Michelle A. Baker

Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone

an upland agricultural stream. We measured solute concentrations ( 15 NO3 , 15 N2 (g), as well as NO3 ,N H3, DOC, DO, Cl − ), 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). NO3 increased at short residence times from 0.32 to 0.54 mg‐ NL −1 until a threshold of 6.9 h and then consistently decreased from 0.54 to 0.03 mg‐ NL −1 . Along these same flow paths, declines were seen in DO (from 8.31 to 0.59 mg‐O2 L −1 ) and DOC (from 3.0 to 1.7 mg‐ CL −1 ). 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 NO3 tracing confirmed that a fraction of the NO3 removal was via denitrification as 15 N2 was produced across the entire gravel bar HZ. Production of 15 N2 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.

ARE RIVERS JUST BIG STREAMS? A PULSE METHOD TO QUANTIFY NITROGEN DEMAND IN A LARGE RIVER

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.

Hydrologic Influences on Groundwater-Surface Water Ecotones: Heterogeneity in Nutrient Composition and Retention

The groundwater-surface water (GW-SW) ecotone, or hyporheic zone, is an active component of stream ecosystems that influences whole-system metabolism and nutrient retention. Because hydrologic fluxes affect the supply of carbon, nutrients, and oxygen to the GW-SW ecotone, the biogeochemical structure of the ecotone (i.e., nutrient content) and the role of the ecotone in nutrient retention are expected to vary under differing hydrologic conditions. In this paper, we employ an inter-basin comparison of headwater streams to assess the influence of ecosystem hydrology on the structure and functioning of GW-SW ecotones. Specifically, we address how differing rate and extent of GW-SW interaction influences heterogeneity in interstitial nutrient content and how variation in GW-SW interaction alters the role of the ecotone in whole-system nutrient retention. A multiple regression model derived from 6 solute-injection experiments identified the extent and rate of hydrologic exchange between the stream and its aquifer as critical variables that determine the retention of biologically important solutes. This approach emphasizes that the nature of GW-SW interaction is established by catchment geology (i.e., alluvial hydrogeologic properties), is modified by changing discharge within a catchment, and is a strong determinant of nutrient retention. At the landscape scale, identifying catchment geologic composition may be a starting point for comparative studies of GW-SW ecotones that could contribute to a more robust model of lotic ecosystem functioning.

Poor Growth of Rainbow Trout Fed New Zealand Mud Snails Potamopyrgus antipodarum

Abstract The New Zealand mud snail (NZMS) Potamopyrgus antipodarum is rapidly invading North American freshwaters, leading to speculation that native fisheries, especially those involving trout, will be negatively impacted. To assess whether trout would consume NZMSs and could assimilate nutrients from them, we conducted a laboratory 15N tracer study, a laboratory feeding study, and bioenergetics modeling with rainbow trout Oncorhynchus mykiss; we also evaluated 5 years of diet and condition data describing rainbow trout and brown trout Salmo trutta collected from a river colonized by NZMSs. The 15N tracer study showed that rainbow trout consumed and to a lesser extent assimilated NZMSs. Rainbow trout fed 15N-labeled NZMSs had muscle isotopic signatures that were 80% higher than those of fish fed unlabeled NZMSs and 30% lower than those of fish fed 15N-labeled amphipods. The feeding study showed that rainbow trout fed an exclusive and unlimited amount of NZMSs lost 0.14–0.48% of their initial body weight ...

Is in-stream N2 fixation an important N source for benthic communities and stream ecosystems?

Abstract We evaluate the current state of knowledge concerning the ecosystem- and community-level importance of N2 fixation in streams. We reviewed the literature reporting N2-fixation contributions to stream N budgets and compared in-stream N2-fixation rates to denitrification and dissolved inorganic N (DIN)-uptake rates. In-stream N2 fixation rarely contributed >5% of the annual N input in N budgets that explicitly measured N2 fixation, but could contribute higher proportions when considered over daily or seasonal time scales. N2-fixation rates were statistically indistinguishable from denitrification and DIN-uptake rates from the same stream reach. However, published N2-fixation rates compiled from a wide variety of streams were significantly lower than denitrification or DIN-uptake rates, which were indistinguishable from one another. The data set we compiled might be biased because the number of published N2-fixation measurements is small (9 studies reporting rates in 22 streams), the range of stream conditions (NO3–-N concentration, discharge, season) under which N2-fixation and other N-processing rates have been measured is limited, and all of the rate estimates have associated methodological artifacts. To broaden our understanding of how N2 fixation contributes to stream ecosystems, studies must measure all rates concurrently across a broad range of stream conditions. In addition, focusing on how N2 fixation supports food webs and contributes to benthic community dynamics will help us understand the full ecological ramifications of N2 fixation in streams, regardless of the magnitude of the N flux into streams from N2 fixation.

Modeling priming effects on microbial consumption of dissolved organic carbon in rivers

Rivers receive and process large quantities of terrestrial dissolved organic carbon (DOC). Biologically available (unstable) DOC leached from primary producers may stimulate (i.e., prime) the consu ...

