02956nas a2200313 4500008004100000245012400041210006900165300001400234490000700248520203700255100001802292700001702310700001802327700001702345700001502362700001802377700001602395700001402411700001702425700001702442700001502459700001902474700001902493700001902512700001402531700001802545700001702563856006202580 2015 eng d00aLong-term changes in structure and function of a tropical headwater stream following a disease-driven amphibian decline0 aLongterm changes in structure and function of a tropical headwat a575 - 5890 v603 a

1. Taxonomic and functional diversity in freshwater habitats is rapidly declining, but we know little about how such declines will ultimately affect ecosystems. Neotropical streams are currently experiencing massive losses of amphibians, with many losses linked to the chytrid fungus, Batrachochytrium dendrobatidis (Bd). 2. We examined the ecological consequences of the disease-driven loss of amphibians from a Panamanian stream. We quantified basal resources, macroinvertebrates, N uptake and fluxes through food-web components and ecosystem metabolism in 2012 and 2014 and compared them to pre-decline (2006) and 2 year post-decline (2008) values from a prior study. 3. Epilithon biomass accrued after the decline, more than doubling between 2006 and 2012, but then decreased fivefold from 2012 to 2014. In contrast, suspended particulate organic matter (SPOM) concentrations declined continuously after the amphibian decline through 2014. 4. Biomass of filter-feeding, grazing and shredding macroinvertebrates decreased from 2006 to 2014, while collector–gatherers increased during the same time period. Macroinvertebrate taxa richness decreased from 2006 (52 taxa) to 2012 (30 taxa), with a subsequent increase to 51 taxa in 2014. 5. Community respiration, which initially decreased after the amphibian decline, remained lower than pre-decline in 2012 but was greater than pre-decline values in 2014. Gross primary production remained low and similar among years, while inline image uptake length in both 2012 and 2014 was longer than pre-decline. Nitrogen flux to epilithon increased after the decline and continued to do so through 2014, but N fluxes to fine particulate organic matter and SPOM decreased and remained low. 6. Our findings underscore the importance of studying the ecological consequences of declining biodiversity in natural systems over relatively long time periods. There was no evidence of functional redundancy or compensation by other taxa after the loss of amphibians, even after 8 years.

1 aRantala, H.M.1 aNelson, A.M.1 aFulgoni, J.N.1 aWhiles, M.R.1 aHall, R.O.1 aDodds, W., K.1 aVerburg, P.1 aHuryn, A.1 aPringle, C..1 aKilham, S.S.1 aLips, K.R.1 aColon-Gaud, C.1 aRugenski, A.T.1 aPeterson, S.D.1 aFritz, K.1 aMcLeran, K.E.1 aConnelly, S. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/fwb.1250506005nas 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/108/1/21403830nas 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-metabolism03136nas 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.2009.54.3.0666