Catchment Export Research Articles

Concentration‐Discharge Patterns Reveal Catchment Controls Over the Stoichiometry of Carbon and Nutrient Supply to Boreal Streams

AbstractCarbon (C), nitrogen (N), and phosphorus (P) export from catchments is strongly regulated by interactions between hydrological flowpaths and their terrestrial use/storage. While concentration‐discharge (c‐Q) relationships have been widely used to understand this interplay for C, N, and P individually, how flow regulates the relative supply of these resources across spatial and temporal scales is not well documented. Here, we analyze c‐Q relationships from 12 years of data to test how seasonal flow regulates the concentrations of inorganic N (Dissolved inorganic nitrogen [DIN]) and P (Dissolved inorganic phosphorus [DIP]), dissolved organic N (DON) and C (dissolved organic carbon [DOC]) and their respective ratios across 12 streams in a boreal landscape. We observed opposing c‐Q relationships between organic and inorganic solutes. DOC and DON tended toward transport limitation with little year‐to‐year change, whereas ammonium (NH4) and DIP were increasingly source limited over time. These different c‐Q relationships translated into large (up to three‐fold) shifts in resource ratios (e.g., DOC:DIN) in response to changes in flow. Our results also highlight strong influences of catchment structure on c‐Q patterns, regardless of solute, season, and longer‐term directional changes. Here, the organic solute c‐Q responses became less transport limited over time; while inorganic solute responses became less source limited with increasing mire/decreasing forest cover. Overall, differences in timing of catchment exports for C, N, and P, create dynamic variation in solute concentrations in streams with subsequent impacts on resource stoichiometry that is central to aquatic ecological processes.

Effects of 66 years of water management and hydroclimatic change on the urban hydrology and water quality of the Panke catchment, Berlin, Germany

Long-term records of combined stream flow and water chemistry can be an invaluable source of information on changes in the quantity and quality of water resources. To understand the effect of hydroclimate and water management on the heavily urbanized Panke catchment in Berlin, Germany, an extensive search, collation and digitization of historic data from various sources was undertaken. This integrated a unique 66-year spatially distributed record of stream water quality, a 21-year record of groundwater quality and a 31-year stream flow record. These data were analysed in the context of hydroclimatic variability, as well as the history and technological evolution of water resource management in the catchment. To contextualize the effect of droughts, “average” and wet years the Standard Precipitation Index (SPI) was applied. As upstream sites have been less regulated by human impacts, the flow regime is most sensitive to changes in hydroclimatic conditions, while downstream sites are more influenced by wastewater effluents, urban storm drains and inter-basin transfers for flood alleviation. However, at all sites, a general increase in maximum event discharge was observed until a recent drought, starting in 2018. In general, water quality in the catchment has gradually improved as a result of management change and increasingly effective wastewater treatment, though in some places legacy and/or contemporary urban and rural groundwater contamination may be affecting the stream. Hydroclimatic changes, particularly drought years can affect water quality classes, and alter the chemostatic/dynamic behaviour of catchment export patterns. These insights from the Panke catchment underline the importance of strategic adaptation and improvement of water treatment and water resource management in order to enhance the quality of urban water courses. It also demonstrates the importance of long-term integrated data sets.

Synthesizing the impacts of baseflow contribution on concentration–discharge (<i>C</i>–<i>Q</i>) relationships across Australia using a Bayesian hierarchical model

Abstract. Understanding concentration–discharge (C–Q) relationships can inform catchment solute and particulate export processes. Previous studies have shown that the extent to which baseflow contributes to streamflow can affect C–Q relationships in some catchments. However, the current understanding on the effects of baseflow contribution in shaping the C–Q patterns is largely derived from temperate catchments. As such, we still lack quantitative understanding of these effects across a wide range of climates (e.g. arid, tropical and subtropical). The study aims to assess how baseflow contributions, as defined by the median and the range of daily baseflow indices within individual catchments (BFI_m and BFI_range, respectively), influence C–Q slopes across 157 catchments in Australia spanning five climate zones. This study focuses on six water quality variables: electrical conductivity (EC), total phosphorus (TP), soluble reactive phosphorus (SRP), total suspended solids (TSS), the sum of nitrate and nitrite (NOx) and total nitrogen (TN). The impact of baseflow contributions is explored with a novel Bayesian hierarchical model. For sediments and nutrient species (TSS, NOx, TN and TP), we generally see largely positive C–Q slopes, which suggest a dominance of mobilization export patterns. Further, for TSS, NOx and TP we see stronger mobilization (steeper positive C–Q slopes) in catchments with higher values in both the BFI_m and BFI_range, as these two metrics are positively correlated for most catchments. The enhanced mobilization in catchments with higher BFI_m or BFI_range is likely due to the more variable flow pathways that occur in catchments with higher baseflow contributions. These variable flow pathways can lead to higher concentration gradients between low flows and high flows, where the former is generally dominated by groundwater/slow subsurface flow while the latter by surface water sources, respectively. This result highlights the crucial role of flow pathways in determining catchment exports of solutes and particulates. Our study also demonstrates the need for further studies on how the temporal variations of flow regimes and baseflow contributions influence flow pathways and the potential impacts of these flow pathways on catchment C–Q relationships.

