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The relative influence of seasonal patterns in hydrological flow and seasonal differences in biological and geochemical activity on stream chemistry patterns is difficult to discern because they covary; temperate systems are characterized by lower mean flow in the summer (i.e. corresponding to deeper flow paths, elevated temperature, and biological activity), and higher mean flow in the winter (i.e. corresponding to shallower flow paths, depressed temperature, and biological dormancy). Using 2018 data, when seasonal stream flow conditions reversed, and two prior conventional water years, the relationship between monthly acid-relevant analyte concentrations and streamflow were compared within and between winter and summer to provide insight into controls on characteristic seasonal chemistry patterns at two mid-Appalachian sites with distinct geology (weatherable mafic and weather resistant siliciclastic). Acid neutralizing capacity (ANC) increased (1) with lower flow, in both seasons and (2) in summer, for all flow conditions. The compounding impacts resulted in a doubling of concentration from typical winter with high flow to summer with low flow at both sites. Base cation patterns tracked ANC at the mafic site, resulting in an ~ 60% increase of from winter with high flow to summer with low flow; distinctions between summer and winter contributed more to the seasonal pattern (72%) than changes in flow. Sulfate increased at the mafic site (1) with higher flow, in both seasons and (2) in winter, for all flow conditions, resulting in an ~ 50% increase from summer with low flow to winter with high flow; distinctions between winter and summer conditions and flow contributed similarly (40–60%) to the typical seasonal chemical pattern. The biogeochemical mechanism driving differences in stream chemistry between summer and winter for the same flow conditions is likely increased rates of natural acidification from elevated soil respiration in summer, resulting in greater bedrock weathering and sulfate adsorption. Findings highlight the significance and consistency of growing vs dormant season variations in temperature and biological activity in driving intra-annual patterns of stream solutes. This data set informs parameterization of hydro-biogeochemical models of stream chemistry in a changing climate at a biologically relevant, seasonal, timescale. [ABSTRACT FROM AUTHOR]

Increased occurrence, size, and intensity of fire result in significant but variable changes to hydrology and material retention in watersheds with concomitant effects on stream biogeochemistry. In arid regions, seasonal and episodic precipitation results in intermittency in flows connecting watersheds to recipient streams that can delay the effects of fire on stream chemistry. We investigated how the spatial extent of fire within watersheds interacts with variability in amount and timing of precipitation to influence stream chemistry of three forested, montane watersheds in a monsoonal climate and four coastal, chaparral watersheds in a Mediterranean climate. We applied state-space models to estimate effects of precipitation, fire, and their interaction on stream chemistry up to five years following fire using 15 + years of monthly observations. Precipitation alone diluted specific conductance and flushed nitrate and phosphate to Mediterranean streams. Fire had positive and negative effects on specific conductance in both climates, whereas ammonium and nitrate concentrations increased following fire in Mediterranean streams. Fire and precipitation had positive interactive effects on specific conductance in monsoonal streams and on ammonium in Mediterranean streams. In most cases, the effects of fire and its interaction with precipitation persisted or were lagged 2–5 years. These results suggest that precipitation influences the timing and intensity of the effects of fire on stream solute dynamics in aridland watersheds, but these responses vary by climate, solute, and watershed characteristics. Time series models were applied to data from long-term monitoring that included observations before and after fire, yielding estimated effects of fire on aridland stream chemistry. This statistical approach captured effects of local-scale temporal variation, including delayed responses to fire, and may be used to reduce uncertainty in predicted responses of water quality under changing fire and precipitation regimes of arid lands. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a widespread contaminant known to pose severe risks to wildlife and human health. While Hg emissions have declined in recent decades, legacy emissions and stored Hg will continue to impact watershed Hg cycling for the foreseeable future. Boreal forests are a major concern due to their capacity for storing Hg, vulnerability to disturbance, and record of high Hg concentrations in fish. Thus, there is a need to better quantify factors that influence Hg export from boreal forest catchments to inform watershed management decisions regarding Hg. Streamflow measurements, as well as approximately bi-weekly sampling for total mercury (THg), methylmercury (MeHg), and supporting stream chemistry were completed in 19 headwater streams near Dryden, Ontario during the ice-free season of 2019. The results were related to landscape and hydrological indices to elucidate the potential factors governing THg and MeHg export across these catchments. This study shows that while Hg concentrations are relatively low (0.50–20.46 ng l−1 THg; < 0.04–1.21 ng l−1 MeHg) across boreal streams in south central Canada, there are significant differences in Hg export. Catchments within boreal shield landscapes dominated by shallow soils and exposed bedrock export more methylmercury than catchments within glaciolacustrine plain landscapes dominated by thicker sand deposits. Coniferous forest cover is more significant than dissolved organic matter concentrations and more reliable than available % wetland cover data, two metrics commonly included in Hg transport models, for predicting THg and MeHg loads. In the absence of substantial mapped wetland cover, wet forest cover, as defined by the proportion of catchment cover by tree species favoring wet conditions, is shown to be an effective alternative metric. Considering the generally detailed and extensive data on tree species coverage available in Canada's managed forests, wet forest cover, in addition to coniferous forest cover, could be useful for modelling Hg transport in boreal forest watersheds. [ABSTRACT FROM AUTHOR]

The biogeochemical model, PnET-BGC, was modified and parameterized using field data from an experimental whole-tree harvest of watershed (W5) in 1983–1984 at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. The model simulated the hydrology, biomass accumulation, and soil solution and stream water chemistry responses to forest cutting. The parameterized model was then applied to other experimentally cut watersheds at the HBEF; including a devegetation experiment (W2; devegetation and herbicide treatment) and a commercial strip-cut (W4) to evaluate the ability of the model to depict ecosystem responses to a range of cutting regimes. Revisions of algorithms of PnET-BGC improved model performance in predicting short- and long-term dynamics of major elements following various approaches to forest cutting. Despite some initial differences in species composition and biomass accumulation rates among the cut watersheds, simulations of total forest biomass for all three treated watersheds (W2, W4 and W5) were consistent with expectations based on the growth trajectory of a second-growth, reference watershed (W6) at the HBEF. The modified two-soil-layer PnET-BGC captured the immediate increase in stream concentrations of NO3−, Ca2+, Mg2+ and Na+ as well as enhanced adsorption of SO42− following cuttings and indicated a greater response for the devegetation and the whole-tree harvest treatments than the sequential strip-cut of W4. Simulations indicated intense NO3− leaching with the devegetation and herbicide treatment and consequent accelerated decline in soil base saturation and a slower recovery pattern during forest regrowth by the end of the simulation period (2100) compared to the other treatments. [ABSTRACT FROM AUTHOR]

