream DOC concentration varying across campaigns, at least one model structure was able to capture longitudinal trends during each campaign. Specifically, our results showed that during snowmelt periods or high-flow conditions (>50 L s−1), accounting for distributed inputs of DRIPs improved simulations of stream DOC concentrations along the reach, because groundwater inputs from DRIPs diluted the DOC in transport. Moreover, accounting for in-stream DOC uptake immediately downstream of DRIPs improved simulations during five sampling campaigns that were performed during spring and summer, indicating that these locations served as a resource of DOC for aquatic biota. These results show that the role of DRIPs in modulating DOC concentration, cycling, and export varies over time and depends strongly on catchment hydrology. Therefore, accounting for DRIPs can improve stream biogeochemistry frameworks and help inform management of riparian areas under current and future climatic conditions.