Towards a history of Holocene P dynamics for the Northern Hemisphere using lake sediment geochemical records

Abstract. Present-day lake water phosphorus (P) enrichment and accelerated P cycling are changes superimposed on a dynamic Holocene history of landscape development following glaciation, changes in climate, and long-term low-intensity human activity. Knowledge of the history of long-term P dynamics is essential for understanding present-day landscape P export and for managing both terrestrial and aquatic environments. This study is the first attempt to constrain the timing and magnitude of terrestrial changes in Holocene P dynamics across the Northern Hemisphere using lake sediment records. Here we reconstruct trajectories in terrestrial Holocene P dynamics for the Northern Hemisphere. We apply a simple process model to published lake sediment geochemical P records from 24 sites, producing records of landscape P yield and reconstructing lake water total phosphorus (TP) concentrations. Individual site trajectories of landscape P yield and lake water TP vary systematically, with differences attributable to local landscape development history. Three distinct traits are apparent. Mountain sites with minimal direct human impact show falling P supply and conform to conceptual models of natural soil development (Trait 1). Lowland sites where substantial (pre-)historic agriculture was present show progressively increasing P supply (Trait 2). Lowland sites may also show a rapid acceleration in P supply over the last few centuries, where high-intensity land use, including settlements and farming, is present (Trait 3). Where data availability permitted comparison, our reconstructed TP records agree well with monitored lake water TP data, and our sediment-inferred P yields are comparable to reported catchment export coefficients. Comparison with diatom-inferred TP reveals good agreement for recent records. Our reconstructions form the first systematic assessment of average terrestrial P export for the Northern Hemisphere over the Holocene and provide the empirical data needed for constraining long-term landscape P cycling models and values for terrestrial P export that could be used for ocean P cycling models. The long-term perspective provided by our sediment-inferred TP can be used to identify pre-disturbance baselines for lake water quality, information essential to target-driven lake management. We find the first detectable anthropogenic impacts on P cycling ca. 6000 BP, with more substantial impacts as early as 3000 BP. Consequently, to characterize pre-disturbance lake P conditions at Trait 2 and Trait 3 sites, it is necessary to consider time periods before the arrival of early farmers. Our use of trait classifications has a predictive power for sites without sediment records, allowing prediction of TP baselines and P trajectories based on regional landscape development history.

The lake as an iron sink - new insights on the role of iron speciation

The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.

Hydrological control of water quality – Modelling base cation weathering and dynamics across heterogeneous boreal catchments

Linking biogeochemical processes to water flow paths and solute travel times is important for understanding internal catchment functioning and control of water quality. Base cation weathering is a process closely linked to key factors affecting catchment functioning, including water pathways, soil contact time, and catchment characteristics, particularly in silicate-dominated areas. However, common process-based weathering models are often calibrated and applied for individual soil profiles, which can cause problems when trying to extrapolate results to catchment scale and assess consequences for stream water and groundwater quality. Therefore, in this work, base cation export was instead modelled using a fully calibrated 3D hydrological model (Mike SHE) of a boreal catchment, which was expanded by adding a relatively simple but still reasonably flexible and versatile weathering module including the base cations Na, K, Mg, and Ca. The results were evaluated using a comprehensive dataset of water chemistry from groundwater and stream water in 14 nested sub-catchments, representing different catchment sizes and catchment characteristics. The strongest correlations with annual and seasonal observations were found for Ca (r = 0.89-0.93, p < 0.05), Mg (r = 0.90-0.95, p < 0.05), and Na (r = 0.80-0.89, p < 0.05). These strong correlations suggest that catchment hydrology and landscape properties primarily control weathering rates and stream dynamics of these solutes. Furthermore, catchment export of Mg, Ca, and K was strongly connected to travel times of discharging stream water (r = 0.78-0.83). Conversely, increasing Na export was linked to a reduced areal proportion of mires (r = -0.79). The results suggest that a significant part (~45%) of the catchment stream export came from deep-soil weathering sources (>2.5 m). These results have implications for terrestrial and aquatic water quality assessments. If deep soils are present, focusing mainly on the shallow soil could lead to misrepresentation of base cation availability and the acidification sensitivity of groundwater and water recipients such as streams and lakes.

