Toward measuring biogeochemistry within the stream‐groundwater interface at the network scale: An initial assessment of two spatial sampling strategies

Abstract

It is important to understand how point measurements across spatially heterogeneous ecosystems are scaled to represent these systems. Stream biogeochemistry presents an illustrative example because water quality concerns within stream networks and recipient water bodies motivate heterogeneous watershed studies. Measurements of the stream water-groundwater (SW-GW) interface (i.e., the shallow stream subsurface) are well-documented for point-scale sampling density measurements (i.e., cm 2 -m 2 features), but poorly characterized for network-scale sampling density measurements (i.e., km 2 ; stream reaches and networks). Sampling the SW-GW interface is more time and labor intensive than surface water sampling, meaning sample point selection must be made with care for network-scale analyses. In this study, we endeavor to determine which of two common spatial sampling schemes is appropriate for characterizing SW-GW interface biogeochemistry across a third-order stream network, focusing on dissolved organic carbon. The first scheme, called Local Sampling, focuses on characterizing small-scale (< 10 m 2 ) variability produced by the local physical and biogeochemical heterogeneity, with fewer points across the stream network. The second scheme, called Longitudinal Sampling, has approximately the same number of measurements distributed over many more points across the stream network with less local variability characterization. This comparison reveals that selection of a Local Sampling versus a Longitudinal Sampling scheme influences the biogeochemical pattern interpretation at the stream network scale. Additionally, this study found that increasing observation efforts at the local scale added limited information for reach-to network-scale biogeochemical patterns, suggesting that emphasis should be placed on characterizing variability across broader spatial scales with the Longitudinal Sampling approach.

Coupled Biotic-Abiotic Processes Control Biogeochemical Cycling of Dissolved Organic Matter in the Columbia River Hyporheic Zone

“…It is also interesting to note that network heterogeneity (indicative of larger transformations and degradation) was always greater in the river than the hyporheic zone, regardless of river stage (Figure 7). These results directly oppose the current dogma of watershed organic matter degradation, where much of DOM turnover is thought to occur due to enhanced microbial activity in the hyporheic zone but agrees with recent work by Lee-Cullin et al (2018), which suggested that hyporheic exchange homogenizes DOM diversity in surface water. Our results suggest abiotic degradation may be playing a significant role in DOM degradation within watershed systems.…”

Stream Dissolved Organic Matter in Permafrost Regions Shows Surprising Compositional Similarities but Negative Priming and Nutrient Effects

“…Because the sources of aquatic DOM vary strongly across ecosystems and through time (e.g., soil, vegetation, and microorganisms), this similarity has mainly been attributed to “filter effects” that select for certain compounds at the dissolution phase and during transport (Gabor, Eilers, et al, 2014; Marín‐Spiotta et al, 2014; Roth et al, 2019; Zark & Dittmar, 2018; Zarnetske et al, 2018). A non‐exclusive hypothesis for this convergence is that autochthonous production of DOM by aquatic autotrophic and heterotrophic organisms contributes new DOM with a common “aquatic” signature (Harjung et al, 2018; Kellerman et al, 2018; Lee‐Cullin et al, 2018). These filtering and aquatic contribution hypotheses have been supported by DOM optical analysis, which shows initial diversity associated with biogeochemical origin followed by convergence toward a more or less universal aquatic DOM signature (Coble et al, 2019; Gabor, Eilers, et al, 2014; Mutschlecner et al, 2018; Wünsch et al, 2019).…”

Spatial and temporal variation in river corridor exchange across a 5th order mountain stream network

“…See detailed descriptions for further information (Dyrness, 1969;Swanson and James, 1975;Swanson and Jones, 2002;Jefferson et al, 2004;Cashman et al, 2009;Deligne et al, We conducted a synoptic study at 46 sites within the HJA during late summer baseflow conditions ( Figure 1). Site selection was done to stratify sampling by stream order so that more headwater sites were sampled than higher order reaches, as suggested by other synoptic investigations of sediment-water interfaces at the basin scale [Ruhala et al, 2017;Lee-Cullin et al, 2018]. We selected low baseflow conditions to maximize our ability to measure QHEF/Q, which is expected to be largest under low discharge conditions [Wondzell, 2011].…”