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The concentrations of Ag, Cd, Cu, Hg(II), monomethylmercury (MeHg), Pb, and Zn were measured and modeled in calanoid copepods collected from the Hudson River plume in April 2005 and May 2006. Copepods in the buoyant plume were significantly (p < 0.05) enriched in Ag, Cu, Pb, Zn, and MeHg (2- to 25-fold), but depleted in Cd (by 25-40%) relative to oceanic copepods. Median burdens of Ag, Cu, and Hg(II) differed in plume copepods collected in April 2005 and May 2006 by factors of 1.5 to 3, reflecting the influence of river discharge and associated physical processes on plankton dynamics and metal bioaccumulation. Only the concentrations of Cu and Zn in plume copepods clearly showed the expected trend of highest levels near the mouth of the Hudson River and lower levels offshore. Spatial trends in concentrations of Ag and Hg(II) in plume copepods were strikingly different from those in suspended particles and the dissolved phase, apparently reflecting differences in metal bioavailability because of shifts in available food types. Modeled concentrations of Cd, Cu, Pb, and Zn in plume copepods were within less than a factor of two of measured values, bur model results showed that Hg(II) was less bioavailable, and Ag and MeHg were more bioavailable in the Hudson River plume than laboratory results would predict. Zooplankton from urban river plumes may accumulate potentially toxic levels of trace metals or act as a trophic link between metal contamination in polluted estuaries and pelagic ecosystems. doi: 10.4319/lo.2010.55.6.2487

We assessed how a 5-yr nutrient enrichment affected the responses of different size classes of primary consumers and predators in a detritus-based headwater stream. We hypothesized that alterations in detritus availability because of enrichment would decrease the abundance and biomass of large-bodied consumers. In contrast, we found that 2 yr of enrichment increased the biomass and abundance of all consumers regardless of body size. Furthermore, during the fourth and fifth year of enrichment, the abundance and biomass of large-bodied primary consumers continued to increase, while small-bodied primary consumers returned to pretreatment levels. The size structure of a dominant primary consumer (Pycnopsyche spp.) also shifted during the 5-yr enrichment: its average and maximum individual body size increased in the treatment stream compared with the reference stream. Positive enrichment effects also occurred on small-bodied predators, but not on large-bodied predators. Thus, enrichment increased prey body size, but these positive effects on large prey did not propagate up to higher trophic levels to affect large predators. Because consumer body size can be an important species-specific trait determining population dynamics and ecosystem processes, these observed shifts in consumer size distributions suggest a potentially important pathway for global increases in nutrient enrichment to alter stream structure and function. doi: 10.4319/lo.2010.55.6.2305

Hyporheic water is thought to infiltrate at the head of a riffle, which in turn is complemented by upwelling back to the stream at the tail of the riffle in a pool--riffle--pool (PRP) sequence. Heat tracing is a potentially useful method to characterize these hyporheic flow paths and quantify associated fluxes. Temperature was monitored within a PRP sequence for several days. Temperature in the hyporheic zone reflected the diel temperature change in the river but not uniformly. The observed thermal pattern exhibited deeper penetration of thermal oscillations below the head pool and shallower penetration below the tail pool. This pattern is consistent with the conceptual model of hyporheic exchange over a PRP sequence. One-dimensional analytical heat transport models were used at different points below the PRP sequence to estimate distributed vertical fluid fluxes. The calculated fluxes exhibit a trend that follows the expected distribution for a PRP sequence but modified for a losing stream (net fluid flux into the streambed). Deviation of both magnitude and distribution of fluxes from the conceptual 'downwelling-to-upwelling' model is due partly to the dominantly losing conditions at the study site, but the trends are consistent with a losing stream undergoing hyporheic exchange. Violation of the assumptions in the analytical models most likely adds error to flux estimates. For this study, flux estimation methods using a temperature time-series amplitude analysis more closely matched field measurements than phase methods. doi: 10.4319/10.2010.55.4.1741