The restoration of drained afforested peatlands, through drain blocking and tree removal, is increasing in response to peatland restoration targets and policy incentives. In the short term, these intensive restoration operations may affect receiving watercourses and the biota that depend upon them. This study assessed the immediate effect of 'forest-to-bog' restoration by measuring stream and river water quality for a 15 month period pre- and post-restoration, in the Flow Country peatlands of northern Scotland. We found that the chemistry of streams draining restoration areas differed from that of control streams following restoration, with phosphate concentrations significantly higher (1.7–6.2 fold, mean 4.4) in restoration streams compared to the pre-restoration period. This led to a decrease in the pass rate (from 100 to 75%) for the target "good" quality threshold (based on EU Water Framework Directive guidelines) in rivers in this immediate post-restoration period, when compared to unaffected river baseline sites (which fell from 100 to 90% post-restoration). While overall increases in turbidity, dissolved organic carbon, iron, potassium and manganese were not significant post-restoration, they exhibited an exaggerated seasonal cycle, peaking in summer months in restoration streams. We attribute these relatively limited, minor short-term impacts to the fact that relatively small percentages of the catchment area (3–23%), in our study catchments were felled, and that drain blocking and silt traps, put in place as part of restoration management, were likely effective in mitigating negative effects. Looking ahead, we suggest that future research should investigate longer term water quality effects and compare different ways of potentially controlling nutrient release. [ABSTRACT FROM AUTHOR]

Improvements in air quality have led to ecosystem recovery from acidic deposition, but the mechanisms and trajectories of this recovery are not fully understood. Here, we present long-term stream response and recovery data for paired watersheds at the Bear Brook Watershed in Maine (BBWM) during declining ambient SO4 and NO3 in precipitation. East Bear (EB) received ambient deposition from 1989 to 2018; West Bear (WB) received artificially elevated N + S from 1989 to 2016. The WB treatment was discontinued after 2016, the beginning of the recovery from both the experimental N + S and ambient decline. Stream SO4 in WB gradually declined after the treatment ended, from ~147 μeq L−1 in 2010–16 to ~126 μeq L−1 in 2017–18. The declining S inputs induced desorption of SO4 from soil phase surfaces, with stream loss far exceeding precipitation input. At the current rate of recovery, it will be many decades before the WB stream returns to pre-treatment SO4 concentrations. In contrast, NO3 is only weakly adsorbed in soil, and WB stream NO3 concentrations rapidly declined from ~39 μeq L−1 in 2010–16 to ~5 μeq L−1 in 2017–18, comparable to the N-limited EB stream. The acid anions are strongly coupled to base cation chemistry in streams, and there was a distinct hysteretic response of Ca and Mg to the chronic acidification, as (Ca + Mg) increased rapidly during the initial years, followed by declining values due to depletion of the soil exchange complex. This 30-year monitoring study (1989–2019) provides insights into recovery mechanisms from acidic deposition and highlights the role of abiotic processes in soil that mediate nutrient cycling and retention. Documenting the rapid response of N alongside the slower recovery for S identifies the temporal resolution necessary for other whole-watershed recovery studies. [ABSTRACT FROM AUTHOR]

Nitrate retention is an important ecosystem service provided by streams which reduces nitrogen loads to downstream ecosystems. However, most inferences about nitrate retention and uptake in streams are based on measurements at sub-annual scales. Our objective was to evaluate variation in nitration retention and solute concentrations at weekly to decadal time scales. We measured nitrate and chloride concentration, nitrate flux and nitrate retention in Emmons Creek, a groundwater-dominated, nitrate-rich stream in central Wisconsin during a 10-year period. We used a two-station mass balance approach to measure nitrate retention while accounting for groundwater inputs. Surface water nitrate concentration, nitrate yield and nitrate retention exhibited strong seasonal variation. Seasonal Kendall tests revealed positive trends for surface water discharge, nitrate and chloride concentrations and groundwater chloride concentration, but not nitrate retention. Generalized additive mixed models (GAMM), accounting for serial autocorrelations, also indicated positive trends in surface water nitrate and chloride concentrations but no trend for nitrate retention. Nitrate retention averaged 370 mg NO3-N m−2 d−1 (± 316 SD) and was highest during the spring (512 mg NO3-N m−2 d−1 ± 319), followed by summer (460 ± 322), fall (340 ± 281) and winter (165 ± 212). Nitrate retention rates in Emmons Creek were relatively high compared to most published estimates. However, our results suggest that Emmons Cr. may have saturated its capacity to retain nitrate during a period when nitrate yield from its watershed increased, which is an important finding given the increases in N availability in many regions of earth. [ABSTRACT FROM AUTHOR]

