@article {KNZ001431, title = {Cross-stream comparison of substrate-specific denitrification potential}, journal = {Biogeochemistry}, volume = {104}, year = {2011}, pages = {381 -392}, abstract = {

Headwater streams have a demonstrated ability to denitrify a portion of their nitrate (NO3 \−) load but there has not been an extensive consideration of where in a stream this process is occurring and how various habitats contribute to total denitrification capability. As part of the Lotic Intersite Nitrogen Experiment II (LINX II) we measured denitrification potential in 65 streams spanning eight regions of the US and draining three land-use types. In each stream, potential denitrification rates were measured in common substrate types found across many streams as well as locations unique to particular streams. Overall, habitats from streams draining urban and agricultural land-uses showed higher potential rates of denitrification than reference streams draining native vegetation. This difference among streams was probably driven by higher ambient nitrate concentrations found in urban or agricultural streams. Within streams, sandy habitats and accumulations of fine benthic organic matter contributed more than half of the total denitrification capacity (mg N removed m\−2 h\−1). A particular rate of potential denitrification per unit area could be achieved either by high activity per unit organic matter or lower activities associated with larger standing stocks of organic matter. We found that both small patches with high rates (hot spots) or more widespread but less active areas (cool matrix) contributed significantly to whole stream denitrification capacity. Denitrification estimated from scaled-up denitrification enzyme assay (DEA) potentials were not always dramatically higher than in situ rates of denitrification measured as 15N gas generation following 24-h 15N\–NO3 tracer additions. In general, headwater streams draining varying land-use types have significant potential to remove nitrate via denitrification and some appear to be functioning near their maximal capacity.

}, keywords = {LTER-KNZ, Comparative, Comparison of potential with realized denitrification, DEA, denitrification, stream, Substrate-specific}, doi = {10.1007/s10533-010-9512-8}, url = {https://link.springer.com/article/10.1007\%2Fs10533-010-9512-8}, author = {Findlay, S.E.G. and Mulholland, P.J. and Hamilton, S. and Tank, J. and Bernot, M. and Burgin, A.J. and Crenshaw, C. and W. K. Dodds and Grimm, N. and W.H. McDowell and Potter, J. and Sobota, D.} } @article {KNZ001360, title = {Nitrous oxide emission from denitrification in stream and river networks}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {108}, year = {2011}, pages = {214 -219}, abstract = {

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3\−) concentrations, but that \<1\% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3\−. We suggest that increased stream NO3\− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg\·y\−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10\% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change. Humans have more than doubled the availability of fixed nitrogen (N) in the biosphere, particularly through the production of N fertilizers and the cultivation of N-fixing crops (1). Increasing N availability is producing unintended environmental consequences including enhanced emissions of nitrous oxide (N2O), a potent greenhouse gas (2) and an important cause of stratospheric ozone destruction (3). The Intergovernmental Panel on Climate Change (IPCC) estimates that the microbial conversion of agriculturally derived N to N2O in soils and aquatic ecosystems is the largest source of anthropogenic N2O to the atmosphere (2). The production of N2O in agricultural soils has been the focus of intense investigation (i.e., \>1,000 published studies) and is a relatively well constrained component of the N2O budget (4). However, emissions of anthropogenic N2O from streams, rivers, and estuaries have received much less attention and remain a major source of uncertainty in the global anthropogenic N2O budget. Microbial denitrification is a large source of N2O emissions in terrestrial and aquatic ecosystems. Most microbial denitrification is a form of anaerobic respiration in which nitrate (NO3\−, the dominant form of inorganic N) is converted to dinitrogen (N2) and N2O gases (5). The proportion of denitrified NO3\− that is converted to N2O rather than N2 (hereafter referred to as the N2O yield and expressed as the mole ratio) partially controls how much N2O is produced via denitrification (6), but few studies provide information on the N2O yield in streams and rivers because of the difficulty of measuring N2 and N2O production in these systems. Here we report rates of N2 and N2O production via denitrification measured using whole-stream 15NO3\−-tracer experiments in 72 headwater streams draining different land-use types across the United States. This project, known as the second Lotic Intersite Nitrogen eXperiment (LINX II), provides unique whole-system measurements of the N2O yield in streams. Although N2O emission rates have been reported for streams and rivers (7, 8), the N2O yield has been studied mostly in lentic freshwater and marine ecosystems, where it generally ranges between 0.1 and 1.0\%, although yields as high as 6\% have been observed (9). These N2O yields are low compared with observations in soils (0\–100\%) (10), which may be a result of the relatively lower oxygen (O2) availability in the sediments of lakes and estuaries. However, dissolved O2 in headwater streams is commonly near atmospheric equilibrium and benthic algal biofilms can produce O2 at the sediment\–water interface, resulting in strong redox gradients more akin to those in partially wetted soils. Thus, streams may have variable and often high N2O yields, similar to those in soils (11). The N2O yield in headwater streams is of particular interest because much of the NO3\− input to rivers is derived from groundwater upwelling into headwater streams. Furthermore, headwater streams compose the majority of stream length within a drainage network and have high ratios of bioreactive benthic surface area to water volume (12).