Colloidal catchment response to snowmelt and precipitation events differs in a forested headwater catchment

AbstractClimate change affects the occurrence of high‐discharge (HD) events and associated nutrient exports in catchment stream water. Information on colloidal events‐based losses of important nutrients, such as organic C(Corg), N, P, and S, remain relatively scarce. We hypothesized that contributions of colloidal exported N, S, and P due to differing hydrological mechanisms vary between HD events in late winter and spring. We examined one combined snowmelt and rainfall event (March 2018) with one rainfall event (May 2018) for temporal Corg, N, P, and S dynamics. The catchment exports of colloids and their subset nanoparticles were analyzed by asymmetric‐flow field flow fractionation (P) and a filtration cascade (N and S). The Corg source in both events was assessed by δ13C composition of the stream water in relation to that of the soil. In winter, &lt;6% of stream water P was transported by colloids (&gt;0.1 μm), but this was 29–64% in spring and was associated with Corg, Fe, and Al. Colloidal N and particulate S (&gt;1 μm) were higher during both events, but the majority of losses were dissolved (&lt;0.1 μm). The δ13C values of dissolved organic matter (13CDOM) showed that in winter, most Corg was exported from the hydrologically connected hillslopes by water flowing through mineral horizons, due to snowmelt. During and after the rainfall events, export from organic horizons dominated the nutrient losses as particulates, including colloids. These events highlight the need for a better quantification of often underreported particulate, colloid, and nanoparticle contributions to weather‐driven nutrient losses from catchments.

Late summer peak in pCO2 corresponds with catchment export of DOC in a temperate, humic lake

ABSTRACT Humic lakes play a key role in the processing of organic carbon (OC) mobilised from their catchments, but knowledge of OC dynamics in lakes within maritime temperate climates is limited. Climate exerts a significant influence on mechanisms of OC capture, storage, and processing on the wet and cloudy west coast of Ireland. We examined a high-frequency dataset of partial pressure of CO2 (pCO2) in the surface waters of Lough Feeagh collected over 1 year. The annual pattern in pCO2 ranged between 491 and 1169 µatm and was strongly related to allochthonous riverine OC inputs. In contrast to observations in colder climates, a single peak in pCO2 occurred in Lough Feeagh in early September. Generalised additive mixed modelling revealed that 2 variables, inflow water colour concentration (a reliable proxy for DOC concentrations) and lake Schmidt stability, together explained 68% of pCO2 variability. Both the statistical analysis and timing of the peaks in inflow DOC and pCO2 strongly suggested that catchment carbon export drove pCO2 supersaturation in the lake, and hence CO2 emissions. We estimated that between 217 and 370 t CO2-C (0.55–0.94 t/ha) was emitted during the study period. These results highlight the interplay between catchment OC fluxes and climate in determining pCO2 dynamics in maritime temperate lakes.

Catchment export of base cations: improved mineral dissolution kinetics influence the role of water transit time

