06735nas a2200373 4500008004100000245012300041210006900164300000800233490000700241520565500248100001505903700001405918700001305932700002105945700002005966700001805986700001706004700001906021700001706040700002006057700001906077700001606096700002006112700001606132700001906148700001806167700001906185700001706204700001506221700001606236700001706252700001806269856007406287 2018 eng d00aPartitioning assimilatory nitrogen uptake in streams: an analysis of stable isotope tracer additions across continents0 aPartitioning assimilatory nitrogen uptake in streams an analysis a1380 v883 a

Headwater streams remove, transform, and store inorganic nitrogen (N) delivered from surrounding watersheds, but excessive N inputs from human activity can saturate removal capacity. Most research has focused on quantifying N removal from the water column over short periods and in individual reaches, and these ecosystem-scale measurements suggest that assimilatory N uptake accounts for most N removal. However, cross-system comparisons addressing the relative role of particular biota responsible for incorporating inorganic N into biomass are lacking. Here we assess the importance of different primary uptake compartments on reach-scale ammonium (NH4+-N) uptake and storage across a wide range of streams varying in abundance of biota and local environmental factors. We analyzed data from 17 15N-NH4+tracer addition experiments globally, and found that assimilatory N uptake by autotrophic compartments (i.e., epilithic biofilm, filamentous algae, bryophytes/macrophytes) was higher but more variable than for heterotrophic microorganisms colonizing detrital organic matter (i.e., leaves, small wood, and fine particles). Autotrophic compartments played a disproportionate role in N uptake relative to their biomass, although uptake rates were similar when we rescaled heterotrophic assimilatory N uptake associated only with live microbial biomass. Assimilatory NH4+-N uptake, either estimated as removal from the water column or from the sum uptake of all individual compartments, was four times higher in open- than in closed-canopy streams. Using Bayesian Model Averaging, we found that canopy cover and gross primary production (GPP) controlled autotrophic assimilatory N uptake while ecosystem respiration (ER) was more important for the heterotrophic contribution. The ratio of autotrophic to heterotrophic N storage was positively correlated with metabolism (GPP: ER), which was also higher in open- than in closed-canopy streams. Our analysis shows riparian canopy cover influences the relative abundance of different biotic uptake compartments and thus GPP:ER. As such, the simple categorical variable of canopy cover explained differences in assimilatory N uptake among streams at the reach scale, as well as the relative roles of autotrophs and heterotrophs in N storage. Finally, this synthesis links cumulative N uptake by stream biota to reach-scale N demand and provides a mechanistic and predictive framework for estimating and modeling N cycling in other streams.

1 aTank, J.L.1 aMarti, E.1 aRiis, T.1 avon Schiller, D.1 aReisinger, A.J.1 aDodds, W., K.1 aWhiles, M.R.1 aAshkenas, L.R.1 aBowden, W.B.1 aCollins, S., M.1 aCrenshaw, C.L.1 aCrowl, T.A.1 aGriffiths, N.A.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMcDowell, W.H.1 aNorman, B.M.1 aRosi, E.J.1 aSimon, K.S.1 aThomas, S.A.1 aWebster, J.R. uhttps://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.128002431nas a2200373 4500008004100000245012200041210006900163300001500232490000700247520133300254100001801587700002401605700001901629700001501648700001601663700001801679700001601697700001701713700001801730700001901748700001901767700001301786700001901799700001701818700002101835700001801856700001801874700002001892700001401912700002401926700001901950700001401969856007401983 2014 eng d00aYou are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies0 aYou are not always what we think you eat selective assimilation  a2757 -27670 v953 a

Analyses of 21 15N stable isotope tracer experiments, designed to examine food web dynamics in streams around the world, indicated that the isotopic composition of food resources assimilated by primary consumers (mostly invertebrates) poorly reflected the presumed food sources. Modeling indicated that consumers assimilated only 33–50% of the N available in sampled food sources such as decomposing leaves, epilithon, and fine particulate detritus over feeding periods of weeks or more. Thus, common methods of sampling food sources consumed by animals in streams do not sufficiently reflect the pool of N they assimilate. Isotope tracer studies, combined with modeling and food separation techniques, can improve estimation of N pools in food sources that are assimilated by consumers. Food web studies that use putative food samples composed of actively cycling (more readily assimilable) and refractory (less assimilable) N fractions may draw erroneous conclusions about diets, N turnover, and trophic linkages of consumers. By extension, food web studies using stoichiometric or natural abundance approaches that rely on an accurate description of food-source composition could result in errors when an actively cycling pool that is only a fraction of the N pool in sampled food resources is not accounted for.

