Are dissolved organic carbon concentrations in riparian groundwater linked to hydrological pathways in the boreal forest?

Abstract

Abstract. The riparian zone (RZ), or near-stream area, plays a fundamental role in the biogeochemistry of headwaters. Here, wet, carbon-rich soils can change groundwater chemistry before it enters the stream. In the boreal forest, the RZ plays an especially important role in the export of dissolved organic carbon (DOC) to streams. However, the RZ is not uniform, and spatial variability of riparian groundwater hydrology and chemistry can be large. Terrestrial topographic depressions create hydrological pathways towards focal points in the RZ, which we refer to as “discrete riparian inflow points” (DRIPs). Combining the chemical function of the RZ and the convergence of hydrological pathways, we hypothesize that DRIPs play a disproportionally large role in conveying DOC to small streams. Earlier work has demonstrated that runoff from DRIPs can make up the majority of riparian flow contributions to streams, but it is currently unknown how their groundwater chemistry differs from the rest of the RZ. Therefore, we ask the following question: are DOC concentrations in riparian groundwater linked to hydrological pathways in the boreal forest? To answer this question, we sampled riparian groundwater during six campaigns across three boreal headwater streams in Sweden. The groundwater wells were distributed into 10 DRIP and non-DRIP pairs (60 wells), following transects from the upland (20 m lateral distance from the stream bank) to the near-stream area (<5 m lateral distance from the stream bank). The variability in DOC, pH, and electrical conductivity (EC) was analyzed using linear mixed-effects models (LMMs). We explained the variability using three factors: distance from the stream, seasonality, and DRIP/non-DRIP. Our results showed that DRIPs provided DOC-rich water (34 mg L−1) with relatively low EC (36 µS cm−1). The “non-DRIP” riparian water had 40 % lower DOC concentrations (20 mg L−1) and a 45 % higher EC (52 µS cm−1) on average. Moreover, groundwater chemistry from DRIPs was spatially and temporally relatively homogeneous. In contrast, non-DRIP water transformed distinctly in the last 25 m towards the stream, and the chemical variability was also larger between seasons. We concluded that hydrological pathways and spatial variability in riparian groundwater DOC concentrations are linked, and that DRIPs can be seen as important control points in the boreal landscape. Characterizing DRIPs in headwater catchments can be useful for upscaling carbon inputs in boreal stream ecosystems and for delineating hydrologically adapted buffers for forest management practices.

Missing connectivity during summer drought controls DOC mobilization and export in a small, forested catchment

“…Analyses of the pool water showed very high DOC concentrations, between 30 and 50 mg L -1 (data not shown). The possibility that these pools contribute to the DOC export later during the event is also supported by the observation that topographic depressions can play a very important role in DOC accumulation and DOC transport to the stream (Ploum et al, 2020). In general, the riparian zone saturates over the course of a precipitation event (Ledesma et al, 2015;Tunaley et al, 2016).…”

“…In addition, a higher DOC production would be expected in the lower catchment due to the elevation differences and subsequently different temperatures (Borken et al, 2011;Tunaley et al, 2016;Wen et al, 2019;Andersson et al, 2000). Another reason for a higher DOC export from the lower catchment would be the importance of large riparian zones for DOC mobilization (Ploum et al, 2020;Mei et al, 2014;Ledesma et al, 2015;Ledesma et al, 2018;Inamdar and Mitchell, 2007;Strohmeier et al, 2013;Musolff et al, 2018). The importance of the riparian zone for in-stream DOC concentrations is confirmed by DOC measurements performed in a low elevation sub-catchment next to HS with more than 40 % hydromorphic soils (Beudert et al, 2012) and with stream DOC concentrations being higher (6.3 mg L -1 ) at baseflow than in the lower catchment.…”

Delineating Source Contributions to Stream Dissolved Organic Matter Composition Under Baseflow Conditions in Forested Headwater Catchments

“…Moreover, the riparian zone surface water sample had the highest AI modwa recorded and relatively high DOC concentration. In fact, these pools may represent a very important DOC source for small headwater streams, when hydrologically connected to the stream (Ploum et al, 2020). Moreover, all DOM sources were…”

Predicting evaporation from mountain streams

“…Mixed-effects models are common in biological science and ecology, but have had fewer applications in hydrology; some hydrological examples include Booker and Dunbar (2008), Howie, Whitfield, and Moore (2020), Kasurak, Kelly, andBrenning (2011), andPloum, Laudon, Peralta-Tapia, andKuglerová (2019).…”