We estimated deposition and resuspension rates of natural particulate organic matter (POM) in Bloomington Creek, Idaho, a mountain stream flowing at 225 L [s.sup.-1]L POM was collected from the water column, fractionated into two size classes--very fine POM (VFPOM, 15-52 [micro]m) and fine POM (FPOM, 53-106 [micro]m), and radiolabeled by using [sup.14]C-dimethylsulfate. The labeled particles in each size class and a conservative tracer were released to the stream in metered pulses and then sampled from the water column at six stations extending 1 km downstream for 4 d. Deposition and resuspension rates were estimated by fitting a one-dimensional advection-dispersion model to [sup.14]C-concentrations measured during and after release. Model-estimated deposition velocities were 0.12 (0.09-0.16, 95% confidence interval) and 0.18 (0.10-0.31) mm [s.sup.-1] for VFPOM and FPOM, respectively. There was some (~0.05 mm [s.sup.-1]) additional short-term (~20 min) detention of VFPOM and FPOM that may have been related to transient storage. For VFPOM, 34% of deposited particles resuspended after a mean residence time of 13 (6.9-25) h, and the remainder resuspended with a residence time of 7.5 (2.9-19) d. For FPOM, these estimates were 17%, 2.4 (1.0-4.9) h, and 2.6 (1.7-4.0) d, respectively. The weighted mean residence times and downstream velocities of particle migration were 5.1 d and 150 m [d.sup.-1] for VFPOM, and 2.2 d and 230 m [d.sup.-1] for FPOM. The migration velocities suggest that a significant fraction of particles exported from headwater streams travel long distances and can reach larger riverine or marine environments before mineralization.

We added dextrose for two 8-week periods (summer and autumn) to a highly heterotrophic headwater stream in North Carolina, U.S.A., to examine the responses to its benthic food web to increased labile carbon. We hypothesized that addition of labile carbon would elevate microbial abundance and activity, resulting in greater resource availability and higher macroinvertebrate production and that the effect of dextrose addition would be less marked during the autumn due to lower ambient stream temperatures and large seasonal inputs of leaf litter. Bacterial densities were significantly higher in the treatment reach during both additions. Thick microbial mats of sheathed bacteria and the aquatic hyphomycete Lemonniera pseudofloscula developed on bedrock outcrops. Increased microbial growth led to higher respiration rates on leaf disks and a threefold increase in instantaneous growth rates of chironomid larvae. The abundance and biomass of invertebrate collector-gatherers and predators increased significantly on bedrock during the summer addition but not in the autumn; however, shredder biomass increased significantly in the autumn. On mixed substrates, shredder abundance and scraper biomass increased significantly during the autumn addition. During both additions, all functional feeding groups, including predators, assimilated isotopically distinct dextrose, despite high standing stocks of coarse particulate matter during the autumn addition. Consumers of epilithon and fine particles showed the greatest response. Assimilation of dextrose and increases in invertebrate abundance and biomass suggest that the added carbon stimulated biological activity even in a stream with abundant organic carbon.

We studied the distribution and seasonal dynamics of five species from the genus Flavobacterium and one species from the genus Marinomonas over the course of a year along a northern Baltic Sea river marine transect. All of the species had been previously demonstrated as important consumers of riverine dissolved organic carbon. Quantitative DNA--DNA hybridization data showed that two of the Flavobacterium spp. and the Marinomonas sp. had highest abundance in the river water (maximum 20,000 cells [ml.sup.-1]). with maximum relative abundance of 0.5-2.5% of the bacterial community. These species declined in abundance from the river to the estuary and the offshore site. Abundance and dynamics in the estuarine environment suggested successful immigration of freshwater bacteria, accompanied by growth in the brackish water environment. Two of the three abundant species showed high cell numbers also during late autumn to early spring in the estuary, indicating a selective advantage when riverine dissolved organic carbon was the main carbon source. The remaining three species showed more episodic abundance close to the detection limit of the method, providing weaker evidence of occurrence in the freshwater environment. Some bacterioplankton consuming riverine organic carbon in the brackish water environment in the northern Baltic are therefore freshwater species, with a selective advantage during winter.

