Hydrologic and biogeochemical influences on carbon processing in the riparian zone of a subarctic stream

Abstract

Small streams and their adjacent riparian zones contribute disproportionately to the processing and transport of C compared to the catchment area and, thereby, may alter the flux of dissolved organic C (DOC) from aquatic ecosystems. We examined the factors influencing C uptake and CO 2 production in the riparian zone of a boreal stream. We tested the hypothesis that enhanced water velocity increases the supply of DOC and nutrients to microorganisms and, therefore, enhances DOC mineralization in the riparian zone. We measured in situ DOC mineralization rate in the riparian zone of a headwater stream at locations that varied with respect to the water velocity of shallow ground water. We added acetate as a tracer to measure C uptake in shallow ground water and monitored CO 2 production during tracer experiments. We assessed relationships of these measures of C dynamics with the time that the tracer remained in shallow ground water surrounding the injection site (plume residence time), pre-injection DOC concentration, nutrient availability, and aromaticity of dissolved organic matter. Acetate uptake rate varied 2 orders of magnitude from −0.002 to −0.19/min, whereas CO 2 production rate varied 5-fold from 0.01 to 0.06/min. Acetate uptake rate was negatively correlated with mean plume residence time, indicating that rapid water velocity increased C decomposition in the riparian zone. Moreover, Damköhler numbers (Da) were generally <1, indicating that substrate uptake was slower than hydrologic transport. Neither acetate nor CO 2 rates were correlated with pre-injection riparian groundwater chemistry. Spatial heterogeneity of water velocity in the subsurface has a critical influence on the processing and subsequent export of C and nutrients from the riparian zone to streams.

Transport and transformation of soil-derived CO2, CH4 and DOC sustain CO2 supersaturation in small boreal streams

“…In addition, previous studies in similar rivers have shown that the combined biological and photochemical degradation of DOC within the stream channel is insufficient to account for the observed CO 2 fluxes (Lapierre and del Giorgio, 2014). It is thus clear that most of the degradation of terrestrial DOC must occur before it reaches the river channel, likely in the hyporheic zone, which is known to be a hotspot of biogeochemical activity (Fiebig et al, 1990;Krause et al, 2011;Rinehart et al, 2015;Schindler and Krabbenhoft, 1998;Stegen et al, 2016). In this regard, the DOC removal that we observe here is commensurate with what has been reported for artificial stream bed reactors (Sleighter et al, 2014).…”

Geostatistical estimation of forest biomass in interior Alaska combining Landsat-derived tree cover, sampled airborne lidar and field observations

“…CPCRW is approximately 10 600 ha in size and divided into 11 watershed units. Many research initiatives at CPCRW involve vegetation and hydrology comparisons among watershed units (Amatya et al, 2016;Rinehart et al, 2015;Tanaka-Oda et al, 2016). Upland areas are dominated by paper birch and aspen on south-facing slopes.…”

Groundwater–surface-water interactions: current research directions

“…DOC processing in these zones must be better understood to predict effects of permafrost thawing associated with climatic warming on DOC fluxes. Rinehart et al (2015) used push-pull slug tests with acetate to investigate the effects of GW-SW interactions on DOC processing in the riparian zone of a boreal catchment. The rate constants for DOC processing did not appear to be related to preinjection water chemistry.…”