A national river water quality database of total reactive phosphorus (TRP) and total phosphorus (TP) and flow was used, together with catchment characteristic datasets (soils, land use and hydroclimatic properties), to derive national fluvial phosphorus (P) flux estimates for Great Britain (England, Wales and Scotland) from 1974 to 2012. These fluvial P fluxes were compared with P imports and exports, in fertilizer, food, feedstuffs, and industrial products, along with coastal direct discharge of wastes, at the British national boundary from 1990 to 2012. The results showed that: (i) Average annual river TP concentrations in Great Britain have declined from a peak of 0.27-0.1 mg P/l and annual river TP flux has declined from 120 to 16 ktonnes P/year (0.49-0.06 tonnes P/km/year); (ii) Average river TRP concentration has declined from a peak of 0.19-0.05 mg P/l and annual river TRP flux has declined from 71 to 10 ktonnes P/year (0.29-0.05 tonnes P/km); (iii) Over the period 2003-2012, even after the introduction of the Urban Waste Water Directive, 60 % of UK's TP flux was still from urban areas; and (iv) In 1990, the fluvial flux of TP from the UK was equivalent to 41 % of imports; by 2012 this had decreased to 15 %. The UK (relative to its boundary) continues to accumulate P and, over the last 15 years, this accumulation has increased at an average rate of 0.6 ktonnes P/year. Enhanced removal of P in waste water treatment has shifted the environmental pathway of sewage P from discharge to rivers to accumulation in sewage sludge, which is largely disposed of on agricultural land, and which could eventually provide a sustained legacy source of P to rivers for decades. However, a substantial proportion of P accumulation is via food waste into landfills. [ABSTRACT FROM AUTHOR]

The absolute concentrations of nitrogen (N) and phosphorus (P) and their relative availabilities (N:P stoichiometry) can influence numerous ecological processes. In streams, N:P stoichiometry is influenced by different hydrologic and biogeochemical processes that also affect the downstream transport of these nutrients to receiving waters. Calcium carbonate (CaCO) deposition, a widespread geochemical process in alkaline streams and other aquatic ecosystems, can lower phosphate concentrations and, potentially, decrease P availability relative to N availability. We evaluated the effects of CaCO deposition on stream water stoichiometry using a 3-year dataset of stream physicochemistry and several metrics of CaCO deposition across three streams in the Huachuca Mountains of southern Arizona, USA. CaCO deposition rates varied across and within streams, with benthic coverage of travertine as high as 70 % and deposition rates up to 8.3 μg Ca L m. Redundancy analysis revealed a strong, negative correlation between stream water phosphate concentrations and CaCO deposition rates, a relationship that also extended to total P concentrations, and a strong, positive correlation between inorganic N concentrations and CaCO deposition rates. Furthermore, we found a significant positive relationship between CaCO deposition rates and N:P ratios. These results support the role of coprecipitation of phosphate with CaCO deposition in reducing P supply. They also suggest that reduced concentrations of P in the water column may reduce biological N uptake, amplifying the stoichiometric signal of CaCO deposition. [ABSTRACT FROM AUTHOR]

Streams are typically supersaturated with carbon dioxide (CO) thus playing a pivotal role in the global carbon cycle. To further our knowledge on stream CO dynamics, we tested the linkages between dissolved CO and dissolved organic matter quality during two time periods (spring and summer). We sampled 48 streams in Ontario, Canada that represent a range of catchment cropland cover (2-80 %), nutrients, dissolved organic carbon concentration (DOC), and dissolved organic matter (DOM) quality. For both sampling periods, DOM composition (quality) was found to be an important predictor of CO concentration in streams, more so than DOC concentration. In general, the more complex terrestrially-derived DOM was associated with elevated CO levels whereas microbial-like DOM had a poor relationship with CO. The DOM characteristics associated with increased CO levels were related to wetlands rather than croplands. Nutrients (nitrogen and phosphorus) were also important predictors of CO, however this relationship was not linear. Since the streams in the sampled network were of mixed land use, it appears that CO levels were affected by both land use types in a composite non-linear way. Overall, our findings suggest that DOM quality is a better predictor of stream CO, however the relationship between DOM and CO is complicated by the presence of breakpoints in the data associated with non-linearity. [ABSTRACT FROM AUTHOR]

The influence of built urban infrastructure on stream chemistry was quantified throughout the drainage network of the tropical Río Piedras watershed, San Juan metropolitan area, Puerto Rico. Urbanization and failing domestic wastewater infrastructure appeared to drive changes in surface water chemistry throughout the watershed. Mean baseflow concentrations of chloride (Cl), ammonium (NH), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and phosphate (PO) all increased with urban infrastructure, while nitrate (NO) and dissolved oxygen (DO) decreased. These patterns in stream chemistry suggest that sewage effluent from failing or illegally connected sewer pipes has a major impact on surface water quality. Concentrations of Cl, DO, and NH in stream water were most strongly related to sewer pipe volume, demonstrating the tight connection between urban infrastructure and stream chemistry. The loading and transformation of NO and NH were modeled along the river network and NH loading rates from the landscape were strongly related to urban infrastructure, whereas NO loading rates showed only weak relationships, highlighting the importance for incorporating NH dynamics into river network models in urban environments. Water quality appears to be severely impacted by sewage in this tropical basin, despite large investments in built infrastructure. The high temperatures in the Río Piedras exacerbate water quality problems by reducing saturation DO levels in streams, and intense rainstorms tax the ability of built infrastructure to adequately manage overland flows. These problems are likely typical of much of the urbanized humid tropics. [ABSTRACT FROM AUTHOR]

Rain-on-snow (ROS) events are major drivers of nitrate (NO-N) export from seasonally snow-covered forested catchments and may cause episodic declines in stream pH. High intensity monitoring of throughfall, snow pack and stream water draining two proximal catchments (Harp 3A and Harp 6A) with very different NO-N export revealed that a very small percentage of ROS-induced stream discharge originates from throughfall and melting snow (new water; average = 6.4 %). However, this new water has a very high concentration of NO-N (throughfall/snowmelt average = 498 μg/L) compared with baseflow (average = 7.3 μg/L in Harp 6A; average = 41 μg/L in Harp 3A) and as a result, throughfall and snowmelt contribute the majority of NO-N export (average = 62 %) during ROS events. In contrast, concentrations of sulphate, dissolved organic carbon and calcium in rain and snowpack are similar to baseflow and therefore ROS-induced declines in pH (often to below pH 6.0) are attributed entirely to increases in NO-N concentration. Differences in absolute magnitude of ROS NO-N export between catchments are explained through differences in baseflow NO-N concentrations. The frequency and magnitude of ROS events in this region are affected by both NO-N deposition and winter temperature, and thus the impact of these events in the future depends on changes in both atmospheric deposition and winter climate. [ABSTRACT FROM AUTHOR]