Taxonomic distinctness is a newer biodiversity measure that emphasizes the average taxonomic relatedness between species in a community. We examined whether species richness (SR) and taxonomic distinctness (TD) were significantly related and whether they showed similar environmental relationships in regional data sets for various groups of freshwater organisms, ranging from lake mollusks and fishes to stream diatoms and invertebrates. We found that the relationship between SR and TD varied widely, ranging from significantly negative through nonsignificant to significantly positive. In general. SR and TD were related to different environmental gradients, although the particular environmental variables accounting for biodiversity patterns varied among data sets and, more importantly, even between different data sets for the same organism groups. SR and TD can provide complementary views of the variability of biodiversity. These findings thus underline the importance of considering a set of different measures in the assessment of community-level biodiversity, as well as considering this variability when determining anthropogenic effects in freshwater ecosystems.

In many dryland rivers with intermittent flow, relatively deep segments of the river channel serve as refugia for aquatic life during protracted intervals between flows. Semipermanent channel segments are known as waterholes in the semiarid Cooper Creek system of western Queensland. Fractional water loss by evaporation was estimated in 15 Cooper waterholes from the increase in conservative ion ([Na.sup.+] and [Cl.sup.-]) concentrations and independently from evaporative fractionation of oxygen and hydrogen isotopes in water. The major solute chemistry and isotope results indicated that evaporative water loss controlled the water levels between flows and that the surface waters were effectively isolated from underlying groundwater that had a distinctive chemical and isotopic composition. Fractional water loss rates combined with stage-volume relationships for each basin showed a mean evaporative loss rate of 2.1 m [yr.sup.1-]; during that time (April-October 2002), pan evaporation averaged 2.5 m [yr.sup.-1]. Site-specific extrapolation of those estimated evaporative loss rates indicated that the waterholes would dry to 10% of their bankfull volumes in 6-23 months, although those estimates were based on sampling in 2002, when pan evaporation rates were 18% higher than the long-term mean. These persistence times show the importance of occasional, irregular flow pulses in sustaining these aquatic refugia; the desiccation of waterholes could become more common if future water withdrawals reduce the frequency and intensity of river flows to the point where they occur less often than annually.

The annual surface water flux of total reactive (i.e., potentially bioavailable) particulate P from the Mississippi River was estimated by measuring the reactive (including labile, iron, organic, and calcium bound) and nonreactive (detrital) P phases in suspended particulates in the Mississippi River. in addition, the transformation of the major sediment P phases resulting from seasonal channel storage and resuspension was examined. Samples were collected during five cruises over 1 yr at marine and riverine sites. Solid-phase and pore-water nutrients were quantified, and solid-phase P pools were measured using a sequential extraction technique. These results indicate that the Mississippi River exports 134 x [l0.sup.6] kg [yr.sub.-1] of total reactive P via surface water. Seasonal hydrological forcing controlled the variability in major P phases found in channel sediments through hydrodynamic sorting. Although the 6-9-month time period during which sediments were stored in the river channel was sufficient to see evidence of early diagenesis in the pore waters, no significant net effect was seen on major P phase distribution. The loss of a significant percentage of labile and iron-bound P appears to be occurring only as these riverine sediments are deposited and reworked on the continental shelf.

We tested whether sediment bacteria abundance (4',6-diamidino-2-phenylindole-stained cell counts) were related to sediment organic content (ash-free dry mass [AFDM]) in 11 nonforested streams of three different Alpine catchments during summer 2003. We used terminal restriction fragment-length polymorphism (T-RLFP, a molecular genetic technique) to test for seasonal and spatial differences in bacterial composition in these same streams. We then related the above parameters, in conjunction with periphyton biomass and hyporheic respiration, to whole stream estimates of gross primary production (GPP) and ecosystem respiration (ER) in a glacial and nonglacial stream, representing environmental extremes, in one of the catchments. The percentage of organic matter of sediments was 4-14% (0.01-0.04 g AFDM ml [sediment.sup.-1]), and counts of bacteria cells per millimeter of sediment averaged 2 x [10.sup.-6]-4 x [10.sup.6]. Bacteria counts correlated with sediment AFDM only for streams in the catchment with highest sediment AFDM levels. Bacteria composition (based on the presence and absence of terminal restriction fragments from T-RFLP analysis) changed seasonally in the different streams and differed between glacial- and groundwater-fed streams. In the one catchment, hyporheic respiration averaged 0.0004 and 0.0003 g [O.sub.2] [h.sup.-1] kg [sediment.sup.-1] and was positively correlated with AFDM ([r.sup.2] = 0.23). Ecosystem metabolism displayed a strong seasonality, with GPP averaging 4.5 and 8.4 and ER averaging 5.4 and 9.9 g [O.sub.2] [m.sup.-2] [d.sup.-1] for the two sites, respectively. thus indicating a predominance of heterotrophy (P : R < 1) in these high-elevation, open-canopied systems. Bacteria play a strong role in the trophic dynamics of alpine streams.