}, keywords = {LTER-KNZ}, doi = {10.1073/pnas.1011464108}, url = {https://www.pnas.org/content/108/1/214}, author = {Beaulieu, J.K. and Tank, J.L. and Hamilton, S.K. and Wollheim, W.M. and Hall, R.O. and Mulholland, P.J. and Peterson, B.J. and L.R. Ashkenas and Cooper, L.W. and Dahm, C.N. and W. K. Dodds and Grimm, N.B. and Johnson, S.L. and W.H. McDowell and Poole, G.C. and Valett, H.M. and Arango, C.P. and Bernot, M.J. and Burgin, A.J. and Crenshaw, C. and Helton, A.M. and Johnson, L. and O{\textquoteright}Brien, J.M. and Potter, J.D. and Sheibley, R.W. and Sobota, D.J. and Thomas, S.M.} } @article {KNZ001423, title = {Thinking outside the channel: modeling nitrogen cycling in networked river ecosystems}, journal = {Frontiers in Ecology and the Environment}, volume = {9}, year = {2011}, pages = {229 -238}, abstract = {

Agricultural and urban development alters nitrogen and other biogeochemical cycles in rivers worldwide. Because such biogeochemical processes cannot be measured empirically across whole river networks, simulation models are critical tools for understanding river-network biogeochemistry. However, limitations inherent in current models restrict our ability to simulate biogeochemical dynamics among diverse river networks. We illustrate these limitations using a river-network model to scale up in situ measures of nitrogen cycling in eight catchments spanning various geophysical and land-use conditions. Our model results provide evidence that catchment characteristics typically excluded from models may control river-network biogeochemistry. Based on our findings, we identify important components of a revised strategy for simulating biogeochemical dynamics in river networks, including approaches to modeling terrestrial\–aquatic linkages, hydrologic exchanges between the channel, floodplain/riparian complex, and subsurface waters, and interactions between coupled biogeochemical cycles.

}, keywords = {LTER-KNZ}, doi = {10.1890/080211}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/080211}, author = {Helton, A.M. and Poole, G.C. and Meyer, J.L. and Wollheim, W.M. and Peterson, B.J. and Mulholland, P.J. and Bernhardt, E.S. and Stanford, J.A. and Arango, C.P. and L.R. Ashkenas and Cooper, L.W. and W. K. Dodds and Gregory, S.V. and O{\textquoteright}Hall, R. and Hamilton, S.K. and Johnson, S.L. and W.H. McDowell and Potter, J.D. and Tank, J.L. and Thomas, S.M. and Valett, H.M. and Webster, J.R. and Lydia H. Zeglin} } @article {KNZ001347, title = {Inter-regional comparison of land-use effects on stream metabolism}, journal = {Freshwater Biology}, volume = {55}, year = {2010}, pages = {1874 -1890}, abstract = {1. Rates of whole-system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first-order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban-influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike{\textquoteright}s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land-use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land-use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land-use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land-use categories showed reduced inter-regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.}, keywords = {LTER-KNZ}, doi = {10.1111/j.1365-2427.2010.02422.x}, author = {Bernot, M.J. and Sobota, D.J. and Hall, R.O. and Mulholland, P.J. and W. K. Dodds and Webster, J.R. and Tank, J.L. and L.R. Ashkenas and Cooper, L.W. and Dahm, C.N. and Gregory, S.V. and Grimm, N.B. and Hamilton, S.K. and Johnson, S.L. and W.H. McDowell and Meyer, J.L. and Peterson, B. and Poole, G.C. and Valett, H.M. and Arango, C.P. and Beaulieu, J.J. and Burgin, A.J. and Crenshaw, C. and Helton, A.M. and Johnson, L. and Merriam, J. and Niederlehner, B.R. and O{\textquoteright}Brien, J.M. and Potter, J.D. and Sheibley, R.W. and Thomas, S.M. and Wilson, K.} } @article {KNZ001251, title = {Nitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification}, journal = {Limnology and Oceanography}, volume = {54}, year = {2009}, pages = {666 -680}, abstract = {