Abstract. Soil mineral weathering is one of the major sources of base cations (BC), which play a dual role in forest ecosystems: they function as plant nutrients and buffer against the acidification of catchment runoff. On a long-term basis, soil weathering rates determine the highest sustainable forest productivity that does not cause acidification. It is believed that the hydrologic residence time plays a key role in determining the weathering rates at the landscape scale. The PROFILE weathering model has been used for almost 30 years to calculate weathering rates in the rooting zone of forest soils. However, the mineral dissolution equations in PROFILE are not adapted for the saturated zone, and employing these equations at the catchment scale results in a significant overprediction of base cation release rates to surface waters. In this study, we use a revised set of PROFILE equations which, among other features, include retardation due to silica concentrations. Relationships between the water transit time (WTT) and soil water concentrations were derived for each base cation, by simulating the soil water chemistry along a one-dimensional flow path, using the mineralogy from a glacial till soil. We show how the revised PROFILE equations are able to reproduce patterns in BC and silica concentrations as well as BC ratios (Ca2+/BC, Mg2+/BC and Na+/BC) that are observed in the soil water profiles and catchment runoff. In contrast to the original set of PROFILE equations, the revised set of equations could reproduce the fact that increasing WTT led to a decreasing Na+/BC ratio and increasing Ca2+/BC and Mg2+/BC ratios. Furthermore, the total release of base cations from a hillslope was calculated using a mixing model, where water with different WTTs was mixed according to an externally modeled WTT distribution. The revised set of equations gave a 50 % lower base cation release (0.23 eq m−2 yr−1) than the original PROFILE equations and are in better agreement with mass balance calculations of weathering rates. Thus, the results from this study demonstrate that the revised mineral dissolution equations for PROFILE are a major step forward in modeling weathering rates at the catchment scale.

A cultivated area forecasting approach in artificial oases under climate change and human activities

The cultivated area in artificial oases is deeply influenced by global climate change and human activities. Thus, forecasting cultivated area in artificial oases under climate change and human activities is of great significance. In this study, an approach named GD-HM-PSWROAM, consisting of general circulation model downscaling (GD), hydrological model (HM), and planting structure and water resource optimal allocation model (PSWROAM), was developed and applied in the irrigation district of the Manas River Basin in Xinjiang Uygur Autonomous Region of China to forecast the cultivated area tendency. Furthermore, the catchment export of the MIKE11 HD/NAM model was set to the Kensiwate hydrological station. The results show that the downscaling effects of temperature can be fairly satisfying, while those of precipitation may be not satisfying but acceptable. Simulation capacity of the MIKE11 HD/NAM model on the discharge in the Kensiwate hydrological station can meet the requirements of running the PSWROAM. The accuracy of the PSWROAM indicated that this model can perform well in predicting the change of cultivated area at the decadal scale. The cultivated area in the Manas River Basin under current human activities may be generally decreasing due to the climate change. But the adverse effects of climate change can be weakened or even eliminated through positive human activities. The cultivated area in the Manas River Basin may even be increasing under assumed human activities and future climate scenarios. The effects of human activities in the future can be generally predicted and quantified according to the cultivated area trends under current human activities and the situations in the study area. Overall, it is rational and acceptable to forecast the cultivated area tendency in artificial oases under future climate change and human activities through the GD-HM-PSWROAM approach.

Role of recent climate change on carbon sequestration in peatland systems

This paper provides information on the impact of recent climate change on carbon sequestration in peatland systems in South West England. This is important because peatlands have the potential to sequester and hold large quantities of anthropogenically released CO2. This paper investigates whether there has been a reduction in the strength of carbon sinks in a valley mire and blanket bog, which occur on the limits of the biogeographical envelop for peatlands in Britain. Past rates of carbon accumulation were calculated from peat depth and the sequential analysis of peat age, bulk density and carbon content from cores taken from valley mire and blanket bog. At the valley mire site contemporary net ecosystem carbon balance (NECB) was calculated by measuring inputs to the peat body, via net primary productivity (NPP), of Sphagna. Losses of C from the peat body were calculated by measuring CH4, and aquatic carbon, calculated from catchment export of particulate and dissolved organic carbon. The study found similar mean rates of carbon accumulation since 1850 of 11.26 t ± 0.68 t CO2e ha−1 yr−1 (307 g C m−2 yr−1) in valley mire and 11.77 t ± 0.88 t CO2e ha−1 yr−1 (321 g C m−2 yr−1) in blanket bog. The mean present-day CO2 sequestration rate for Sphagna on valley mire was calculated to be 9.13 t ± 0.98 t CO2e ha−1 yr−1 (249 g C m−2 yr−1). Both past and contemporary rates of CO2 sequestration were found to be at the maxima of those reported for temperate peatlands. NPP was found to vary according to microform with higher rates of carbon sequestration found in lawn and hummock microforms compared with pools. Our work suggests that recent changes in the climate appear to have had limited impact on the strength of peatland carbon sinks in South West England.

