The Spatial Structure of Variability in a Semi-Arid, Fluvial Ecosystem

The arrangement and composition of flowpath types within a given network are thought to govern its functioning. This concept assumes that different flowpath types are functionally distinct. We investigated this assumption in a fluvial ecosystem by comparing the riparian zone, parafluvial zone (in-channel gravel bars), and surface stream. We hypothesized that differences in advection, uptake, and sorption would render material cycles more (a) open and (b) mutable in the surface stream, whereas the converse would occur in the riparian zone, and an intermediate state would be seen in the intervening parafluvial zone. To test our first hypothesis, we predicted that spatial heterogeneity in solute concentrations would be least in the surface stream, greater in the parafluvial zone, and greatest in the riparian zone. Using a null model, we ascertained that this pattern was shown by all solute species we examined (nitrate, ammonium, total dissolved inorganic nitrogen [DIN], dissolved organic N, total dissolved N, soluble reactive phosphorus, dissolved organic carbon, and chloride). To test our second hypothesis, we predicted that temporal change in spatial heterogeneity would be greatest in the surface stream, less in the parafluvial zone, and least in the riparian zone. Nitrate, DIN, and chloride showed this pattern. In particular, surface stream inorganic N was less spatially variable following months of high rainfall. According to an extant hypothesis, these results suggest that inorganic N processing may be a stable function in this ecosystem. Other solute species did not support our second prediction, perhaps because their retention and release dynamics are influenced principally by geochemistry. Generally, our findings indicate that a geomorphic template can generate spatial patterns in ecosystem function, warranting an expansion of the spiraling framework to a variety of flowpath types

Nutrient dynamics in river bed sediments: effects of hydrological disturbances using experimental flow manipulations

International audiencePurpose River sediments play a crucial role in the storage and transformation of organic matter (OM). Nutrient dy- namics are controlled by the interaction of several key parameters, i.e. river discharge, channel geometry and ver- tical exchanges of water (upwelling vs. downwelling zones). The main aim of this study was to evaluate the effect of channel forms and discharge variation on nutrient spiralling in the hyporheic zone (HZ) of streams. Materials and methods Four experimental flow manipula- tions (EFM) were carried out at two reaches with different channel forms (straight vs. sinuous) in an oligotrophic sub- tropical river in Australia. Flow manipulation consisted of reducing the river width with a temporary dam, diverting and concentrating the main water flux on two different geomorphological units (riffle vs. gravel bar), in order to simulate flooding conditions. Hyporheic waters were ana- lysed for their physicochemical characteristics and nutrient (nitrates + nitrites 0 NOx and soluble reactive phosphorus [SRP]) and OM contents at two depths (10 and 50 cm) within the bed sediments, both upstream and downstream of the geomorphological units. Results and discussion The physicochemical parameters clearly demonstrated the existence of hyporheic flow paths, characterized by the alternation of downwelling and upwell- ing areas, with more consistent gradients in gravel bars than in riffles. The HZ acted as source for NOx and SRP, but this role varied between geomorphological units and reaches. The effect of EFM differed between sampling points, irre- spective of the type of geomorphological unit. In gravel bars, a flush out during high discharge was observed for NOx, SRP and particulate organic matter (POM) at the sinuous channel, whereas storage and removal were recorded at the straight channel for SRP and NOx, respec- tively. At the riffle of the sinuous channel, very fine POM accumulated, while removal was noticed for POM. In con- trast, at the riffle of the straight channel, SRP accumulated in the HZ and NOx was removed out of the HZ. Conclusions Nutrient dynamics in the HZ and the response to flow increases were not governed by the geomorphological unit type. Other parameters that determine water residence time in the sediments, such as local heterogeneity in sediment characteristics (grain size, porosity and hydraulic conductivi- ty), channel sinuosity, reach slope and the size and form of the gravel bar, may be more significant explanatory variables for understanding OM and nutrient dynamics in the HZ. This study emphasizes the need for caution in making generalisa- tions about the role of river sediment in nutrient storage and the impact of floods on nutrient dynamics