In extreme environments, retention of nutrients within stream ecosystems contributes to the persistence of aquatic biota and continuity of ecosystem function. In the McMurdo Dry Valleys, Antarctica, many glacial meltwater streams flow for only 5-12 weeks a year and yet support extensive benthic microbial communities. We investigated N[O.sup.-.sub.3]-uptake and denitrification in Green Creek by analyzing small-scale microbial mat dynamics in mesocosms and reach-scale nutrient cycling in two whole-stream N[O.sup.-.sub.3] enrichment experiments. Nitrate uptake results indicated that microbial mats were nitrogen (N)-limited, with N[O.sup.-.sub.3] uptake rates as high as 16 nmol N [cm.sub.-2] [h.sub.-1]. Denitrification potentials associated with microbial mats were also as high as 16 nmol N [cm.sub.-2] [h.sub.-1]. During two whole-stream N[O.sup.-.sub.3]--enrichment experiments, a simultaneous pulse of N[O.sup.-.sub.2] was observed in the stream water. The one-dimensional solute transport model with inflow and storage was modified to simulate two storage zones: one to account for short time scale hydrologic exchange of stream water into and out of the benthic microbial mat, the other to account for longer time scale hydrologic exchange with the hyporheic zone. Simulations indicate that injected N[O.sup.-.sub.3] was removed both in the microbial mat and in the hyporheic zone and that as much as 20% of the N[O.sup.-.sub.3] that entered the microbial mat and hyporheic zone was transformed to N[O.sup.-.sub.2] by dissimilatory reduction. Because of the rapid hydrologic exchange in microbial mats, it is likely that denitrification is limited either by biotic assimilation, reductase limitation, or transport limitation (reduced N[O.sup.- .sub.2] is transported away from reducing microbes).

We investigated the effect of benthic substratum type (sand and rocks) and nutrient supply (N and P) on biofilm structure and heterotrophic metabolism in a field experiment in a forested Mediterranean stream (Fuirosos). Rock and sand colonization and biofilm formation was intensively studied for 44 d at two stream reaches: control and experimental (continuous addition of phosphate, ammonia, and nitrate). Structural (C, N, and polysaccharide content and bacterial and chlorophyll density) and metabolic biofilm parameters ([beta]-glucosidase, peptidase, and phosphatase enzyme activities) were analyzed throughout the colonization process. The epilithic biofilm (grown on rocks) had a higher peptidase activity at the impacted reach, together with a higher algal and bacterial biomass. The positive relationship between the peptidase activity per cell and the N content of the epilithic biofilm suggested that heterotrophic utilization of proteinaceous compounds from within the biofilm was occurring. In contrast, nutrient addition caused the epipsammic biofilm (grown on sand) to exhibit lower [beta]-glucosidase and phosphatase activities, without a significant increase in bacterial and algal biomass. The differential response to nutrient addition was related to different structural characteristics within each biofilm. The epipsammic biofilm had a constant and high C : N ratio (22.7) throughout the colonization. The epilithic biofilm had a higher C : N ratio at the beginning of the colonization (43.2) and evolved toward a more complex structure (high polysaccharide content and low C : N ratio) during later stages. The epipsammic biofilm was a site for the accumulation and degradation of organic matter: polysaccharides and organic phosphorus compounds had higher degradation activities.