1 aDodds, W., K.1 aCollins, Scott., L.1 aHamilton, S.K.1 aTank, J.L.1 aJohnson, S.1 aWebster, J.R.1 aSimon, K.S.1 aWhiles, M.R.1 aRantala, H.M.1 aMcDowell, W.H.1 aPeterson, S.D.1 aRiis, T.1 aCrenshaw, C.L.1 aThomas, S.A.1 aKristensen, P.B.1 aCheever, B.M.1 aFlecker, A.S.1 aGriffiths, N.A.1 aCrowl, T.1 aRosi-Marshall, E.J.1 aEl-Sabaawi, R.1 aMarti, E. uhttps://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/13-2276.106005nas a2200433 4500008004100000245007700041210006900118300001300187490000800200520484900208100001905057700001505076700001905091700001905110700001505129700002105144700001905165700001905184700001705203700001505220700001805235700001605253700001805269700001905287700001605306700001705322700001705339700001705356700001705373700001705390700001705407700001605424700001805440700001705458700001905475700001705494700001705511856004305528 2011 eng d00aNitrous oxide emission from denitrification in stream and river networks0 aNitrous oxide emission from denitrification in stream and river  a214 -2190 v1083 a

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).

1 aBeaulieu, J.K.1 aTank, J.L.1 aHamilton, S.K.1 aWollheim, W.M.1 aHall, R.O.1 aMulholland, P.J.1 aPeterson, B.J.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aDodds, W., K.1 aGrimm, N.B.1 aJohnson, S.L.1 aMcDowell, W.H.1 aPoole, G.C.1 aValett, H.M.1 aArango, C.P.1 aBernot, M.J.1 aBurgin, A.J.1 aCrenshaw, C.1 aHelton, A.M.1 aJohnson, L.1 aO'Brien, J.M.1 aPotter, J.D.1 aSheibley, R.W.1 aSobota, D.J.1 aThomas, S.M. uhttps://www.pnas.org/content/108/1/21402203nas a2200385 4500008004100000245009000041210006900131300001300200490000600213520111200219100001701331700001601348700001601364700001901380700001901399700002101418700002001439700001901459700001701478700001901495700001701514700001801531700001801549700001701567700001901584700001801603700001901621700001701640700001501657700001701672700001701689700001801706700002201724856007101746 2011 eng d00aThinking outside the channel: modeling nitrogen cycling in networked river ecosystems0 aThinking outside the channel modeling nitrogen cycling in networ a229 -2380 v93 a

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.

1 aHelton, A.M.1 aPoole, G.C.1 aMeyer, J.L.1 aWollheim, W.M.1 aPeterson, B.J.1 aMulholland, P.J.1 aBernhardt, E.S.1 aStanford, J.A.1 aArango, C.P.1 aAshkenas, L.R.1 aCooper, L.W.1 aDodds, W., K.1 aGregory, S.V.1 aO’Hall, R.1 aHamilton, S.K.1 aJohnson, S.L.1 aMcDowell, W.H.1 aPotter, J.D.1 aTank, J.L.1 aThomas, S.M.1 aValett, H.M.1 aWebster, J.R.1 aZeglin, Lydia, H. uhttps://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/08021103830nas a2200493 4500008004100000245007100041210006900112300001500181490000700196520248100203100001702684700001702701700001502718700002102733700001802754700001802772700001502790700001902805700001702824700001502841700001802856700001602874700001902890700001802909700001902927700001602946700001702962700001602979700001702995700001703012700001903029700001703048700001703065700001703082700001603099700001603115700002303131700001803154700001703172700001903189700001703208700001503225856009603240 2010 eng d00aInter-regional comparison of land-use effects on stream metabolism0 aInterregional comparison of landuse effects on stream metabolism a1874 -18900 v553 a1. 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’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.1 aBernot, M.J.1 aSobota, D.J.1 aHall, R.O.1 aMulholland, P.J.1 aDodds, W., K.1 aWebster, J.R.1 aTank, J.L.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aGregory, S.V.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMcDowell, W.H.1 aMeyer, J.L.1 aPeterson, B.1 aPoole, G.C.1 aValett, H.M.1 aArango, C.P.1 aBeaulieu, J.J.1 aBurgin, A.J.1 aCrenshaw, C.1 aHelton, A.M.1 aJohnson, L.1 aMerriam, J.1 aNiederlehner, B.R.1 aO'Brien, J.M.1 aPotter, J.D.1 aSheibley, R.W.1 aThomas, S.M.1 aWilson, K. uhttp://knz.lternet.edu/content/inter-regional-comparison-land-use-effects-stream-metabolism01846nas a2200145 4500008004100000245010700041210006900148300001500217490000700232520135400239100001801593700001501611700001801626856005601644 2009 eng d00aThe influence of land use on stream biofilm nutrient limitation across eight North American ecoregions0 ainfluence of land use on stream biofilm nutrient limitation acro a1081 -10940 v663 a

