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A comparative (15)N-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.

Nitrous oxide (N 2 O) 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 N 2 O via microbial denitrification that converts N to N 2 O and dinitrogen (N 2 ). The fraction of denitrified N that escapes as N 2 O rather than N 2 (i.e., the N 2 O yield) is an important determinant of how much N 2 O is produced by river networks, but little is known about the N 2 O yield in flowing waters. Here, we present the results of whole-stream 15 N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N 2 O at rates that increase with stream water nitrate (NO 3 − ) concentrations, but that <1% of denitrified N is converted to N 2 O. Unlike some previous studies, we found no relationship between the N 2 O yield and stream water NO 3 − . We suggest that increased stream NO 3 − loading stimulates denitrification and concomitant N 2 O production, but does not increase the N 2 O yield. In our study, most streams were sources of N 2 O 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 N 2 O in river networks, equivalent to 10% of the global anthropogenic N 2 O emission rate. This estimate of stream and river N 2 O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.H umans 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 (N 2 O), 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 N 2 O in soils and aquatic ecosystems is the largest source of anthropogenic N 2 O to the atmosphere (2). The production of N 2 O 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 N 2 O budget (4). However, emissions of anthropogenic N 2 O from streams, rivers, and estuaries have received much less attention and remain a major source of uncertainty in the global anthropogenic N 2 O budget.Microbial denitrification is a large source of N 2 O emissions in terrestrial and aquatic ecosystems. Most microbial denitrification is a form of anaerobic respiration in which nitrate (NO 3 − , the dominant form of inorganic N) is converted to dinitrogen (N 2 ) and N 2 O gases (5). The proportion of denitrified NO 3 − that is converted to N 2 O rather than N 2 (h...

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SUMMARY1. 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 > I), 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 hydrauh'cs are important controls on the rate of ecosystem metabolism in streams over very extensive geographic areas.

1. ,One of two things can happen to allochthonous material once it enters a stream: it can be broken down or it can be transported downstream. The efficiency with which allochthonous material is used is the result of these two opposing factors: breakdown and transport. 2. ,The present synthesis of new and published studies at Coweeta Hydrologic Laboratory compares biological use versus transport for four categories of particulate organic material: (1) large wood (logs); (2) small wood (sticks); (3) leaves; and (4) fine particulate organic matter (FPOM). 3. ,Over 8_years, logs showed no breakdown or movement. 4. ,The breakdown rate of sticks (≤3_cm diameter) ranged from 0.00017 to 0.00103_day−1, while their rate of transport, although varying considerably with discharge, ranged from 0 to 0.1_m_day−1. 5. ,Based on 40 published measurements, the average rate of leaf breakdown was 0.0098_day−1. The leaf transport rate depended on stream size and discharge. 6. ,The average respiration rate of FPOM was 1.4_mg_O2_g_AFDM−1_day−1 over a temperature range of 6–22_°C, which implies a decomposition rate of 0.00104_day−1. Transport distances of both corn pollen and glass beads, surrogates of natural FPOM, were short (<_10_m) except during high discharge. 7. , Estimates of transport rate were substantially larger than the breakdown rates for sticks, leaves and FPOM. Thus, an organic particle on the stream bottom is more likely to be transported than broken down by biological processes, although estimates of turnover length suggest that sticks and leaves do not travel far. However, once these larger particles are converted to refractory FPOM, either by physical or biological processes, they may be transported long distances before being metabolized.

SUMMARY 1. 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. 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. . 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.