We used radiocarbon ([[DELTA].sup.14]C) and stable isotopic ([[delta].sup.13]C, [[delta].sup.15]N) signatures of bacterial nucleic acids to estimate the sources and ages of organic matter (OM) assimilated by bacteria in the Hudson River and York River estuary. Dualisotope plots of [[DELTA].sup.14]C and [[delta].sup.13]C coupled with a three-source mixing model resolved the major OM sources supporting bacterial biomass production (BBP). However, overlap in the stable isotopic ([[delta].sup.13]C and [[delta].sup.15]N) values of potential source end members (i.e., terrestrial, freshwater phytoplankton, and marsh-derived) prohibited unequivocal source assignments for certain samples. In freshwater regions of the York, terrigenous material of relatively recent origin (i.e.. decadal in age) accounted for the majority of OM assimilated by bacteria (49-83%). Marsh and freshwater planktonic material made up the other major source of OM, with 5-33% and 6-25% assimilated, respectively. In the mesohaline York, BBP was supported primarily by estuarine phytoplankton-derived OM during spring and summer (53-87%) and by marsh-derived OM during fall (as much as 83%). Isotopic signatures from higher salinity regions of the York suggested that BBP there was fueled predominantly by either estuarine phytoplankton-derived OM (July and November) or by material advected in from the Chesapeake Bay proper (October). In contrast to the York, BBP in the Hudson River estuary was subsidized by a greater portion (up to ~25%) of old (~24,000 yr BP) allochthonous OM, which was presumably derived from soils. These findings collectively suggest that bacterial metabolism and degradation in rivers and estuaries may profoundly alter the mean composition and age of OM during transport within these systems and before its export to the coastal ocean.

We measured denitrification and total nitrate uptake rates in a small stream (East Fork of Walker Branch in eastern Tennessee) using a new field [sup.15]N tracer addition and modeling approach that quantifies these rates for entire stream reaches. The field experiment consisted of an 8-h addition of 99 atom% [K.sup.15]N[O.sub.3] and a conservative solute tracer. Two [sup.15]N tracer addition experiments were performed on consecutive days, the first under ambient N[O.sup.-.sub.3] concentrations (23 [micro]g N [L.sup.-1]) and the second with a N[O.sup.-.sub.3] addition of approximately 500 [micro]g N [L.sup.-1]. We fit first-order N[O.sup.-.sub.3] uptake and two-box denitrification models to the longitudinal measurements of tracer [.sup.15]N in dissolved N[O.sup.-.sub.3], [N.sub.2], and [N.sub.2]O in stream water to determine rates. Total N[O.sup.-.sub.3] uptake rates were 0.028 [m.sup.-1] (0.32 [micro]g N [m.sup.-2] [s.sup.-1]) and 0.01 [m.sup.-1] (1.6 [micro]g N [m.sup.-2] [s.sup.-1]) under ambient N[O.sup.-.sub.3] and with N[O.sup.-.sub.3] addition, respectively. Denitrification rates were 0.0046 [m.sup.-1] (uncertainty range of 0.002 to 0.008 [m.sup.-1]) and 9 x [10.sup.-5] [m.sup.-1] (uncertainty range of 3 X [10.sup.-5] to 21 x [10.sup.-5] [m.sup.-1]) under ambient N[O.sup.-.sub.3] and with N[O.sup.-.sub.3] addition, respectively. Denitrification resulted almost exclusively in [N.sub.2] production (>99%) and comprised about 16% ([+ or -] 10%) of total N[O.sup.-.sub.3] uptake rate under ambient N[O.sup.-.sub.3] concentrations and about 1% ([+ or -] 1%) of total N[O.sub.3] uptake rate with N[O.sup.-.sub.3] addition. Denitrification rate expressed on a mass flux basis was about 12 [micro]mol [m.sup.-2] [h.sup.-1] under ambient N[O.sup.-.sub.3] concentrations, a value within the range reported for other streams with low N[O.sup.-.sub.3] concentrations.

