[1] Biogeochemical reactions associated with stream nitrogen cycling, such as nitrification and denitrification, can be strongly controlled by water and solute residence times in the hyporheic zone (HZ , and hydraulic transport parameters (head, flow rates, flow paths, and residence time distributions) of the reach and along HZ flow paths of an instrumented gravel bar. HZ exchange was observed across the entire gravel bar (i.e., in all wells) with flow path lengths up to 4.2 m and corresponding median residence times greater than 28.5 h. The HZ transitioned from a net nitrification environment at its head (short residence times) to a net denitrification environment at its tail (long residence times). NO 3 − increased at short residence times from 0.32 to 0.54 mg-N L −1 until a threshold of 6.9 h and then consistently decreased from 0.54 to 0. ). The rates of the DO and DOC removal and net nitrification were greatest during short residence times, while the rate of denitrification was greatest at long residence times.15 NO 3 − tracing confirmed that a fraction of the NO 3 − removal was via denitrification as 15 N 2 was produced across the entire gravel bar HZ. Production of 15 N 2 across all observed flow paths and residence times indicated that denitrification microsites are present even where nitrification was the net outcome. These findings demonstrate that the HZ is an active nitrogen sink in this system and that the distinction between net nitrification and denitrification in the HZ is a function of residence time and exhibits threshold behavior. Consequently, incorporation of HZ exchange and water residence time characterizations will improve mechanistic predictions of nitrogen cycling in streams.Citation: Zarnetske, J. P., R. Haggerty, S. M. Wondzell, and M. A. Baker (2011), Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone,

Given recent focus on large rivers as conduits for excess nutrients to coastal zones, their role in processing and retaining nutrients has been overlooked and understudied. Empirical measurements of nutrient uptake in large rivers are lacking, despite a substantial body of knowledge on nutrient transport and removal in smaller streams. Researchers interested in nutrient transport by rivers (discharge >10000 L/s) are left to extrapolate riverine nutrient demand using a modeling framework or a mass balance approach. To begin to fill this knowledge gap, we present data using a pulse method to measure inorganic nitrogen. (N) transport and removal in the Upper Snake River, Wyoming, USA (seventh order, discharge 12000 L/s). We found that the Upper Snake had surprisingly high biotic demand relative to smaller streams in the same river network for both ammonium (NH4+) and nitrate (NO3-). Placed in the context of a meta-analysis of previously published nutrient uptake studies, these data suggest that large rivers may have similar biotic demand for N as smaller tributaries. We also found that demand for different forms of inorganic N (NH4+ vs. NO3-) scaled differently with stream size. Data from rivers like the Upper Snake and larger are essential for effective water quality management at the scale of river networks. Empirical measurements of solute dynamics in large rivers are needed to understand the role of whole river networks (as opposed to stream reaches) in patterns of nutrient export at regional and continental scales.

Ecosystem metabolism, that is, gross primary productivity (GPP) and ecosystem respiration (ER), controls organic carbon (OC) cycling in stream and river networks and is expected to vary predictably with network position. However, estimates of metabolism in small streams outnumber those from rivers such that there are limited empirical data comparing metabolism across a range of stream and river sizes. We measured metabolism in 14 rivers (discharge range 14-84 m 3 s -1 ) in the Western and Midwestern United States (US). We estimated GPP, ER, and gas exchange rates using a Lagrangian, 2-station oxygen model solved in a Bayesian framework. GPP ranged from 0.6-22 g O 2 m -2 d -1 and ER tracked GPP, suggesting that autotrophic production supports much of riverine ER in summer. Net ecosystem production, the balance between GPP and ER was 0 or greater in 4 rivers showing autotrophy on that day. River velocity and slope predicted gas exchange estimates from these 14 rivers in agreement with empirical models. Carbon turnover lengths (that is, the distance traveled before OC is mineralized to CO 2 ) ranged from 38 to 1190 km, with the longest turnover lengths in high-sediment, arid-land rivers. We also compared estimated turnover lengths with the relative length of the river segment between major tributaries or lakes; the mean ratio of carbon turnover length to river length was 1.6, demonstrating that rivers can mineralize much of the OC load along their length at baseflow. Carbon mineralization velocities ranged from 0.05 to 0.81 m d -1 , and were not different than measurements from small streams. Given high GPP relative to ER, combined with generally short OC spiraling lengths, rivers can be highly reactive with regard to OC cycling.