Seasonal patterns of dissolved organic matter (DOM) were evaluated for multiple watershed sources and stream water during baseflow and stormflow to investigate the influence of hydrologic flow paths and key phenological events. Watershed sources sampled were throughfall, litter leachate, soil water, and deep groundwater. DOM data for a 4-year period (2008-2011) included: DOC concentrations and spectrofluorometric indices such as a, humification index, protein-like and humic-like DOM. Seasons were defined as-winter (December-February), spring (March-May), summer (June-September) and autumn (October and November). Seasonal differences in DOM were most pronounced for surficial flow paths (e.g., stormflow, litter leachate, throughfall and soil water) but muted or absent for groundwater and baseflow. This was attributed to the loss of DOM by sorption on mineral soil surfaces and/or microbial breakdown. DOM in summer stormflow had higher DOC concentrations and was more humic in character versus DOM in spring and winter runoff. Storm events in early autumn produced a sharp increase in DOC concentrations and % protein-like DOM for stream waters and litter leachate. Elevated DOC concentrations for early spring throughfall were attributed to leaching of organic exudates associated with leaf emergence. Our results underscore that watershed and ecosystem studies need to pay a greater attention to surficial flow paths and runoff sources (including stormflow) for understanding seasonal patterns of DOM. Understanding the influence of phenological episodes such as autumn leaf-fall for DOM is important considering that these transitional events may be especially affected by climate change. [ABSTRACT FROM AUTHOR]

Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59-71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, δC increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, $$ P_{{{\text{CO}}_{2} }} $$ increased up to 12 times atmospheric values leading to substantial degassing. [ABSTRACT FROM AUTHOR]

Elemental ratios of calcium (Ca), strontium (Sr) and barium (Ba) in plant tissue have been used to help identify sources of Ca in trees. However, recent work has clearly shown that ratios of Ca:Sr or Ca:Ba in plant tissues differ from soil values and discrimination factors (alteration of elemental ratios of Ca, Sr and Ba) have been established to account for differences between soil and foliar values. In this study, concentrations of Ca, Sr and Ba (and molar ratios) in bulk deposition, throughfall, soil water and stream water were measured along with tree chemistry and biomass (foliage, branches, bark, wood, roots and litter) at a forested catchment in central Ontario, Canada. Concentrations of Ca, Sr and Ba vary greatly among the eight dominant tree species and plant tissue and molar ratios of Ca:Sr or Ca:Ba vary more than tenfold. In general Ca:Sr ratios were highest in foliage (up to 3,175) and lowest in wood (low as 91) and the mass-weighted Ca:Sr ratio of the total tree biomass was approximately 340. The molar ratio of Ca:Sr in bulk precipitation was ~820, increased to over 1,000 in throughfall and then decreased steadily through the various soil horizons, falling to 320 in the B horizon. Conservative mass balances for the forest floor for Ca, Sr and Ba suggest that more than half the Ca, Sr and Ba annual uptake by trees is obtained from mineral soil, which is consistent with the observation that the weighted-average ratios of Ca:Sr, Ca:Ba or Ba:Sr in the forest biomass fall between values in water draining the forest floor and water draining the rooting zone (B horizon). This work shows that discrimination between Ca, Sr and Ba is primarily internal to trees and discrimination factors between soil and foliage should be interpreted with caution. [ABSTRACT FROM AUTHOR]

Humid tropical forests play a dominant role in many global biogeochemical cycles, yet long-term records of tropical stream chemistry and its response to disturbance events such as severe storms and droughts are rare. Here we document the long-term variability in chemistry of two streams in the Luquillo Mountains, Puerto Rico over a period of 27 years. Our two focal study watersheds, the Río Icacos and Quebrada Sonadora, both drain several hundred hectares of tropical wet forests, and each received direct hits from Hurricanes Hugo (1989) and Georges (1998). They differ primarily in lithology (granitic vs. volcaniclastic) and elevation. Changes in major cations, anions, silica, and dissolved organic carbon were minimal over the study period, but the concentrations of nitrate show a strong response to hurricane disturbance and the longest time to recovery. Potassium also showed a large, although less consistent, response to disturbance. In the granitic terrain, nitrate concentrations exceeded long-term pre-hurricane background levels for over a decade, but were elevated in the volcaniclastic terrain for only 1–2 years. Lithology appears to be the primary driver explaining the different response trajectories of the two watersheds. In the granitic terrain, which showed slow recovery to pre-hurricane conditions, the quartz diorite bedrock weathers to produce coarser soils, deeper groundwater flow paths, and riparian zones with sharp spatial variation in redox conditions and very high nitrogen levels immediately adjacent to the stream. Groundwater flow paths are shallow and the levels of N in streamside groundwater are much lower in the volcaniclastic terrain. The recovery of vegetation following hurricane disturbance appears similar in the two watersheds, suggesting that the extent of structural damage to canopy trees determines the magnitude of NO 3 increases, but that the duration of elevated concentrations in stream water is a function of lithology. [ABSTRACT FROM AUTHOR]

Stream chemistry in permafrost regions is regulated by a variety of drivers that affect hydrologic flowpaths and watershed carbon and nutrient dynamics. Here we examine the extent to which seasonal dynamics of soil active layer thickness and wildfires regulate solute concentration in streams of the continuous permafrost region of the Central Siberian Plateau. Samples were collected from 2006 to 2012 during the frost-free season (May–September) from sixteen watersheds with fire histories ranging from 3 to 120 years. The influence of permafrost was evident through significantly higher dissolved organic carbon (DOC) concentrations in the spring, when only the organic soil horizon was accessible to runoff. As the active layer deepened through the growing season, water was routed deeper through the underlying mineral horizon where DOC underwent adsorption and concentrations decreased. In contrast, mean concentrations of major cations (Ca 2+ > Na + > Mg 2+ > K +) were significantly higher in the summer, when contact with mineral horizons in the active zone provided a source of cations. Wildfire caused significantly lower concentrations of DOC in more recently burned watersheds, due to removal of a source of DOC through combustion of the organic layer. An opposite trend was observed for dissolved inorganic carbon and major cations in more recently burned watersheds. There was also indication of talik presence in three of the larger watersheds evidenced by Cl − concentrations that were ten times higher than those of other watersheds. Because climate change affects both fire recurrence intervals as well as rates of permafrost degradation, delineating their combined effects on solute concentration allows forecasting of the evolution of biogeochemical cycles in this region in the future. [ABSTRACT FROM AUTHOR]