We measured denitrification rates using a field 15NO3\− tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban-urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (SWdenn) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N2 production rates far exceeded N2O production rates in all streams. The fraction of total NO3\− removal from water due to denitrification ranged from 0.5\% to 100\% among streams (median of 16\%), and was related to NH4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO3\− concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO3\− concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis-Menten equation. Although Uden increased with increasing NO3\− concentration, the efficiency of NO3\− removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO3\− load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO3\− concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO3\− concentration.

}, keywords = {LTER-KNZ}, doi = {10.4319/lo.2009.54.3.0666}, url = {https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.0666}, author = {Mulholland, P.J. and Hall, R.O. and Sobota, D.J. and W. K. Dodds and Findlay, S.E.G. and Grimm, N.B. and Hamilton, S.K. and W.H. McDowell and O{\textquoteright}Brien, J.M. and Tank, J.L. and L.R. Ashkenas and Cooper, L.W. and Dahm, C.N. and Gregory, S.V. and Johnson, S.L. and Meyer, J.L. and Peterson, B.J. and Poole, G.C. and Valett, H.M. and Webster, J.R. and Arango, C.P. and Beaulieu, J.J. and Bernot, M.J. and Burgin, A.J. and Crenshaw, C.L. and Helton, A.M. and Johnson, L.T. and Niederlehner, B.R. and Potter, J.D. and Sheibley, R.W. and Thomas, S.M.} } @article {KNZ001250, title = {Nitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake}, journal = {Limnology and Oceanography}, volume = {54}, year = {2009}, pages = {653 -665}, abstract = {

We measured uptake length of 15NO3\− in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO3\− uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban-urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO3\− concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO3\− uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79\% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO3\− concentrations, showing a loss in removal efficiency in streams with high NO3\− concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO3\− removal. The fraction of catchment area as agriculture and suburban-urban land use weakly predicted NO3\− uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO3\− uptake lengths via directly increasing both gross primary production and NO3\− concentration. Gross primary production shortened SWtot, while increasing NO3\− lengthened SWtot resulting in no net effect of land use on NO3\− removal.

}, keywords = {LTER-KNZ}, doi = {10.4319/lo.2009.54.3.0653}, url = {https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.0653}, author = {Hall, R.O. Jr. and Tank, J.L. and Sobota, D.J. and Mulholland, P.J. and O{\textquoteright}Brien, J.M. and W. K. Dodds and Webster, J.R. and Valett, H.M. and Poole, G.C. and Peterson, B.J. and Meyer, J.L. and W.H. McDowell and Johnson, S.L. and Hamilton, S.K. and Grimm, N.B. and Gregory, S.V. and Dahm, C.N. and Cooper, L.W. and L.R. Ashkenas and Thomas, S.M. and Sheibley, R.W. and Potter, J.D. and Niederlehner, B.R. and Johnson, L.T. and Helton, A.M. and Crenshaw, C.M. and Burgin, A.J. and Bernot, M.J. and Beaulieu, J.J. and Arango, C.P.} } @article {KNZ001164, title = {Stream denitrification across biomes and its response to anthropogenic nitrate loading}, journal = {Nature}, volume = {452}, year = {2008}, pages = {202 -207}, abstract = {

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing1, 2 and terrestrial ecosystems are becoming increasingly nitrogen-saturated3, causing more bioavailable nitrogen to enter groundwater and surface waters4, 5, 6. Large-scale nitrogen budgets show that an average of about 20\–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins7, 8, indicating that substantial sinks for nitrogen must exist in the landscape9. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification6, 10, 11. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