We conducted an in-stream tracer experiment with Br and [sup.15]N-enriched N[O.sup.-.sub.3] to determine the rates of denitrification and related processes in a gaining N[O.sup.-.sub.3]-rich stream in an agricultural watershed in the upper Mississippi basin in September 2001. We determined reach-averaged rates of N fluxes and reactions from isotopic analyses of N[O.sup.-.sub.3], N[O.sup.-.sub.2], [N.sub.2], and suspended particulate N in conjunction with other data in a 1.2-km reach by using a forward time-stepping numerical simulation that included groundwater discharge, denitrification, nitrification, assimilation, and air-water gas exchange with changing temperature. Denitrification was indicated by a systematic downstream increase in the [[delta].sup.15]N values of dissolved [N.sub.2]. The reach-averaged rate of denitrification of surface-water N[O.sup.-.sub.3] indicated by the isotope tracer was approximately 120 [+ or -] 20 [micro]mol [m.sup.-2] [h.sup.-1] (corresponding to zero- and first-order rate constants of 0.63 [micro]mol [L.sup.-1] [h.sup.-1] and 0.009 [h.sup.-1], respectively). The overall rate of N[O.sup.-.sub.3] loss by processes other than denitrification (between 0 and about 200 [micro]mol [m.sup.-2] [h.sup.-1]) probably was less than the denitrification rate but had a large relative uncertainty because the N[O.sup.-.sub.3] load was large and was increasing through the reach. The rates of denitrification and other losses would have been sufficient to reduce the stream N[O.sup.-.sub.3] load substantially in the absence of N[O.sup.-.sub.3] sources, but the losses were more than offset by nitrification and groundwater N[O.sup.-.sub.3] inputs at a combined rate of about 500-700 [micro]mol [m.sup.-2] [h.sup.-1]. Despite the importance of denitrification, the overall mass fluxes of [N.sub.2] were dominated by discharge of denitrified groundwater and air-water gas exchange in response to changing temperature, whereas the flux of [N.sub.2] attributed to denitrification was relatively small. The in-stream isotope tracer experiment provided a sensitive direct reach-scale measurement of denitrification and related processes in a N[O.sup.-.sub.3]-rich stream where other mass-balance methods were not suitable because of insufficient sensitivity or offsetting sources and sinks. Despite the increasing N[O.sup.-.sub.3] load in the experimental reach, the isotope tracer data indicate that denitrification was a substantial permanent sink for N leaving this agricultural watershed during low-flow conditions.

Trace element signatures can be found in otoliths to give an indication of the natal river of juvenile American shad (Alosa sapidissima), based on studies done using samples from the Connecticut, Delaware and Hudson rivers. A permanent record of environmental conditions may be found in the fish otoliths, which reflect physical and chemical characteristics of the ambient water. This can be used to accurately determine a river of origin for fish.

We determined the concentrations and compositions of coarse particulate ([is greater than] 63 [Mu] m), fine particulate (0.1-63 [Mu] m), and dissolved (0.001-0.1 [Mu] m) organic matter collected along a river reach extending from a first-order stream in the Bolivian Andes, through the Beni River system, to the lower Madeira and Amazon Rivers. Dissolved organic carbon (DOC) concentrations increased down the total reach from ~80 to 350 [Mu] M. The percentage of total DOC with a molecular weight greater than ~1,000 atomic mass units that could be isolated by ultrafiltration also increased downstream from 40 to 80%. Weight percentages of organic carbon in the ultrafiltered isolates also grew downstream from 5% at the uppermost station to 37% in the Amazon mainstem. Organic carbon composed only 0.4-1.2 weight percentage of the total mass of the fine particulate fraction, which accounted for 70-80% of the total organic carbon (TOC) in transport through the highly turbid (~600-2000 mg [L.sup.-1]) Beni sequence. Observed compositional differences were related primarily to the size fractions in which the organic matter occurred. On average, coarse particulate organic material exhibited an atomic C:N of 24, whereas ultrafiltered DOM was nitrogen poor, (C:N)a = 34, and fine particulate material was nitrogen rich, (C:N)a = 15. The lignin and stable-carbon isotopic compositions of these fractions indicate tree leaves and other nonwoody tissues from C3 land plants as predominant sources. Three molecular parameters demonstrate that the coarse, fine, and dissolved fractions of individual water samples are increasingly degraded downstream. Elemental nitrogen, amino acids, and basic amino acids are all preferentially associated with fine minerals. Observed geographical patterns included more positive [[Delta].sup.13]C values in particulate organic matter from high altitude sites and an increase in the abundance and degradation of ultrafiltered dissolved organic matter down the drainage system. Many of these compositional patterns are imprinted within materials carried by low-order, high-altitude tributaries and appear to reflect processes occurring on the landscape.