Effects of periphyton stoichiometry on mayfly excretion rates and nutrient ratios

Abstract We evaluated the effects of periphyton elemental composition on the rate and ratios of N and P excretions by heptageniid mayfly larvae (Ephemeroptera). We predicted that excretion N:P ratios would relate primarily to mismatches between periphyton N:P and mayfly N:P. We immersed periphyton grown in a stream above (inlet: high N) and below (outlet: low N) a lake in P-amended or control (no P) water for 12 h to increase periphyton P content and reduce periphyton N:P. We then measured release rates and ratios of N and P from Cinygmula sp. larvae that had consumed periphyton from 1 of the 4 treatments. Reducing periphyton N:P increased consumer P excretion rates (range for P-amended periphyton: 0.055 ± 0.010 to 0.076 ± 0.022 μg P g−1 dry mass [DM] h−1; range for control periphyton: 0.027 ± 0.008 to 0.031 ± 0.006 μg P g−1 DM h−1). Mayflies that consumed P-amended periphyton from the inlet excreted N at a higher rate than did mayflies that consumed P-amended periphyton from the outlet (inlet P-amended: 0.661 ± 0.244 μg N g−1 DM h−1; outlet P-ammended: 0.245 ± 0.020 μg N g−1 DM h−1). We found some evidence for N:P imbalance between consumer and resource as a predictor of excretion N:P, but other factors also appear to influence nutrient release rates. Mayflies retained significantly more P than predicted by a stoichiometric mass-balance model, a result that suggests that mayflies might be able to store P when it available in their food and use it for growth when needed. Our model also suggests that assimilation efficiencies of N and P are more important determinants of excretion N:P than elemental imbalances between mayflies and their food. We conclude that elemental imbalances between lotic macroinvertebrates and their food resources play an important role in governing nutrient release rates and ratios, but that resource characteristics (e.g., periphyton community composition) and organismal physiology (e.g., nutrient assimilation efficiency) also must be considered. The possibly substantial role of these additional factors in determining nutrient excretion bears further investigation.

Disruptions of stream sediment size and stability by lakes in mountain watersheds: potential effects on periphyton biomass

Abstract The location of a stream reach relative to other landforms in a watershed is an important attribute. We hypothesized that lakes disrupt the frequency of finer, more mobile sediments and thereby change sediment transport processes such that benthic substrates are more stable (i.e., less mobile) below lakes than above lakes. In turn, we hypothesized that this reduced mobility would lead to greater periphyton biomass below lakes. We tested these hypotheses in study reaches above and below lakes in 3 mountain watersheds. To expand this comparison, we analyzed the relationship between sediment attributes and periphyton biomass in one watershed with and one watershed without a lake. We hypothesized that no clear pattern or change in sediment size or chlorophyll a (chl a) would be observed over a 3-km-long study reach without a lake. In contrast, we expected a clear discontinuity in both sediment size and chl a in a 7-km-long study reach interrupted by a lake. Average median sediment size (D50) was significantly larger (p < 0.01) in lake-outlet than lake-inlet reaches (41 mm vs 10 mm). Bed sediments in lake-outlet reaches were immobile during bankfull flows, whereas sediments at lake-inlet reaches were mobile during bankfull flows. Chlorophyll a was ≥10× greater in lake-outlet reaches than in lake-inlet reaches, although this difference was not statistically significant (p = 0.17). The longitudinal analysis clearly showed geomorphic transitions in sediment size and mobility downstream of mountain lakes, and these geomorphic transitions might be associated with changes in periphyton biomass. Geomorphic transitions can alter sediment transport and should be considered in concert with other factors that are considered more commonly in benthic ecology, such as light, nutrients, and temperature.

Stream Nitrogen Inputs Reflect Groundwater Across a Snowmelt-Dominated Montane to Urban Watershed

Snowmelt dominates the hydrograph of many temperate montane streams, yet little work has characterized how streamwater sources and nitrogen (N) dynamics vary across wildland to urban land use gradients in these watersheds. Across a third-order catchment in Salt Lake City, Utah, we asked where and when groundwater vs shallow surface water inputs controlled stream discharge and N dynamics. Stream water isotopes (δ(2)H and δ(18)O) reflected a consistent snowmelt water source during baseflow. Near-chemostatic relationships between conservative ions and discharge implied that groundwater dominated discharge year-round across the montane and urban sites, challenging the conceptual emphasis on direct stormwater inputs to urban streams. Stream and groundwater NO3(-) concentrations remained consistently low during snowmelt and baseflow in most montane and urban stream reaches, indicating effective subsurface N retention or denitrification and minimal impact of fertilizer or deposition N sources. Rather, NO3(-) concentrations increased 50-fold following urban groundwater inputs, showing that subsurface flow paths potentially impact nutrient loading more than surficial land use. Isotopic composition of H2O and NO3(-) suggested that snowmelt-derived urban groundwater intercepted NO3(-) from leaking sewers. Sewer maintenance could potentially mitigate hotspots of stream N inputs at mountain/valley transitions, which have been largely overlooked in semiarid urban ecosystems.

iSAW: Integrating Structure, Actors, and Water to Study Socio-Hydro-Ecological Systems

Urbanization, climate, and ecosystem change represent major challenges for managing water resources. Although water systems are complex, a need exists for a generalized representation of these systems to identify important components and linkages to guide scientific inquiry and aid water management. We developed an integrated Structure-Actor-Water framework (iSAW) to facilitate the understanding of and transitions to sustainable water systems. Our goal was to produce an interdisciplinary framework for water resources research that could address management challenges across scales (e.g., plot to region) and domains (e.g., water supply and quality, transitioning, and urban landscapes). The framework was designed to be generalizable across all human–environment systems, yet with sufficient detail and flexibility to be customized to specific cases. iSAW includes three major components: structure (natural, built, and social), actors (individual and organizational), and water (quality and quantity). Key linkages among these components include: (1) ecological/hydrologic processes, (2) ecosystem/geomorphic feedbacks, (3) planning, design, and policy, (4) perceptions, information, and experience, (5) resource access and risk, and (6) operational water use and management. We illustrate the flexibility and utility of the iSAW framework by applying it to two research and management problems: understanding urban water supply and demand in a changing climate and expanding use of green storm water infrastructure in a semi-arid environment. The applications demonstrate that a generalized conceptual model can identify important components and linkages in complex and diverse water systems and facilitate communication about those systems among researchers from diverse disciplines.