Assessing the impact of long-term changes in climate and atmospheric deposition on a shallow alpine lake from southeast Tibet

Regional warming and atmospheric nitrogen deposition have been widely recorded to impact remote catchments and alpine lakes; however, their independent roles and interactions have rarely been identified. Here, we combined down-core analyses of sedimentary mercury (Hg) and aluminum (Al) with multiple proxies (i.e. nitrogen stable isotope, chlorophyll a pigments, diatoms) for a radiometrically-dated sediment core of an alpine lake in southeast Tibet to track the atmospheric deposition of pollutants, and to examine possible effects of climate and catchment forcing over the past three centuries. The sediment data revealed that airborne deposition of Hg was recorded from the ~1860s, with an accelerating increase in anthropogenic Hg flux since the ~1960s. A synchronous decrease in reconstructed lake-water TOC indicated that acid deposition may have affected lake-water carbon concentrations and impaired catchment export of decomposed organic matter (OM). A moderate depletion of bulk sediment δ15N started from the ~1820s, but was followed by an enriching trend after the ~1970s. This positive shift of δ15N was associated with elevated sediment OM and decreased catchment runoff of clastic materials (as inferred by Al). Sediment OM content displayed an accelerating increase from the ~1960s, with an increased input of autochthonous sources (i.e. lower bulk sediment C:N ratios), such as algae (as inferred by sedimentary chlorophyll a pigments). Meanwhile, climate warming and decreased lake-water TOC enhanced the production of algae, which was characterized by a more enriched δ15N signal than that of allochthonous OM. Furthermore, atmospheric acid deposition was significantly related to diatom assemblage changes, with an increase in acidophilous taxa. Our sediment evidence revealed the dominating impact of climate and catchment processes on lake-water chemistry and algal shifts in the context of atmospheric nitrogen deposition, and highlighted an increasing link of external forcing with in-lake processes in enriching sediment δ15N signal over the last few decades.

Ecohydrological determinants of seasonality and export of total organic carbon in Narew River with high peatland contribution (north-eastern Poland)

This study used a long-term monitoring data to analyse the abundance, seasonal dynamics, spatial distribution and catchment export of total organic carbon (TOC) in one of the largest undisturbed peatland river catchment in Central Europe. Mean annual TOC concentrations in the Narew River basin were higher than 10–11mgCL−1 which is similar to nemoral or Baltic rivers. The maximal monthly TOC concentrations occurred in May with a one-month delay from the spring maximum of water outflow. High TOC concentrations were accompanied by naturally increased organic nitrogen and phosphorous concentrations resulting in decreased water quality class despite the anthropogenic pressure is low. Mean annual export of TOC from the Narew River catchment amounted to 2.15gCm−2, which is nearly 25% higher than for the Vistula River, the recipient. TOC export form the analysed river basin in dry years was 30% lower and 50% higher in wet years than the multiannual average. The primary cause of high TOC concentrations in the Narew River catchment and high TOC export is the occurrence of biogeochemically active wetlands, functioning not only thanks to spring flooding of the valleys, but also based on permanent intensive groundwaters supply from surrounding plateaus. This emphasises the importance of basin hydrology in the regulation of carbon cycling in the temperate lowland catchments. Our results also indicate that in environmental zonation of Europe Narew River catchment should be classified as belonging to the Baltic Province (Nemoral environmental zone) rather then to Continental zone. This is because it is characterized with high riverine TOC concentrations and natural water oxygen depletion in summers. Thus, Environmental Stratification of Europe developed for environmental statistics and monitoring should be re-evaluated. Climate change is likely to affect the organic carbon circulation in peatland catchments and reduce TOC fluxes into river network.

National scale evaluation of the InVEST nutrient retention model in the United Kingdom

A wide variety of tools aim to support decision making by modelling, mapping and quantifying ecosystem services. If decisions are to be properly informed, the accuracy and potential limitations of these tools must be well understood. However, dedicated studies evaluating ecosystem service models against empirical data are rare, especially over large areas. In this paper, we report on the national-scale assessment of a new ecosystem service model for nutrient delivery and retention, the InVEST Nutrient Delivery Ratio model. For 36 river catchments across the UK, we modelled total catchment export of phosphorus (P) and/or nitrogen (N) and compared model outputs to measurements derived from empirical water chemistry data.The model performed well in terms of relative magnitude of nutrient export among catchments (best Spearman's rank correlation for N and P, respectively: 0.81 and 0.88). However, there was wide variation among catchments in the accuracy of the model, and absolute values of nutrient exports frequently showed high percentage differences between modelled and empirically-derived exports (best median absolute percentage difference for N and P, respectively: ±64%, ±44%). The model also showed a high degree of sensitivity to nutrient loads and hydrologic routing input parameters and these sensitivities varied among catchments.These results suggest that the InVEST model can provide valuable information on nutrient fluxes to decision makers, especially in terms of relative differences among catchments. However, caution is needed if using the absolute modelled values for decision-making. Our study also suggests particular attention should be paid to researching input nutrient loadings and retentions, and the selection of appropriate input data resolutions and threshold flow accumulation values. Our results also highlight how availability of empirical data can improve model calibration and performance assessment and reinforce the need to include such data in ecosystem service modelling studies.