Nutrient diffusing substrata were used to determine the influence of inorganic nitrogen (N) and phosphorus (P) availability on community respiration (CR), gross primary production (GPP), and chlorophyll a (chl a) on inorganic and organic substrata. We incubated substrata in nine streams each in a total of eight ecoregions (n = 72 streams) located in a range of native vegetation, agriculture, and urban land-use types. On organic substrata, CR was nutrient-limited in 94% of reference streams but showed significant nutrient limitation in only 60% and 65% of agricultural and urban streams, respectively. The relative magnitude of nutrient limitation for CR on organic substrata decreased with increasing percent modified land use in the basin (agriculture + urban). On inorganic and organic substrata, GPP and chl a were rarely nutrient-limited across all ecoregions and land-use types, although the magnitude of nutrient limitation increased with increasing light availability. The effect of human land use on nutrient limitation of biofilm CR, GPP, and chl a was influenced by ecoregion, yet heterotrophic biofilms were consistently most sensitive to nutrient enrichment across ecoregions. Both heterotrophic and autotrophic biofilm constituents should be considered to fully understand stream ecosystem responses to nutrient enrichment.

1 aJohnson, L.T.1 aTank, J.L.1 aDodds, W., K. uhttp://www.nrcresearchpress.com/doi/10.1139/F09-06503136nas a2200481 4500008004100000245009500041210006900136300001300205490000700218520179900225100002102024700001502045700001702060700001802077700002002095700001602115700001902131700001902150700001802169700001502187700001902202700001702221700001502238700001802253700001802271700001602289700001902305700001602324700001702340700001802357700001702375700001902392700001702411700001702428700001902445700001702464700001802481700002302499700001702522700001902539700001702558856007902575 2009 eng d00aNitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification0 aNitrate removal in stream ecosystems measured by 15N addition ex a666 -6800 v543 a

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.

1 aMulholland, P.J.1 aHall, R.O.1 aSobota, D.J.1 aDodds, W., K.1 aFindlay, S.E.G.1 aGrimm, N.B.1 aHamilton, S.K.1 aMcDowell, W.H.1 aO'Brien, J.M.1 aTank, J.L.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aGregory, S.V.1 aJohnson, S.L.1 aMeyer, J.L.1 aPeterson, B.J.1 aPoole, G.C.1 aValett, H.M.1 aWebster, J.R.1 aArango, C.P.1 aBeaulieu, J.J.1 aBernot, M.J.1 aBurgin, A.J.1 aCrenshaw, C.L.1 aHelton, A.M.1 aJohnson, L.T.1 aNiederlehner, B.R.1 aPotter, J.D.1 aSheibley, R.W.1 aThomas, S.M. uhttps://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.066603146nas a2200469 4500008004100000245009200041210006900133300001300202490000700215520184400222100001502066700001502081700001702096700002102113700001802134700001802152700001802170700001702188700001602205700001902221700001602240700001902256700001802275700001902293700001602312700001802328700001502346700001702361700001902378700001702397700001902414700001702433700002302450700001802473700001702491700001902508700001702527700001702544700001902561700001702580856007902597 2009 eng d00aNitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake0 aNitrate removal in stream ecosystems measured by 15N addition ex a653 -6650 v543 a

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.