The influence of biogeochemical processes on nitrogen and organic matter transformation and transport was investigated for two urban streams receiving groundwater discharge during the dry summer baseflow period. A multiple lines of evidence approach involving catchment-, and stream reach-scale investigations were undertaken to describe the factors that influence pore water biogeochemical processes. At the catchment-scale gaining stream reaches were identified from water table mapping and groundwater discharge estimated to be between 0.1 and 0.8 m m d from baseflow analysis. Sediment temperature profiles also suggested that the high groundwater discharge limited stream water infiltration into the sediments. At the stream reach-scale, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations were higher in stream water than in groundwater. However, DOC and DON concentrations were greatest in sediment pore water. This suggests that biodegradation of sediment organic matter contributes dissolved organic matter (DOM) to the streams along with that delivered with groundwater flow. Pore water ammonium (NH) was closely associated with areas of high pore water DOM concentrations and evidence of sulfate (SO) reduction (low concentration and SO:Cl ratio). This indicates that anoxic DOM mineralization was occurring associated with SO reduction. However the distribution of anoxic mineralization was limited to the center of the streambed, and was not constrained by the distribution of sediment organic matter which was higher along the banks. Lower sediment temperatures measured along the banks compared to the center suggests, at least qualitatively, that groundwater discharge is higher along the banks. Based on this evidence anoxic mineralization is influenced by groundwater residence time, and is only measurable along the center of the stream where groundwater flux rates are lower. This study therefore shows that the distribution of biogeochemical processes in stream sediments, such as anoxic mineralization, is strongly influenced by both the biogeochemical conditions and pore water residence time. [ABSTRACT FROM AUTHOR]

The Okavango River, in semi-arid northwestern Botswana, flows for over 400 km in a pristine wetland developed on a large (>22,000 km) alluvial fan (Okavango Delta). An annual flood pulse inundates the floodplains of the wetlands and travels across the Delta in 4-6 months. In this study, we assess the effects of long hydraulic residence time, variable hydrologic interaction between river-floodplain-wetland and evapotranspiration on carbon cycling. We measured dissolved inorganic carbon (DIC) concentrations and stable carbon isotopes of DIC (δC) from river water when the Delta was not flooded (low water) and during flooding (high water). During low water, the average DIC concentration was 31 % higher and the δC 2.1 ‰ more enriched compared to high water. In the lower Delta with seasonally flooded wetlands, the average DIC concentration increased by 70 % during low water and by 331 % during high water compared to the Panhandle with permanently flooded wetlands. The increasing DIC concentration downriver is mostly due to evapoconcentration from transpiration and evaporation with increased transit time. The average δC between low and high water decreased by 3.7 ‰ in the permanently flooded reaches compared to an increase of 1.6 ‰ in the seasonally flooded reaches. The lower δC during high water in the permanently flooded reaches suggest that DIC influx from the floodplain-wetland affects river's DIC cycling. In contrast, higher river channel elevations relative to the wetlands along seasonal flooded reaches limit hydrologic interaction and DIC cycling occurs mostly by water column processes and river-atmospheric exchange. We conclude that river-wetlands interaction and evapoconcentration are important factors controlling carbon cycling in the Okavango Delta. [ABSTRACT FROM AUTHOR]

Peatland headwater streams are consistently supersaturated with respect to gaseous C and are known to degas CO and CH directly to the atmosphere. Using a combination of injection of a purposeful gas tracer (propane) and a soluble tracer (NaCl) we carried out 49 measurements of the gas transfer coefficient on 12 representative stream reaches to quantify the gas transfer rates of CO and CH in headwater (1st-3rd order) streams draining six UK peatlands. These were compared to measured stream reach physical variables, such as discharge and water travel time. Whilst we found that evasion rates were highly variable in space and time, $$ {\text{K}}_{{{\text{CO}}_{2} }} $$ (gas transfer coefficient of CO) was positively related to discharge. Individual study sites showed a high degree of variability in gas transfer rates; at all 49 sites median/mean values for $$ {\text{K}}_{{{\text{CO}}_{2} }} $$ were 0.087/0.157 and $$ {\text{K}}_{{{\text{CH}}_{4} }} $$ 0.092/0.176 min. Median/mean instantaneous CO and CH evasion rates were 133/367 and 0.22/1.45 μg C m s, respectively. Methane evasion rates were therefore more than two orders of magnitude lower than CO, with CH invasion (rather than evasion) measured on 37 % of occasions. Our gas flux measurements from peatland headwater streams are higher than values previously used to estimate landscape scale fluxes and emphasise the importance of the evasion flux term in the overall carbon balance. [ABSTRACT FROM AUTHOR]

Inland freshwaters transform and retain up to half of the carbon that enters from the terrestrial environment and have recently been recognized as important components of regional and global carbon budgets. However, the importance of small streams to these carbon budgets is not well understood due to the lack of globally-distributed data, especially from streams draining agricultural landscapes. We quantified organic carbon pools and heterotrophic metabolism seasonally in 6 low-order streams draining row-crop fields in northwestern Indiana, USA, and used these data to examine patterns in organic carbon spiraling lengths (S; km), downstream velocities (V; m/d), and turnover rates (K; day). There were seasonal differences in S, with the longest spiraling lengths in winter (range: 7.7-54.4 km) and the shortest in early and late summer (range: 0.2-9.0 km). This seasonal pattern in S was primarily driven by differences in discharge, suggesting that hydrology tightly controls the fate of organic carbon in these streams. K did not differ seasonally, and variability (range: 0.0007-0.0193 day) was controlled by differences in stream water soluble reactive phosphorus concentrations. Compared to previous studies conducted primarily in forested streams, agricultural streams tended to be less retentive of organic carbon. These systems function predominantly as conduits transporting organic carbon to downstream ecosystems, except during low, stable-flow periods (i.e., late summer) when agricultural streams can be as retentive of organic carbon as forested headwaters. High organic carbon retention in the late summer has implications for coupled carbon and nitrogen cycling (i.e., denitrification), which may play an important role in removing nitrate from stream water during periods of low flow. [ABSTRACT FROM AUTHOR]