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[1] Application of transient storage models has become popular for characterizing hydrologic and biogeochemical processes in streams. The typical transient storage model represents exchange between the main channel and a single storage zone, essentially lumping together different exchange processes. Here we present a method to inform a transient storage model that accounts for two storage zones (2-SZ) to discriminate between surface transient storage (STS) exchange and exchange with hyporheic transient storage (HTS). This method requires that, in addition to tracer breakthrough curves from the main channel, cross-sectional stream velocity distributions and stream tracer concentration time series data from several main channel locations and adjacent representative STS zones be collected. We apply this method to a constant rate conservative tracer injection in a first-order stream and to an instantaneous slug conservative tracer injection in a fourth-order stream. The 2-SZ model simulations matched observed breakthrough curves of tracer concentration in the main channel and general STS behavior well. Additionally, we compared the optimized parameter sets of the 2-SZ model to one-storage zone model (1-SZ) simulations and found that the lumped storage terms of the 1-SZ model described the time scales of 2-SZ model HTS exchange and attributed the time scales of observed STS exchange to longitudinal dispersion. With additional field data collection efforts and data processing, this method can provide much more useful results than the 1-SZ approach to those interested in discriminating between surface and subsurface transient storage dynamics of streams, which is important for discerning processes important to the cycling and fate of biogeochemicals.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. In groundwater ecosystems, in situ primary production is low, and metabolism depends on organic matter inputs from other regions of the catchment. Heterotrophic metabolism and biogeochemistry in the floodplain groundwater of a headwater catchment (Rio Calaveras, New Mexico, USA) were examined to address the following questions: (1) How do groundwater metabolism and biogeochemistry vary spatially and temporally? (2) What factors influence groundwater metabolism? (3) What is the energy source for groundwater metabolism?At Rio Calaveras, surface discharge and water table elevation increased at the onset of spring snowmelt. Groundwater biogeochemical changes in response to snowmelt included increases in dissolved oxygen and dissolved organic carbon (DOC) concentrations. Dissolved organic carbon concentration then decreased exponentially with time, suggesting that newly saturated floodplain sediments were a major source of DOC. Organic matter content in seasonally saturated sediments averaged 3% by mass, and -0.05 mg C/g dry sediment was water soluble. Microorganisms from these sediments were able to consume an average of 45% of the leached DOC. These results show that snowmelt imports DOC to groundwater and that a substantial amount can be consumed by biota. These results may be important ecologically if the growth and abundance of groundwater organisms are limited by DOC availability.The influence on groundwater heterotrophic metabolism of DOC availability, inorganic nitrogen (N), inorganic phosphorus (P), temperature, and season were assessed using laboratory manipulations of aquifer sediments and seasonal measurements in field microcosms. Augmentation with DOC (10 mgC/L above background) nearly doubled respiration rate during base flow but did not influence respiration during snowmelt. In contrast, addition of N and P did not influence respiration at any time. Respiration rate during snowmelt was significantly higher than at base flow and was not influenced by any combination of DOC, N, P, or temperature. The hypothesis that groundwater metabolism is limited by DOC availability during base flow was supported. Hydrologic linkage between soils and groundwater represents a critical flux of DOC that supports metabolism in unconfined alluvial aquifers.

Summary Severe or extreme droughts occurred about 10% of the time over a 105‐year record from central New Mexico, U.S.A., based on the Palmer Drought Severity Index. Drought lowers water tables, creating extensive areas of groundwater recharge and fragmenting reaches of streams and rivers. Deeper groundwater inputs predominate as sources of surface flows during drought. Nutrient inputs to streams and rivers reflect the biogeochemistry of regional ground waters with longer subsurface residence times. Inputs of bioavailable dissolved organic carbon to surface waters decrease during drought, with labile carbon limitation of microbial metabolism a byproduct of drought conditions. Decreased inputs of organic forms of carbon, nitrogen and phosphorus and a decrease in the organic : inorganic ratio of nutrient inputs favours autotrophs over heterotrophs during drought. The fate of autotrophic production during drought will be strongly influenced by the structure of the aquatic food web within impacted sites.

An in situ acetate injection was used to determine the influence of labile dissolved organic carbon (DOC) availability on microbial respiration in the hyporheic zone of a headwater stream. We added bromide as a conservative tracer and acetate as an organic substrate to the hyporheic zone of Rio Calaveras, New Mexico, via an injection well. Tracer was observed in four of eight capture wells. Three of the four wells showed increases in bromide without concurrent increases in acetate concentration, suggesting 100% acetate retention. One well had 38% acetate retention. Pore velocity and acetate retention were negatively correlated, suggesting hydrologic control of acetate retention. Acetate did not significantly sorb to the sandy hyporheic sediments at this site, indicating biological consumption of acetate. Acetate addition stimulated total CO 2 production along monitored flowpaths and led to changes in solutes associated with microbial terminal electron-accepting processes (TEAPs). Dissolved oxygen (DO), nitrate, and sulfate significantly decreased, and ferrous iron and methane significantly increased compared to background concentrations in most wells. These results support the hypothesis that microbial respiration in the hyporheic zone is limited by labile DOC availability. Furthermore, we have shown that a suite of metabolic processes, from aerobic respiration to methanogenesis, cooccur and that anaerobic processes dominate heterotrophic metabolism in the hyporheic zone of Rio Calaveras.In the past 10-15 yr, the importance of the hyporheic zone (i.e., subsurface water containing at least 10% surface water, sensu Triska et al. 1989) to lotic biogeochemistry and ecology has been documented for streams and rivers worldwide (e.g., Jones and Holmes 1996;Brunke and Gonser 1997;Gibert et al. 1997). Hyporheic sediments are metabolically active, are an important site for organic matter decomposition, and can have a large impact on respiration (i.e., decrease P : R ratios) in measures of whole-stream ecosystem metabolism (Grimm and Fisher 1984;Mulholland et al. 1997;Nageli and Uehlinger 1997). Microbial metabolism in the hyporheic zone depends on transport of organic substrates and electron acceptors from the surface stream and/ or nearby groundwater (e.g., Findlay 1995;Jones et al. 1995a). Since subsurface organisms are mostly heterotrophic (Jones et al. 1995b) and require allochthonous organic matter sources, it is commonly hypothesized that microbial metabolism in the hyporheic zone is limited by organic matter availability (e.g., Jones 1995). The relationship between hyporheic zone aerobic respiration and organic matter availability has been examined using sediment microcosms (e.g., Pusch and Schwoerbel 1994;Jones 1995 AcknowledgmentsThe authors thank Christine Fellows, Chelsea Crenshaw, Sonya Sanchez, Lisa Roberts, and Danielle Boling for help in the field. Sonya Sanchez conducted the acetate sorption assays. John Morrice and Padinare Unnikrishna gave valuable advice on subsurface injection techniques. Arma...