1 aHall, R.O.1 aTank, J.L.1 aSobota, D.J.1 aMulholland, P.J.1 aO'Brien, J.M.1 aDodds, W., K.1 aWebster, J.R.1 aValett, H.M.1 aPoole, G.C.1 aPeterson, B.J.1 aMeyer, J.L.1 aMcDowell, W.H.1 aJohnson, S.L.1 aHamilton, S.K.1 aGrimm, N.B.1 aGregory, S.V.1 aDahm, C.N.1 aCooper, L.W.1 aAshkenas, L.R.1 aThomas, S.M.1 aSheibley, R.W.1 aPotter, J.D.1 aNiederlehner, B.R.1 aJohnson, L.T.1 aHelton, A.M.1 aCrenshaw, C.M.1 aBurgin, A.J.1 aBernot, M.J.1 aBeaulieu, J.J.1 aArango, C.P. uhttps://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.2009.54.3.065302900nas a2200481 4500008004100000245009100041210006900132300001300201490000800214520160100222100002101823700001701844700001601861700001501877700001901892700001901911700001501930700001901945700001701964700001501981700001801996700002002014700001802034700001602052700001802068700001902086700001602105700001702121700001802138700001702156700001902173700001702192700001702209700001702226700001602243700002302259700001802282700001702300700001902317700001702336700001702353856004802370 2008 eng d00aStream denitrification across biomes and its response to anthropogenic nitrate loading0 aStream denitrification across biomes and its response to anthrop a202 -2070 v4523 a

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.