We added [sup.15][NH.sub.4]Cl over 6 weeks to Upper Ball Creek, a second-order deciduous forest stream in the Appalachian Mountains, to follow the uptake, spiraling, and fate of nitrogen in a stream food web during autumn. A priori predictions of N flow and retention were made using a simple food web mass balance model. Values of [[Delta].sup.15]N were determined for stream water ammonium, nitrate, dissolved organic nitrogen, and various compartments of the food web over time and distance and then compared to model predictions. Ammonium uptake lengths were shortest at the beginning of the tracer addition (28 m) and increased through time (day 20 = 82 m, day 41 = 94 m), and ammonium residence time in stream water ranged from 4 min on day 0 to 15 min on day 41. Whole-stream ammonium uptake rates, determined from the decline in [sup.15][NH.sub.4] in water over the stream reach, decreased from 191 mg N [m.sup.-2] [d.sup.-1] on day 0 to 83.2 mg N [m.sup.-2] [d.sup.-2] on day 41. Temporal trends in the [NH.sub.4] mass transfer coefficient ([v.sub.f]) were similar to uptake rates; [v.sub.f] was highest on day 0 (7.4 X [10.sup.-4] m [s.sup.-1]) and lower on days 20 and 41 (2.7 and 2.8 X [10.sup.-4] m [s.sup.-1], respectively). Rates of nitrification were estimated to be very low throughout the tracer addition and accounted for [is less than]3% of [sup.15][NH.sub.4] uptake on day 0. It appears that most of the N in epilithon was actively cycling based on comparisons of [sup.15]N in stream water and biomass at the end of the experiment. In contrast, for allochthonous organic matter, we found that microbial [sup.15]N represented 69% of the label in wood, 20% in leaves, and 31% in fine benthic organic matter (FBOM). Despite higher [[Delta].sup.15]N values in primary producers, [sup.15][NH.sub.4] uptake rates per unit stream bottom area were generally lower in epilithon compared to the detrital compartments, a result of the lower biomass of epilithon. Turnover times were similar for epilithon (47 d), leaves (38 d), and FBOM (53 D) based on the decline in [sup.15]N tracer over the first 28 d after the addition stopped. Incorporation of [sup.15]N varied among biomass compartments involved in ammonium uptake from water. Primary producers were more highly labeled than allochthonous organic matter. Epilithon [[Delta].sup.15]N values were higher than leaves or FBOM, appeared to reach isotopic equilibrium by day 42, and followed model-predicted trends. The grazing mayfly Stenonema was more highly labeled than the epilithon, which suggests selective feeding or assimilation of the more highly labeled algal-bacterial portion of the epilithon. Leaves had very low [[Delta].sup.15]N values, and [[Delta].sup.15]N values for the shredding stonefly Tallaperla were close to model predictions and followed labeling in leaves. Total retention of [[Delta].sup.15]N at the end of the experiment by the nine largest biomass compartments within the study reach accounted for only 12.3% of added [sup.15]N, with leaves and FBOM representing the largest portions. Export of [sup.15.]N by suspended particulate and dissolved N accounted for an additional 11% and 30% of added [sup.15]N, respectively. Results from [sup.15]N-tracer addition in Upper Ball Creek demonstrate the high ammonium demand associated with microbes colonizing leaf detritus and the resultant linkage to invertebrate shredders. In Upper Ball Creek in autumn, spiraling of [NH.sub.4] is very tight, [NH.sub.4] residence time in water is short, and uptake rates are very high. Analyses of N spiraling in unimpacted streams provide an ecological foundation for assessment of spiraling in high-N streams.