Impact of soil water erosion processes on catchment export of soil aggregates and associated SOC

Soil erosion is a complex process which causes loss of soil mineral material together with soil organic carbon (SOC). Little is known about the impact of soil water erosion on catchment exports of soil aggregates and associated SOC, especially at a small agricultural catchment. SOC delivered from a small catchment located in Northeast China was assessed for three erosive rainfall events in 2011. The quantity and size of the exported soil aggregates and their concentration of SOC were assessed along with runoff rate, sediment loss and rainfall characteristics to further the understanding of SOC erosion and its potential fate. Aggregate size distribution in sediments was different from that in source soil and was significantly correlated to sediment concentration in runoff. Fine size aggregate fractions in the sediments were the most abundant. The SOC was enriched in the sediments and indicated a high selectivity of the erosion processes. The SOC concentration differed among different sizes of aggregate fractions with coarse size aggregate fractions generally having the largest concentration. We concluded that the soil water erosion processes impact the aggregate size distribution and SOC enrichment in the delivered sediment, which subsequently influence the catchment exports of SOC. Additional measurements over several rainy seasons are required to further quantify the impact of soil water erosion on catchment exports of soil aggregates and associated SOC.

Riparian zones attenuate nitrogen loss following bark beetle‐induced lodgepole pine mortality

AbstractA North American bark beetle infestation has killed billions of trees, increasing soil nitrogen and raising concern for N loss impacts on downstream ecosystems and water resources. There is surprisingly little evidence of stream N response in large basins, which may result from surviving vegetation uptake, gaseous loss, or dilution by streamflow from unimpacted stands. Observations are lacking along hydrologic flow paths connecting soils with streams, challenging our ability to determine where and how attenuation occurs. Here we quantified biogeochemical concentrations and fluxes at a lodgepole pine‐dominated site where bark beetle infestation killed 50–60% of trees. We used nested observations along hydrologic flow paths connecting hillslope soils to streams of up to third order. We found soil water NO3 concentrations increased 100‐fold compared to prior research at this and nearby southeast Wyoming sites. Nitrogen was lost below the major rooting zone to hillslope groundwater, where dissolved organic nitrogen (DON) increased by 3–10 times (mean 1.65 mg L−1) and NO3‐N increased more than 100‐fold (3.68 mg L−1) compared to preinfestation concentrations. Most of this N was removed as hillslope groundwater drained through riparian soils, and NO3 remained low in streams. DON entering the stream decreased 50% within 5 km downstream, to concentrations typical of unimpacted subalpine streams (~0.3 mg L−1). Although beetle outbreak caused hillslope N losses similar to other disturbances, up to 5.5 kg ha−1y−1, riparian and in‐stream removal limited headwater catchment export to &lt;1 kg ha−1y−1. These observations suggest riparian removal was the dominant mechanism preventing hillslope N loss from impacting streams.

Water browning mediates predatory decimation of the Arctic fairy shrimp Branchinecta paludosa

Summary Increased concentration of dissolved organic carbon (DOC) in northern watersheds is a concern. Such browning affects aquatic food webs and biodiversity in multiple ways, not least through increased attenuation of short‐wave UV radiation, which may facilitate establishment of new predators. We examined the effects of browning on occurrence of the predatory larvae of the phantom midge, Chaoborus nyblaei, and quantified their impact on the Arctic fairy shrimp Branchinecta paludosa in ponds in alpine southern Norway. The predator was present only above certain levels of DOC (here represented by total organic carbon, TOC). Comet assays revealed a 50‐fold higher DNA damage in the population inhabiting a clearwater pond (3.9 mg TOC L−1), compared to that from a brownwater site (10.5 mg TOC L−1). Laboratory experiments corroborated the key role of browning in protection against UV‐related DNA damage. Chaoborus nyblaei was a highly effective predator on B. paludosa, which suffered major population declines in ponds where the predator was present. Indeed, extinction of B. paludosa has been observed at lower altitudes of the surveyed area over the past decades, possibly in parallel with warming and increased vegetation cover, as inferred from analyses of Normalized Difference Vegetation Index (NDVI) for the surveyed area. These shifts could clearly have favoured establishment of C. nyblaei in the region. Our results point to climate‐induced cascading effects in alpine ponds, whereby increased warming and terrestrial C‐fixation cause increased catchment export of DOC, leading to increased browning of water and associated food‐web changes.

