Multiscale effects of surface–subsurface exchange on stream water nutrient concentrations

Abstract

Stream-riparian ecosystems are landscapes composed of dynamic interacting terrestrial and aquatic patches. Patch composition and configuration affects both the form of transported materials and the amount of nutrient retention and export. We describe spatial patterns of nutrients in the surface water of an arid-land stream using surveys conducted at 3 different scales, ranging from 30 m to 10 km in extent and from 1 m to 25 m in grain. We then relate these patterns to connections with subsurface patches at channel subunit, channel unit, and reach scales. Our objectives were to compare spatial variation in nutrients across scales, to determine the causes of downstream changes in nutrient concentration in terms of intervening patches, and to investigate whether subsurface patches at different scales behaved similarly in terms of net nutrient processing.Nutrients varied spatially at all scales sampled. The highest variation was observed in nitrate-N (NO 3 -N) in the survey with the smallest grain (CV ϭ 161%) and the lowest was observed in soluble reactive P (SRP) in the same survey (CV ϭ 17%). We hypothesized that downstream changes in nutrient concentrations were caused by upwelling of high-nutrient water from the subsurface. To test this hypothesis, we identified locations of hydrologic inputs to surface water from the subsurface using geomorphic features of the stream such as gravel bar edges (channel subunit scale), riffle-run transitions (channel unit scale), and permanent groundwater sources (reach scale). As surface water passed over these locations, nutrient concentrations generally increased, particularly during late succession when subsurface patches acted as sources of NO 3 -N at all 3 scales and as sources of SRP at the channel unit and reach scales. A hierarchical approach allowed us to decompose effects of subsurface upwellings at different scales and to consider interactions between them. Processes occurring in subsurface patches influenced surface water nutrient patterns at scales from a few meters to several kilometers.

Buffered, lagged, or cooled? Disentangling hyporheic influences on temperature cycles in stream channels

“…In alluvial rivers, large wood provides roughness elements that deflect flows, scour depressions, create gravel bars, facilitate channel braiding, [Abbe and Montgomery, 1996;Gurnell et al, 2002], and encourage channel avulsions that lead to the formation of anabranched channel patterns [Nanson and Knighton, 1996]. Associated complex bed topography and channel patterns enhance hydraulic gradients within the hyporheic zone and substantially increase hyporheic exchange [Dent et al, 2001;Cardenas et al, 2004;Lautz et al, 2006;Wondzell, 2006] while creating features such as spring channels that express resulting surface water variation in diel temperature cycles. Therefore despite the fact that the low-flow main channel of the Umatilla River is largely unshaded (Figure 3), riparian vegetation is apt to influence temperature cycles in the main stem Umatilla River by creating geomorphic features that enhance hyporheic exchange and support dynamic temperature mosaics within stream reaches (Figure 6b).…”

“…The magnitude of hyporheic exchange in a river system depends on characteristics of the river channel and alluvial aquifer, such as the range, frequency, and spatial variation in hydraulic conductivity and hydraulic gradients, as well as variations in floodplain and streambed topography and geomorphology [Wondzell and Swanson, 1999;Dent et al, 2001;Gooseff et al, 2006;Wondzell, 2006]. Thus the thermal regimes of the channel and hyporheic zone are apt to be most tightly integrated in rivers with highly conductive floodplain aquifers, and complex channel patterns or streambed topography, all of which enhance rates of hyporheic exchange [Cardenas et al, 2004;Poole et al, 2006].…”

Evidence for self-organization in determining spatial patterns of stream nutrients, despite primacy of the geomorphic template

“…We calculated water permanence (percentage of time with surface water present within the 22-mo study period) along the stream using these survey data. The spatial locations of upwelling along the 10-km stretch of the stream were obtained from Dent et al (10); they recorded upwelling zones at the reach, channel unit, channel subunit, and particle scale. Upwelling zones at the reach scale occur at transitions from unconstrained to constrained valleys.…”

“…Laterally, streams are hydrologically connected to the riparian zone, the floodplain, and the upland portions of catchments (8), with riparian zones acting as "nutrient filters" that remove various chemical constituents as water moves from uplands to the stream (9). Vertically, subsurface water from the hyporheic zone can also alter the biogeochemical signature of surface water (10). At broader spatial scales, geomorphic features, such as slope breaks and canyons, determine the locations of upwelling and down-welling zones (11).…”

Landscape challenges to ecosystem thinking: Creative flood and drought in the American Southwest

“…Unfortunately, few landscape ecologists have used streams as research laboratories for testing concepts of general application. Exceptions have viewed streams as arenas for study and have resolved spatial heterogeneity within the bounds of the linear channel (Sinsabaugh et al, 1991;Dent et al, 2001). This conceptual evolution of stream ecology as a sub-discipline has been mirrored in the structure of research in the Sycamore Creek Project in Arizona, USA.…”