1 aMulholland, P.J.1 aHelton, A.M.1 aPoole, G.C.1 aHall, R.O.1 aHamilton, S.K.1 aPeterson, B.J.1 aTank, J.L.1 aAshkenas, L.R.1 aCooper, L.W.1 aDahm, C.N.1 aDodds, W., K.1 aFindlay, S.E.G.1 aGregory, S.V.1 aGrimm, N.B.1 aJohnson, S.L.1 aMcDowell, W.H.1 aMeyer, J.L.1 aValett, H.M.1 aWebster, J.R.1 aArango, C.P.1 aBeaulieu, J.J.1 aBernot, M.J.1 aBurgin, A.J.1 aCrenshaw, C.1 aJohnson, L.1 aNiederlehner, B.R.1 aO'Brien, J.M.1 aPotter, J.D.1 aSheibley, R.W.1 aSobota, D.J.1 aThomas, S.M. uhttps://www.nature.com/articles/nature0668602306nas a2200181 4500008004100000245012700041210006900168300001500237490000700252520163700259100001701896700001801913700001801931700001901949700001601968700001501984856012501999 2003 eng d00aComparing denitrification estimates for a Texas estuary by using acetylene inhibition and membrane inlet mass spectrometry0 aComparing denitrification estimates for a Texas estuary by using a5950 -59560 v693 aCharacterizing denitrification rates in aquatic ecosystems is essential to understanding how systems may respond to increased nutrient loading. Thus, it is important to ensure the precision and accuracy of the methods employed for measuring denitrification rates. The acetylene (C2H2) inhibition method is a simple technique for estimating denitrification. However, potential problems, such as inhibition of nitrification and incomplete inhibition of nitrous oxide reduction, may influence rate estimates. Recently, membrane inlet mass spectrometry (MIMS) has been used to measure denitrification in aquatic systems. Comparable results were obtained with MIMS and C2H2 inhibition methods when chloramphenicol was added to C2H2 inhibition assay mixtures to inhibit new synthesis of denitrifying enzymes. Dissolved-oxygen profiles indicated that surface layers of sediment cores subjected to the MIMS flowthrough incubation remained oxic whereas cores incubated using the C2H2 inhibition methods did not. Analysis of the microbial assemblages before and after incubations indicated significant changes in the sediment surface populations during the long flowthrough incubation for MIMS analysis but not during the shorter incubation used for the C2H2 inhibition method. However, bacterial community changes were also small in MIMS cores at the oxygen transition zone where denitrification occurs. The C2H2 inhibition method with chloramphenicol addition, conducted over short incubation intervals, provides a cost-effective method for estimating denitrification, and rate estimates are comparable to those obtained by the MIMS method.1 aBernot, M.J.1 aDodds, W., K.1 aGardner, W.S.1 aMcCarthy, M.J.1 aSobolev, D.1 aTank, J.L. uhttp://knz.lternet.edu/content/comparing-denitrification-estimates-texas-estuary-using-acetylene-inhibition-and-membrane02610nas a2200337 4500008004100000245007800041210006900119300001500188490000700203520162900210100001801839700001901857700001501876700001701891700001801908700001901926700001701945700001501962700002001977700001801997700001602015700001902031700001802050700001402068700001902082700001602101700001802117700001702135700001902152856010102171 2003 eng d00aFactors affecting ammonium uptake in streams - an inter-biome perspective0 aFactors affecting ammonium uptake in streams an interbiome persp a1329 -13520 v483 a1. 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.1 aWebster, J.R.1 aMulholland, P.1 aTank, J.L.1 aValett, H.M.1 aDodds, W., K.1 aPeterson, B.J.1 aBowden, W.B.1 aDahm, C.N.1 aFindlay, S.E.G.1 aGregory, S.V.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMarti, E.1 aMcDowell, W.H.1 aMeyer, J.L.1 aMorrall, D.D.1 aThomas, S.A.1 aWollheim, W.M. uhttp://knz.lternet.edu/content/factors-affecting-ammonium-uptake-streams-inter-biome-perspective02464nas a2200133 4500008004100000245008400041210006900125300001500194490000700209520197500216100001502191700001802206856010602224 2003 eng d00aResponses of heterotrophic and autotrophic biofilms to nutrients in ten streams0 aResponses of heterotrophic and autotrophic biofilms to nutrients a1031 -10490 v483 a1. Nutrient diffusing substrata were used to determine the effect of added inorganic nitrogen (N) and phosphorus (P) on the development of epilithic and epixylic biofilms in 10 North American streams. Four treatments of diffusing substrata were used: Control (agar only), N addition (0.5 m NaNO3), P addition (0.5 m KH2PO4), and N + P combined (0.5 m NaNO3 + 0.5 m KH2PO4). Agar surfaces were covered with glass fibre filters (for epilithon) or discs of untreated white oak wood veneer (for epixylon). 2. We found that if algae showed significant response to nutrient addition, N limitation (either N alone or N with P) was the most frequent response both on GF/F filters and on wood. Despite the low dissolved nutrient concentrations in our study streams, more than a third of the streams did not show any response to N or P addition. In fact, P was never the sole limiting nutrient for algal biofilms in this study. 3. Nutrient addition influenced algal colonisation of inorganic versus organic substrata in different ways. The presence of other biofilm constituents (e.g. fungi or bacteria) may influence whether algal biomass on wood increased in response to nutrient addition. Algae on organic and inorganic substrata responded similarly to nutrient addition in only one stream. 4. Fungal biomass on wood was nutrient limited in six of 10 study streams. N limitation of fungal biomass (with or without secondary P limitation) was most frequent, but P limitation did occur in two streams. 