The seasonal pattern and primary mechanism of nitrogen (N) export by surface runoff from the Wuchuan subwatershed (WCW), an agricultural upper watershed (1.88 km) located in southeast China, were investigated based on extensive streamwater measurements in 2004-2005 under subtropical climatic conditions. The results disclosed a highly variable but strong linkage between hydrological and anthropogenic controls and N export. N export via surface runoff presented a significant seasonal pattern caused by changes in rainfall and watershed N input. Approximately 75% of the annual N export (67 kg ha) was flushed by those storm runoff mainly occurred during the wet season (March through September). The WCW dataset of N concentrations and loads during both baseflow and stormflow implied an interactive effects of anthropogenetic N input and hydrology conditions: N export was flush-driven in late spring, summer and autumn (wet season), but highly related with soil N in winter and early spring. Compared to undisturbed watersheds under similar rainfall conditions, WCW exported a considerable amount of N due to intensive fertilizer application (a mean of 690 kg N ha year, commonly as surface applications). This work provides a first characterization of a small agricultural Chinese catchment under subtropical climates and its associated N export behavior. [ABSTRACT FROM AUTHOR]

The nitrogen (N) budget calculation approach is a useful means of evaluating the impact of human activity on the N cycle. Field scale N budget calculations may ignore the interactions between landscapes, and regional scale calculations rely on statistical data and indirect parameters. Watershed scale budget calculations allow for a more direct quantification of N inputs and outputs. We conducted N budget calculations for a rice paddy-dominated agricultural watershed in eastern China for 2007-2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH) volatilization and household interviews about N-related agricultural activities. The results showed that although total N input to the watershed was up to 280 kg N ha year, riverine discharge was only 4.2 kg N ha year, accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha year after reservoir storage and/or denitrification removal. The low riverine N output was because of the characteristics of the rice paddy-dominated landscape, which intercepts run-off and enhances soil denitrification. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. Major N outputs included food/feed export, NH volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge. [ABSTRACT FROM AUTHOR]

Hood Canal, Washington, USA, is a poorly ventilated fjord-like sub-basin of Puget Sound that commonly experiences hypoxia. This study examined the influence of watershed soils, vegetation, physical features, and population density on nitrogen (N) export to Hood Canal from 43 tributaries. We also linked our watershed study to the estuary using a salinity mass balance model that calculated the relative magnitude of N loading to Hood Canal from watershed, direct precipitation, and marine sources. The overall flow-weighted total dissolved N (TDN) and particulate N input concentrations to Hood Canal were 152 and 49 μg l, respectively. Nitrate and dissolved organic N comprised 64 and 29% of TDN, respectively. The optimal regression models for TDN concentration and areal yield included a land cover term suggesting an effect of N-fixing red alder ( Alnus rubra) and a human population density term (suggesting onsite septic system (OSS) discharges). There was pronounced seasonality in stream water TDN concentrations, particularly for catchments with a high prevalence of red alder, with the lowest concentrations occurring in the summer and the highest occurring in November-December. Due to strong seasonality in TDN concentrations and in particular stream flow, over 60% of the TDN export from this watershed occurred during the 3 month period of November-January. Entrainment of marine water into the surface layer of Hood Canal accounted for ≈98% of N loading to the euphotic zone of this estuary, and in a worst case scenario OSS N inputs contribute ≈0.5% of total N loading. Domestic wastewater discharges and red alders appear to be a very important N source for many streams, but a minor nutrient source for the estuary as a whole. [ABSTRACT FROM AUTHOR]

Plant productivity in many tropical savannas is phosphorus limited. The biogeochemical cycling of P in these ecosystems, however, has not been well quantified. In the present study, we characterized P stocks and fluxes in a well-preserved small watershed in the Brazilian Cerrado. As the Cerrado is also a fire-dominated ecosystem, we measured the P stocks and fluxes in a cerrado stricto sensu plot with complete exclusion of fire for 26 years (unburned plot) and then tested some predictions about the impacts of fire impacts on P cycling in an experimental plot that was burned three times since 1992 (burned plot). The unburned area is an ecosystem with large soil stocks of total P (1,151 kg ha up to 50 cm depth), but the largest fraction is in an occluded form. Readily extractable P was found up to 3 m soil depth suggesting that deep soil is more important to the P cycle than has been recognized. The P stock in belowground biomass (0-800 cm) was 9.9 kg ha. Decomposition of fine litter released 0.97 kg P hayear. Fluxes of P through bulk atmospheric deposition, throughfall and litter leachate were very low (0.008, 0.006 and 0.028 kg hayear, respectively) as was stream export (0.001 kg hayear). Immobilization of P by microbes during the rainy season seems to be an important mechanism of P conservation in this ecosystem. Fire significantly increased P flux in litter leachate to 0.11 kg hayear, and added 1.2 kg ha of P in ash deposition after fire. We found an increase of P concentration in soil solution at 100 cm depth (from 0.03 μg l in unburned plot to 0.3 μg l in the burned plot). In surface soils (0-10 cm) of the burned plot, fire decreased the concentrations of extractable organic-P fractions, but did not significantly increase inorganic-P fractions. The reduction of extractable soil organic P in the burned plot in topsoil and the increase of P in the soil solution at greater depths indicated a reduction of P availability and may increase P fixation in deep soils. Repeated fire events over the long term may result in significant net loss of available forms of phosphorus from this ecosystem. [ABSTRACT FROM AUTHOR]