Hydrologic control of dissolved organic matter concentration and quality in a semiarid artificially drained agricultural catchment

AbstractAgricultural practices have altered watershed‐scale dissolved organic matter (DOM) dynamics, including in‐stream concentration, biodegradability, and total catchment export. However, mechanisms responsible for these changes are not clear, and field‐scale processes are rarely directly linked to the magnitude and quality of DOM that is transported to surface water. In a small (12 ha) agricultural catchment in eastern Washington State, we tested the hypothesis that hydrologic connectivity in a catchment is the dominant control over the concentration and quality of DOM exported to surface water via artificial subsurface drainage. Concentrations of dissolved organic carbon (DOC) and humic‐like components of DOM decreased while the Fluorescence Index and Freshness Index increased with depth through the soil profile. In drain discharge, these characteristics were significantly correlated with drain flow across seasons and years, with drain DOM resembling deep sources during low‐flow and shallow sources during high flow, suggesting that DOM from shallow sources bypasses removal processes when hydrologic connectivity in the catchment is greatest. Assuming changes in streamflow projected for the Palouse River (which contains the study catchment) under the A1B climate scenario (rapid growth, dependence on fossil fuel, and renewable energy sources) apply to the study catchment, we project greater interannual variability in annual DOC export in the future, with significant increases in the driest years. This study highlights the variability in DOM inputs from agricultural soil to surface water on daily to interannual time scales, pointing to the need for a more nuanced understanding of agricultural impacts on DOM dynamics in surface water.

Permafrost conditions in peatlands regulate magnitude, timing, and chemical composition of catchment dissolved organic carbon export.

Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2-3 g C m(-2) ) dominated by the snow melt period (~70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m(-2) ), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and ~10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.

Forest harvest effects on mercury in streams and biota in Norwegian boreal catchments

Forest harvesting practices can potentially increase mercury run-off from catchments. A paired catchment experiment was conducted in a boreal forest in southern Norway, to test effects of forest harvest operations on (i) concentrations and fluxes of methylmercury (MeHg), total mercury (HgT), nutrients and dissolved organic matter (TOC), and on (ii) MeHg bioaccumulation in stream foodwebs.Thirty percent of a catchment was harvested in winter time with snow cover but no soil frost, resulting in wheel tracks and soil compaction. Pre-harvest differences included higher streamwater MeHg, HgT and TOC, and lower pH in the treated catchment compared to the reference.No significant treatment effects on concentrations of MeHg, HgT and TOC were detected, whereas concentrations of nutrients (ammonium, nitrate, phosphorus (P)) increased significantly. Estimated catchment export of nitrate and ammonium increased fourfold, as a combined effect of changed discharge and concentrations. Export of MeHg and HgT increased weakly, primarily because of an increase in discharge.Levels of MeHg in stream invertebrates mirrored differences in aquatic MeHg between the two streams, resulting in higher MeHg in biota in the harvest catchment in pre-harvest conditions. After harvest, MeHg levels in primary consumers (herbivorous stoneflies) were no longer different between the two streams, despite continued exposure to higher aqueous MeHg in the harvested catchment. Simultaneously, dietary biomarkers (δ15N signature, lipid- and algal fatty acid content) in the stoneflies had changed significantly, consistent with higher nutrient loadings and associated higher diet availability in the harvested stream.The results of our experiment do not support that forest management has a strong impact on catchment MeHg production. Catchment disturbance through forest harvesting may decrease MeHg in aquatic biota, because of higher stream productivity and associated higher quality of dietary sources, at least in the short-term. Other studies on catchment MeHg production to disturbance have shown a range in responses, from strong to none. So far, no factor has emerged to explain such range in responses. Predictions of forest management effects on MeHg in streamwater and aquatic food webs are hampered by limited understanding of catchment controls on MeHg production.