5. Our results show that biomass responses to nutrient addition by the heterotrophic and autotrophic components of the epixylic biofilm were different, though both experienced the same stream nutrient conditions. For algae and fungi growing on wood, limiting nutrients were rarely similar. Only three of nine streams showed the same biomass response to nutrient addition, including two that showed no significant change in biomass despite added nutrients.1 aTank, J.L.1 aDodds, W., K. uhttp://knz.lternet.edu/content/responses-heterotrophic-and-autotrophic-biofilms-nutrients-ten-streams02845nas a2200373 4500008004100000245012600041210006900167300001300236490000700249520170000256653001301956653002401969653002101993653002302014653001102037653001802048100002102066700001502087700001802102700001702120700001802137700001802155700001602173700001902189700001802208700001402226700001902240700001602259700001602275700001902291700001702310700001902327856012502346 2002 eng d00aCan uptake length in streams be determined by nutrient addition experiments? Results from an inter-biome comparison study0 aCan uptake length in streams be determined by nutrient addition  a544 -5600 v213 aNutrient 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.10aammonium10anitrogen limitation10anutrient cycling10anutrient spiraling10astream10auptake length1 aMulholland, P.J.1 aTank, J.L.1 aWebster, J.R.1 aBowden, W.B.1 aDodds, W., K.1 aGregory, S.V.1 aGrimm, N.B.1 aHamilton, S.K.1 aJohnson, S.L.1 aMarti, E.1 aMcDowell, W.H.1 aMerriam, J.1 aMeyer, J.L.1 aPeterson, B.J.1 aValett, H.M.1 aWollheim, W.M. uhttp://knz.lternet.edu/content/can-uptake-length-streams-be-determined-nutrient-addition-experiments-results-inter-biome03298nas a2200409 4500008004100000245005500041210005500096300001300151490000700164520221800171653002402389653001302413653001702426653001202443653001302455653001902468653001202487653001302499100001802512700001702530700001702547700001602564700001602580700001902596700001802615700001402633700001902647700001602666700001602682700002102698700001902719700001502738700002502753700001802778700001702796856007502813 2002 eng d00aN uptake as a function of concentration in streams0 aN uptake as a function of concentration in streams a206 -2200 v213 aDetailed 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–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.10aadvective transport10aammonium10aareal uptake10abenthos10akinetics10amass transport10aNitrate10anitrogen1 aDodds, W., K.1 aLópez, A.J.1 aBowden, W.B.1 aGregory, S.1 aGrimm, N.B.1 aHamilton, S.K.1 aHershey, A.E.1 aMarti, E.1 aMcDowell, W.B.1 aMeyer, J.L.1 aMorrall, D.1 aMulholland, P.J.1 aPeterson, B.J.1 aTank, J.L.1 avan der Hoek, D.C.J.1 aWebster, J.R.1 aWollheim, W. uhttp://knz.lternet.edu/content/n-uptake-function-concentration-streams01724nas a2200289 4500008004100000245006800041210006800109300001100177490000800188520088600196100001901082700001901101700002101120700001801141700001601159700001501175700001401190700001701204700001701221700001801238700001901256700001801275700001901293700001601312700001801328856008801346 2001 eng d00aControl of nitrogen export from watersheds by headwater streams0 aControl of nitrogen export from watersheds by headwater streams a86 -900 v2923 aA 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.1 aPeterson, B.J.1 aWollheim, W.M.1 aMulholland, P.J.1 aWebster, J.R.1 aMeyer, J.L.1 aTank, J.L.1 aMarti, E.1 aBowden, W.B.1 aValett, H.M.1 aHershey, A.E.1 aMcDowell, W.H.1 aDodds, W., K.1 aHamilton, S.K.1 aGregory, S.1 aMorrall, D.D. uhttp://knz.lternet.edu/content/control-nitrogen-export-watersheds-headwater-streams02733nas a2200265 4500008004100000245006800041210006700109300001500176490000700191520194500198100002102143700001802164700001502182700001602197700001802213700001902231700001402250700001902264700001702283700001802300700001902318700001502337700001902352856009602371 2001 eng d00aInter-biome comparison of factors controlling stream metabolism0 aInterbiome comparison of factors controlling stream metabolism a1503 -15170 v463 a1. 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.1 aMulholland, P.J.1 aFellows, C.S.1 aTank, J.L.1 aGrimm, N.B.1 aWebster, J.R.1 aHamilton, S.K.1 aMarti, E.1 aAshkenas, L.R.1 aBowden, W.B.1 aDodds, W., K.1 aMcDowell, W.H.1 aPaul, M.J.1 aPeterson, B.J. uhttp://knz.lternet.edu/content/inter-biome-comparison-factors-controlling-stream-metabolism02546nas a2200325 4500008004100000245006100041210006100102300001300163490000600176520164900182653002001831653001401851653001901865653002001884653001101904653002201915100001801937700002201955700001801977700001301995700001702008700001502025700001702040700001802057700001602075700001502091700001702106700001702123856008002140 2000 eng d00aQuantification of the nitrogen cycle in a prairie stream0 aQuantification of the nitrogen cycle in a prairie stream a574 -5890 v33 aNitrogen (N) was added for 35 days in the form of 15NH4Cl to Kings Creek on Konza Prairie, Kansas. Standing stocks of N in key compartments (that is, nutrients, detritus, organisms) were quantified, and the amount of labeled N entering the compartments was analyzed. These data were used to calculate turnover and flux rates of N cycling through the food web, as well as nutrient transformation rates. Inorganic N pools turned over much more rapidly in the water column of this stream than in pelagic systems where comparable measurements have been made. As with other systems, the mass of ammonium was low but it was the key compartment mediating nutrient flux through the ecosystem, whereas dissolved organic N, the primary component of N flux through the system, is not actively cycled. Nitrification was also a significant flux of N in the stream, with rates in the water column and surface of benthos accounting for approximately 10% of the total ammonium uptake. Primary consumers assimilated 67% of the inorganic N that entered benthic algae and microbes. Predators acquired 23% of the N that consumers obtained. Invertebrate collectors, omnivorous crayfish (Orconectes spp.), and invertebrate shredders dominated the N flux associated with primary consumers. Mass balance calculations indicated that at least 23% of the 309 mg of 15N added during the 35 days of release was retained within the 210-m stream reach during the release. Overall, the rates of turnover of N in organisms and organic substrata were significantly greater when C:N was low. This ratio may be a surrogate for biological activity with regard to N flux in streams.10aaquatic insects10aC:N ratio10anitrogen cycle10anitrogen export10astream10aTrophic structure1 aDodds, W., K.1 aEvans-White, M.A.1 aGerlanc, N.M.1 aGray, L.1 aGudder, D.A.1 aKemp, M.J.1 aLo'pez, A.L.1 aStagliano, D.1 aStrauss, E.1 aTank, J.L.1 aWhiles, M.R.1 aWollheim, W. uhttp://knz.lternet.edu/content/quantification-nitrogen-cycle-prairie-stream