}, keywords = {LTER-KNZ}, doi = {10.1038/nature06686}, url = {https://www.nature.com/articles/nature06686}, author = {Mulholland, P.J. and Helton, A.M. and Poole, G.C. and Hall, R.O. Jr. and Hamilton, S.K. and Peterson, B.J. and Tank, J.L. and L.R. Ashkenas and Cooper, L.W. and Dahm, C.N. and W. K. Dodds and Findlay, S.E.G. and Gregory, S.V. and Grimm, N.B. and Johnson, S.L. and W.H. McDowell and Meyer, J.L. and Valett, H.M. and Webster, J.R. and Arango, C.P. and Beaulieu, J.J. and Bernot, M.J. and Burgin, A.J. and Crenshaw, C. and Johnson, L. and Niederlehner, B.R. and O{\textquoteright}Brien, J.M. and Potter, J.D. and Sheibley, R.W. and Sobota, D.J. and Thomas, S.M.} } @article {KNZ00988, title = {Estimation of stream nutrient uptake from nutrient addition experiments}, journal = {Limnology and Oceanography Methods}, volume = {3}, year = {2005}, pages = {174 -182}, abstract = {Nutrient uptake in streams is often quantified by determining nutrient uptake length. However, current methods for measuring nutrient uptake length are often impractical, expensive, or demonstrably incorrect. We have developed a new method to estimate ambient nutrient uptake lengths using field experiments involving several levels of nutrient addition. Data analysis involves plotting nutrient addition uptake lengths versus added concentration and extrapolating to the negative ambient concentration. This method is relatively easy, inexpensive, and based on sound theoretical development. It is more accurate than the commonly used method involving a single nutrient addition. The utility of the method is supported by field studies directly comparing our new method with isotopic tracer methods for determining uptake lengths of phosphorus, ammonium, and nitrate. Our method also provides parameters for comparing potential nutrient limitation among streams.}, keywords = {LTER-KNZ}, doi = {10.4319/lom.2005.3.174}, author = {Payn, R.A. and Webster, J.R. and Mulholland, P.J. and Valett, H.M. and W. K. Dodds} } @article {KNZ00870, title = {Factors affecting ammonium uptake in streams - an inter-biome perspective}, journal = {Freshwater Biology}, volume = {48}, year = {2003}, pages = {1329 -1352}, abstract = {1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of 15N-ammonium and downstream measurements of 15N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake.}, keywords = {LTER-KNZ}, doi = {10.1046/j.1365-2427.2003.01094.x}, author = {Webster, J.R. and P. Mulholland and Tank, J.L. and Valett, H.M. and W. K. Dodds and Peterson, B.J. and W.B. Bowden and Dahm, C.N. and S.E.G. Findlay and Gregory, S.V. and Grimm, N.B. and Hamilton, S.K. and Johnson, S.L. and Marti, E. and W.H. McDowell and Meyer, J.L. and Morrall, D.D. and Thomas, S.A. and Wollheim, W.M.} } @article {KNZ00810, title = {Can uptake length in streams be determined by nutrient addition experiments? Results from an inter-biome comparison study}, journal = {Journal of the North American Benthological Society}, volume = {21}, year = {2002}, pages = {544 -560}, abstract = {Nutrient uptake length is an important parameter for quantifying nutrient cycling in streams. Although nutrient tracer additions are the preferred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more frequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (SW') and uptake length determined by tracer additions (SW) predicted that SW' should be consistently longer than SW, and that the overestimate of uptake length by SW' should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data from an interbiome study of NH4+ uptake length in which 15NH4+ tracer and short-term NH4+ addition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely confirmed the theoretical predictions: SW' was consistently longer than SW and SW':SW ratios were directly related to the level of NH4+ addition and to indicators of N limitation. The experimentally derived SW':SW ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed that tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or different times in the same stream, however, provided that nutrient additions are kept as low as possible and of similar magnitude.}, keywords = {LTER-KNZ, ammonium, nitrogen limitation, nutrient cycling, nutrient spiraling, stream, uptake length}, doi = {10.2307/1468429}, author = {Mulholland, P.J. and Tank, J.L. and Webster, J.R. and W.B. Bowden and W. K. Dodds and Gregory, S.V. and Grimm, N.B. and Hamilton, S.K. and Johnson, S.L. and Marti, E. and W.H. McDowell and Merriam, J. and Meyer, J.L. and Peterson, B.J. and Valett, H.M. and Wollheim, W.M.