The Cerrado is the second largest Brazilian biome and contains the headwaters of three major hydrological basins in Brazil. In spite of the biological and ecological relevance of this biome, there is little information about how land use changes affect the chemistry of low-order streams in the Cerrado. To evaluate these effects streams that drain areas under natural, rural, and urban land cover were sampled near Brasília, Brazil. Water samples were collected between September 2004 and December 2006. Chemical concentrations generally followed the pattern of Urban > Rural > Natural. Median conductivity of stream water of 21.6 (interquartile: 22.7) μS/cm in urban streams was three and five-fold greater relative to rural and natural areas, respectively. In the wet season, despite of increasing discharge, concentration of many solutes were higher, particularly in rural and natural streams. Streams also presented higher total dissolved N (TDN) loads from natural to rural and urban although DIN:DON ratios did not differ significantly. In natural and urban streams TDN was 80 and 77% dissolved organic N, respectively. These results indicate that alterations in land cover from natural to rural and urban are changing stream water chemistry in the Cerrado with increasing solute concentrations, in addition to increased TDN output in areas under urban cover, with potential effects on ecosystem function. [ABSTRACT FROM AUTHOR]

Concentrations of cations (Na, Ca, Mg, K, NH), anions (HCO, Cl, NO, SO, PO) and suspended sediments in the Madeira River water were determined near the city of Porto Velho (RO), in order to assess variation in water chemistry from 2004 to 2007. Calcium and bicarbonate were the dominant cation and anion, respectively. Significant seasonal differences were found, with highest concentrations occurring during the dry season, as expected from the drainage of Andean carbonate-rich substratum. Interannual variations were also observed, but became significant only when annual average discharge was 25% less than normal. Under this atypical discharge condition, bicarbonate was replaced by sulfate, and higher suspended sediment concentrations and loads were also observed. Compared to previously published studies, it appears that no significant changes in water chemistry have occurred during the last 20-30 years, although differences in approaches and sampling designs among this and previous studies may not allow detection of modest changes. The calculated suspended sediment load reported here is close to the values presented elsewhere, reinforcing the relative importance of this river as a sediment supplier for the Amazon Basin. Seasonality has a significant control on the chemistry of Madeira River waters, and severe decrease in discharge due to anthropogenic changes, such as construction of reservoirs or the occurrence of drier years-a plausible consequence of global climate change-may lead to modification in the chemical composition as well in the sediment deliver to the Amazon River. [ABSTRACT FROM AUTHOR]

Biogeochemistry is hosting this special thematic issue devoted to studies of land-water interactions, as part of the Large-scale Biosphere-Atmosphere Experiment in Amazônia (LBA). This compilation of papers covers a broad range of topics with a common theme of coupling land and water processes, across pristine and impacted systems. Findings highlighted that hydrologic flowpaths are clearly important across basin size and structure in determining how water and solutes reach streams. Land-use changes have pronounced impacts on flowpaths, and subsequently, on stream chemistry, from small streams to large rivers. Carbon is produced and transformed across a broad array of fluvial environments and wetlands. Surface waters are not only driven by, but provide feedback to, the atmosphere. [ABSTRACT FROM AUTHOR]

We used stream chemistry and hydrogeomorphology data from 549 stream and 447 river sites to estimate NO-N removal in the Upper Mississippi, Missouri, and Ohio Rivers. We used two N removal models to predict NO-N input and removal. NO-N input ranged from 0.01 to 338 kg km d in the Upper Mississippi River to <0.01-54 kg km d in the Missouri River. Cumulative river network NO-N input was 98700-101676 Mg year in the Ohio River, 85961-89288 Mg year in the Upper Mississippi River, and 59463-61541 Mg year in the Missouri River. NO-N output was highest in the Upper Mississippi River (0.01-329 kg km d), followed by the Ohio and Missouri Rivers (<0.01-236 kg km d) sub-basins. Cumulative river network NO-N output was 97499 Mg year for the Ohio River, 84361 Mg year for the Upper Mississippi River, and 59200 Mg year for the Missouri River. Proportional NO-N removal (PNR) based on the two models ranged from <0.01 to 0.28. NO-N removal was inversely correlated with stream order, and ranged from <0.01 to 8.57 kg km d in the Upper Mississippi River to <0.001-1.43 kg km d in the Missouri River. Cumulative river network NO-N removal predicted by the two models was: Upper Mississippi River 4152 and 4152 Mg year, Ohio River 3743 and 378 Mg year, and Missouri River 2277 and 197 Mg year. PNR removal was negatively correlated with both stream order ( r = −0.80-0.87) and the percent of the catchment in agriculture ( r = −0.38-0.76). [ABSTRACT FROM AUTHOR]

Much of the research on the chemistry of snow and surface waters of the western US, Europe, and Asia has been conducted in high-elevation catchments above treeline. Here we provide information on the solute content of the seasonal snowpack at the Soddie site on Niwot Ridge, Colorado, a subalpine site near treeline. We focus on the storage and release of both inorganic and organic solutes to the soils underneath the snowpack, and subsequent effects on the chemical and nutrient content of the underlying soil solution and the adjacent headwater stream. The concentration of inorganic nitrogen (N) stored in the seasonal snowpack at the Soddie site of about 11 μeq L−1 was on the upper end of values reported for the northern hemisphere when compared to most areas of the Alps, Himalayas, and Tien Shan mountain ranges, but consistent with other reports of snowpacks in the Rocky Mountains. The storage of inorganic N in the snowpack at maximum accumulation averaged about 17 meq m−2, or 170 eq ha−1 (on the order of 2 kg-N ha−1). Solutes were released from storage in the form of an ionic pulse, with a maximum concentration factor of about four. In contrast to the seasonal snowpack, the dominant form of N in the soil solution was dissolved organic N. Thus, soils underlying the seasonal snowpack appear to assimilate inorganic N released from storage in the snowpack and convert it to organic N. A two component mixing model suggests that the majority of streamflow was this year’s snowmelt that had infiltrated the subsurface and undergone subsequent biological and geochemical reactions. The inorganic N in surface waters at the headwaters of Como creek were always near or below detection limits, suggesting that this area at treeline is still N-limited. [ABSTRACT FROM AUTHOR]

