Groundwater, Soil, and Vegetation Interactions at Discrete Riparian Inflow Points (DRIPs) and Implications for Boreal Streams

Abstract

Hydrological processes at hillslope and catchment scales explain a large part of stream chemistry dynamics through source-transport mechanisms from terrestrial to aquatic ecosystems. Riparian zones play a central role, as they exert a strong influence on the chemical signature of groundwater discharge to streams. Especially important are riparian areas where upslope subsurface flow paths converge, because they connect a large part of the catchment to a narrow section of the stream. Recent research shows that both in terrestrial and aquatic ecosystems, riparian convergence zones fulfill important biogeochemical functions that differ from surrounding riparian zones. Most catchment-scale conceptual frameworks focus on generalized hillslope-riparian-stream transects and do not explicitly consider riparian convergence zones. This study integrates collective work on hydrology, groundwater chemistry, vegetation and soils of discrete riparian inflow points (DRIPs) in a boreal landscape. We show that compared to adjacent riparian forests, DRIPs have groundwater levels that are consistently near the surface, and supply organic-rich water to streams. We suggest that interactions between hydrology, wetland vegetation, and peat soil development that occur in DRIPs leads to their unique groundwater chemistry and runoff dynamics. Stream-based studies show that across flow conditions, groundwater inputs from DRIPs to headwater reaches influence stream temperature, water chemistry and biology. As such, accounting for DRIPs can complement existing hillslope and stream observations, which would allow better representation of chemical and biological interactions associated with convergence of subsurface flow paths in riparian zones.

Riparian Microtopography Affects Event‐Driven Stream DOC Concentrations and DOM Quality in a Forested Headwater Catchment

“…DOC concentrations in the shallow groundwater exceeded the concentrations found in the stream during baseflow and precipitation events investigated in previous studies (Blaurock et al., 2021; Da Silva et al., 2021). The convergence of DOC‐rich subsurface flow pathways (Ploum et al., 2021) or the accumulation of DOC‐rich surface water (Scheliga et al., 2019) could have led to the higher DOC concentrations in the topographic depressions at site P . Moreover, the large oscillations of the water table at site P could indicate a larger dominant carbon source layer and a more frequent activation of it (Ledesma et al., 2015; Ledesma, Futter, et al., 2018).…”

“…Such areas can be narrow soil layers or topographical depressions that determine where water accumulates in the catchment (Ploum et al, 2021;Werner et al, 2021). Ploum et al (2021) presented the concept of discrete riparian inflow points consisting of topographic depressions that are consistently wetter than the surrounding areas. As a consequence, hydrological pathways are developing that are subsequently enhancing DOC export.…”

“…Our finding underpins recent observations demonstrating DOC export is often linked to specific source areas in the riparian zone, which contribute disproportionally to DOC export. Such areas can be narrow soil layers or topographical depressions that determine where water accumulates in the catchment (Ploum et al, 2021;Werner et al, 2021). Ploum et al (2021) presented the concept of discrete riparian inflow points consisting of topographic depressions that are consistently wetter than the surrounding areas.…”

Dynamic disequilibrium: Recent widespread increases in vegetation cover on subarctic floodplains of Interior Alaska

“…Boreal forest wildfire and successional processes have also been the subject of numerous investigations in the subarctic (Baltzer et al, 2021; Kasischke et al, 2010; Mack et al, 2021; Walker et al, 2021). However, comparatively little is known about vegetation dynamics in subarctic riparian zones, despite their disproportionately high importance as “hotspots” of hydrological processes (Ploum et al, 2021), biogeochemical cycling (Blackburn et al, 2017), species diversity (Andersson et al, 2000; Johansson et al, 1996; Johnson & Almlöf, 2016; Nilsson & Svedmark, 2002), wildlife habitat use (Cooke & Tauzer, 2020), and ecological disturbance (Scrimgeour et al, 1994).…”

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

“…The organization of groundwater flow paths can also regulate spatial patterns of stream DOC concentration by conveying substantial fluxes of water from large upslope contributing areas through wet corridors to discrete sections of the stream (Jencso et al, 2010;McGlynn and McDonnell, 2003), referred to as discrete riparian inflow points (DRIPs; Ploum et al, 2018). DRIPs have been shown to have high groundwater concentrations of DOC associated with sustained water-saturated conditions, moss-dominated vegetation, and organic matter accumulation (Demars et al, 2020;Ploum et al, 2021). Further, the strong connectivity between DRIPs and adjacent streams makes DRIPs important sources of DOC that can be adsorbed, photodegraded, or mineralized by aquatic microbial communities (Berggren et al, 2009;Mineau et al, 2016).…”