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

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.