The effects of disturbance on the biogeochemical processes that affect the sulfur (S) cycle in forested ecosystems are important, but have been studied in only a few locations. In this investigation, the mechanisms that caused large decreases in stream $SO42-$ concentrations after clearcutting a small forested catchment in the Catskill Mountains of southeastern New York in 1997 were identified through an examination of pH and $SO42-$ concentrations in soil solutions, bulk deposition of $SO42-$ in throughfall collectors, adsorbed $SO42-$ concentrations in buried soil bags, and spatial variations in $SO42-$ concentrations in shallow groundwater. The load of $SO42-$–S in stream water during the first 2 years after clearcutting was about 2 kg ha-1 year-1 less than the background value of 8–10 kg ha-1 year-1. The 10 and 19% decrease in net throughfall flux of $SO42-$–S during the 2nd and 3rd year after the clearcut, respectively, reflects reduced dry deposition of S after removal of the canopy, but this decrease accounts for 0 and 43%, respectively, of the decrease in $SO42-$ load in streamflow for these 2 years. The pH of B-horizon soil water decreased from 4.5 to 4.0 within 8 months after the clearcut, and $SO42-$ concentrations decreased from 45 µmol L-1 to less than 20 µmol L-1 during this time. A strong correlation between $SO42-$ concentrations and pH values (r2 = 0.71, p < 0.01) in B-horizon soil water during the post-harvest period (1997–1999) reflects increased $SO42-$ adsorption in response to soil acidification. Sulfate concentrations in groundwater from 21 spatially distributed wells were inversely related to a topographic index that served as a surrogate for soil wetness; thus, providing additional evidence that $SO42-$ adsorption was the dominant cause of the decreased $SO42-$ concentrations in the stream after clearcutting. These results are consistent with those from a 1985 whole-tree harvest at the Hubbard Brook Experimental Forest in New Hampshire in which increased $SO42-$ adsorption resulting from decreased soil pH was the primary cause of decreased $SO42-$ concentrations in stream water. [ABSTRACT FROM AUTHOR]

We investigated the influences of forest or pasture land use and stream size on particulate and dissolved material concentrations in a network of second to third order streams in Rondônia, in the Brazilian Amazon. During the dry season, a second order stream originating in pasture had lower concentrations of dissolved oxygen and nitrate, higher concentrations of chlorophyll, total suspended solids, particulate organic carbon, particular organic nitrogen, ammonium, and phosphate than a second order stream originating in forest. Where the second order forest stream exited forest and entered pasture, concentrations of dissolved oxygen dropped from 6 mg/L to almost 0 mg/L and nitrate concentrations dropped from 12 μM to 2 μM over a reach of 2 km. These changes indicated a strong influence of land use. During the rainy season, differences among reaches of all particulate and dissolved materials were diminished. Concentrations of oxygen, chlorophyll, total suspended solids, particulate organic carbon and nitrogen, nitrate, ammonium, and phosphate in the third order pasture stream more closely resembled the second order forest stream than the second order pasture stream, suggesting that conditions in the channels of larger pasture streams more strongly control concentrations of these materials. If this pattern is widespread in stream networks of regions that consist of a mosaic of forest and pasture lands, it may have important consequences for understanding the effects of deforestation on larger rivers of the Amazon Basin. This would indicate that the effects of forest clearing on the concentrations of many suspended and dissolved materials will be most easily detected in very small streams but potentially difficult to detect in larger streams and rivers. [ABSTRACT FROM AUTHOR]

Understanding the factors regulating the concentrations of basic cations in soils and surface waters is critical if rates of recovery are to be predicted in response to decreases in acidic deposition. Using a dynamic simulation model (PnET-BGC), we evaluated the extent to which atmospheric deposition of strong acids and associated leaching by strong anions, atmospheric deposition of basic cations through changes in emissions of particulate matter, and historical forest cutting have influenced soil pools of exchangeable basic cations and the acid-base status of stream water at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Historical deposition of basic cations was reconstructed from regression relationships with particulate matter emissions. Simulation results indicate that the combination of these factors has resulted in changes in the percent soil base saturation, and stream pH and acid neutralizing capacity (ANC) from pre-industrial estimates of ∼20%, ∼6.3 and ∼45 μeq L-1, respectively, to current values of ∼10%, ∼5.0 and ∼-5 μeq L-1, respectively. These current values fall within the critical thresholds at which forest vegetation and aquatic biotic are at risk from soil and surface water acidification due to acidic deposition. While the deposition of strong acid anions had the largest impact on the acid-base status of soil and stream water, the reduction in deposition of basic cations associated with reductions in particulate emissions was estimated to have contributed about 27% of the depletion in soil Ca2+ exchange pool and 15% of the decreases in stream water concentrations of basic cations. Decline in stream water concentrations of basic cation occurred under both increasing and decreasing exchangeable pools, depending on the process controlling the acid base status of the ecosystem. Model calculations suggest that historical forest cutting has resulted in only slight decreases in soil pools of exchangeable basic cations, and has had a limited effect on stream ANC over the long-term. [ABSTRACT FROM AUTHOR]

A synthesis of the biogeochemistry of Ca was done during 1963-1992 in reference and human-manipulated forest ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that there has been a marked decline in concentration and input of Ca in bulk precipitation, an overall decline in concentration and output of Ca in stream water, and marked depletion of Ca in soils of the HBEF since 1963. The decline in streamwater Ca was related strongly to a decline in SO sub(4) super(2-) + NO sub(3) super(-) in stream water during the period. The soil depletion of Ca was the result of leaching due to inputs of acid rain during the past 50 yr or so, to decreasing atmospheric inputs of Ca, and to changing amounts of net storage of Ca in biomass.As a result of the depletion of Ca, forest ecosystems at HBEF are much more sensitive to continuing inputs of strong acids in atmosphericdeposition than expected based on long-term patterns of sulfur biogeochemistry. [ABSTRACT FROM AUTHOR]