Hydrological and biogeochemical controls on watershed dissolved organic matter transport: pulse‐shunt concept

Abstract

Hydrological precipitation and snowmelt events trigger large "pulse" releases of terrestrial dissolved organic matter (DOM) into drainage networks due to an increase in DOM concentration with discharge. Thus, low-frequency large events, which are predicted to increase with climate change, are responsible for a significant percentage of annual terrestrial DOM input to drainage networks. These same events are accompanied by marked and rapid increases in headwater stream velocity; thus they also "shunt" a large proportion of the pulsed DOM to downstream, higher-order rivers and aquatic ecosystems geographically removed from the DOM source of origin. Here we merge these ideas into the "pulse-shunt concept" (PSC) to explain and quantify how infrequent, yet major hydrologic events may drive the timing, flux, geographical dispersion, and regional metabolism of terrestrial DOM. The PSC also helps reconcile long-standing discrepancies in C cycling theory and provides a robust framework for better quantifying its highly dynamic role in the global C cycle. The PSC adds a critical temporal dimension to linear organic matter removal dynamics postulated by the river continuum concept. It also can be represented mathematically through a model that is based on stream scaling approaches suitable for quantifying the important role of streams and rivers in the global C cycle. Initial hypotheses generated by the PSC include: (1) Infrequent large storms and snowmelt events account for a large and underappreciated percentage of the terrestrial DOM flux to drainage networks at annual and decadal time scales and therefore event statistics are equally important to total discharge when determining terrestrial fluxes. (2) Episodic hydrologic events result in DOM bypassing headwater streams and being metabolized in large rivers and exported to coastal systems. We propose that the PSC provides a framework for watershed biogeochemical modeling and predictions and discuss implications to ecological processes.

Carbon and nutrient export regimes from headwater catchments to downstream reaches

“…Many other studies have found changes in DOC quantity and quality during in-stream transport (e.g., Lambert et al, 2016). During storm events, DOC dynamics exhibited a majority of accretion patterns, as expected from DOC dynamics in headwater catchments, suggesting limited in-stream or point-source influences (Raymond et al, 2016).…”

Where Carbon Goes When Water Flows: Carbon Cycling across the Aquatic Continuum

“…The relatively slow mobilization of inorganic constituents such as phosphate and silicate relative to C and N-containing molecules (Figure 3) exerts an important control on nutrient limitation for in situ primary production both within inland waters and marine receiving waters. Raymond et al (2016) recently described these mobilization dynamics within the context of the "Pulse Shunt Concept (PSC), " whereby OM is pulsed into streams during extreme hydrologic events (e.g., heavy rainfall or large snow/ice melting events) and subsequently shunted from headwater streams into larger rivers prior to significant microbial or photo-oxidative transformation. This shunting of OM can alter downstream reactivity gradients and also allow for the export of reactive OM that maintains terrestrial signatures (Raymond et al, 2016).…”

Riparian Corridors: A New Conceptual Framework for Assessing Nitrogen Buffering Across Biomes

“…In the other direction, stream contributions to the riparian zone of particulate organic nitrogen can be substantial, but mainly during floods (Tabacchi et al, 1998;Pinay et al, 2002). Conversely, downstream sections (transition and deposition sectors) receive mostly dissolved and particulate organic and inorganic nitrogen from the stream, in particular during floods (Tabacchi et al, 1998;Pinay et al, 2002;Raymond et al, 2016). These downstream sections also receive dissolved nitrogen via subsurface flow through the hyporheic zone Kolbe et al, 2016).…”