} } @article {KNZ00807, title = {A cross-system comparison of bacterial and fungal biomass in detritus pools of headwater streams}, journal = {Microbial Ecology}, volume = {43}, year = {2002}, pages = {55 -66}, keywords = {LTER-KNZ}, doi = {10.1007/s00248-001-1020-x}, author = {Findlay, S.E.G. and Tank, J. and Dye, S. and Vallett, H.M. and Mulholland, P.J. and W.H. McDowell and Johnson, S. and Hamilton, S.K. and Edmonds, J. and W. K. Dodds and W.B. Bowden} } @article {KNZ00806, title = {N uptake as a function of concentration in streams}, journal = {Journal of the North American Benthological Society}, volume = {21}, year = {2002}, pages = {206 -220}, abstract = {Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO3- and NH4+ at constant addition rates in the spring and summer of 1998. NH4+ uptake was also measured with 15N-NH4+ tracer additions and short-term unlabeled NH4+ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH4+ uptake rate per unit area (Ut) was positively correlated to nutrient concentration across all sites (r2 = 0.41, log{\textendash}log relationship). Relationships between concentration and Ut were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, Ut was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient Ut. Linear extrapolation of Ut assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient Ut by \~{}3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 μmol/L. The absolute concentration at which Ut was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH4+ at ambient concentrations in all streams were higher than would be expected, assuming Ut does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, Ut is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), Ut may not saturate, even at high concentrations.}, keywords = {LTER-KNZ, advective transport, ammonium, areal uptake, benthos, kinetics, mass transport, Nitrate, nitrogen}, doi = {10.2307/1468410}, author = {W. K. Dodds and L{\'o}pez, A.J. and W.B. Bowden and Gregory, S. and Grimm, N.B. and Hamilton, S.K. and Hershey, A.E. and Marti, E. and W.B. McDowell and Meyer, J.L. and Morrall, D. and Mulholland, P.J. and Peterson, B.J. and Tank, J.L. and van der Hoek, D.C.J. and Webster, J.R. and Wollheim, W.} } @article {KNZ00783, title = {Control of nitrogen export from watersheds by headwater streams}, journal = {Science}, volume = {292}, year = {2001}, pages = {86 -90}, abstract = {A comparative 15N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.}, keywords = {LTER-KNZ}, doi = {10.1126/science.1056874}, author = {Peterson, B.J. and Wollheim, W.M. and Mulholland, P.J. and Webster, J.R. and Meyer, J.L. and Tank, J.L. and Marti, E. and W.B. Bowden and Valett, H.M. and Hershey, A.E. and W.H. McDowell and W. K. Dodds and Hamilton, S.K. and Gregory, S. and Morrall, D.D.} } @article {KNZ00780, title = {Inter-biome comparison of factors controlling stream metabolism}, journal = {Freshwater Biology}, volume = {46}, year = {2001}, pages = {1503 -1517}, abstract = {1. We studied whole-ecosystem metabolism in eight streams from several biomes in North America to identify controls on the rate of stream metabolism over a large geographic range. The streams studied had climates ranging from tropical to cool-temperate and from humid to arid and were all relatively uninfluenced by human disturbances. 2. Rates of gross primary production (GPP), ecosystem respiration (R) and net ecosystem production (NEP) were determined using the open-system, two-station diurnal oxygen change method. 3. Three general patterns in metabolism were evident among streams: (1) relatively high GPP with positive NEP (i.e. net oxygen production) in early afternoon, (2) moderate primary production with a distinct peak in GPP during daylight but negative NEP at all times and (3) little or no evidence of GPP during daylight and a relatively constant and negative NEP over the entire day. 4. Gross primary production was most strongly correlated with photosynthetically active radiation (PAR). A multiple regression model that included log PAR and stream water soluble reactive phosphorus (SRP) concentration explained 90\% of the variation in log GPP. 5. Ecosystem respiration was significantly correlated with SRP concentration and size of the transient storage zone and, together, these factors explained 73\% of the variation in R. The rate of R was poorly correlated with the rate of GPP. 6. Net ecosystem production was significantly correlated only with PAR, with 53\% of the variation in log NEP explained by log PAR. Only Sycamore Creek, a desert stream in Arizona, had positive NEP (GPP: R > 1), supporting the idea that streams are generally net sinks rather than net sources of organic matter. 7. Our results suggest that light, phosphorus concentration and channel hydraulics are important controls on the rate of ecosystem metabolism in streams over very extensive geographic areas.}, keywords = {LTER-KNZ}, doi = {10.1046/j.1365-2427.2001.00773.x}, author = {Mulholland, P.J. and Fellows, C.S. and Tank, J.L. and Grimm, N.B. and Webster, J.R. and Hamilton, S.K. and Marti, E. and L.R. Ashkenas and W.B. Bowden and W. K. Dodds and W.H. McDowell and Paul, M.J. and Peterson, B.J.} }