[show abstract][hide abstract] ABSTRACT: Significant concern has emerged over the past decades regarding decreases in available base cations (that is, calcium, magnesium, potassium, and sodium) in forest soils and surface waters. Base cations (BCs) are important for buffering against changes in soil and water acidity, and their concentrations can be indicative of environmental management problems such as those linked to acid deposition and land use. Climate variability is also a potentially large factor influencing the dynamics of BCs in soils and surface waters, but our understanding of these interactions at broad scales remains elusive. We used a hierarchical Bayesian model to evaluate the long-term (1990–2010) patterns and drivers of BC concentrations for 60 stream and river monitoring stations across Sweden. Results indicated that the long-term trends in concentration, and the associated environmental drivers, differed among individual BCs and geographical regions. For example, we found that concentrations of Ca2+, Mg2+, K+, and Na+ have decreased in southern Sweden since 1990 and that this is strongly related to concurrent declines in sulfate (SO42−) over the same period of record. In contrast, concentrations of Ca2+, Mg2+, K+, and Na+ in northern Sweden did not exhibit significant directional trends, despite declines in SO42−, nitrate (NO3−), and chloride (Cl−) over the same period. Instead, BC dynamics in the north are characterized by inter-annual variability that is most closely linked to climate variables. Results suggest that the interaction between climatic variability and historical acid deposition determines the regional pattern and long-term trends of BC concentrations across streams and rivers of Sweden. Understanding the strength of the interaction between climate features and historic deposition will greatly improve our ability to predict long-term trends of Ca2+, Mg2+, K+, and Na+ and their inter-annual dynamics in the future.
[show abstract][hide abstract] ABSTRACT: Riparian zones (RZ) are a major factor controlling
water chemistry in forest streams. Base cations’ (BC)
concentrations, fluxes, and cycling in the RZ merit attention
because a changing climate and increased forest harvesting
could have negative consequences, including re-acidification,
for boreal surface waters. We present a two-year study of
BC and silica (Si) flow-weighted concentrations from 13 RZ
and 14 streams in different landscape elements of a boreal
catchment in northern Sweden. The spatial variation in BC
and Si dynamics in both RZ and streams was explained
by differences in landscape element type, with highest concentrations
in silty sediments and lowest concentrations in
peat-dominated wetland areas. Temporal stability in BC and
Si concentrations in riparian soil water, remarkably stable
Mg/Ca ratios, and homogeneous mineralogy suggest that
patterns found in the RZ are a result of a distinct mineralogical
upslope signal in groundwater. Stream water Mg/Ca ratios
indicate that the signal is subsequently maintained in the
streams. Flow-weighted concentrations of Ca, Mg, and Na
in headwater streams were represented by the corresponding
concentrations in the RZ, which were estimated using
the Riparian Flow-Concentration Integration Model (RIM)
approach. Stream and RZ flow-weighted concentrations differed
for K and Si, suggesting a stronger biogeochemical influence
on these elements, including K recirculation by vegetation
and retention of Si within the RZ. Potential increases
in groundwater levels linked to forest harvesting or changes
in precipitation regimes would tend to reduce BC concentrations
from RZ to streams, potentially leading to episodic
[show abstract][hide abstract] ABSTRACT: Riverine transport of iron (Fe) and arsenic (As) is affected by their associations with natural organic matter (NOM) and suspended iron (oxy)hydroxides. Speciation has a strong influence on element transport from the headwaters to the ocean because NOM may be transported over longer distances compared to iron (oxy)hydroxides. We show that Fe speciation changes along the flow path of a boreal watercourse, as water moves from NOM-rich, acidic first-order streams with pH as low as 3.9 to less acidic higher-order systems (up to pH 6.4). Analysis by Flow Field-Flow Fractionation and chemical equilibrium modelling revealed that Fe from wetland-dominated headwaters was mainly exported as Fe-NOM complexes; in catchments with a stream order >1 and with higher pH, Fe was present in Fe-NOM complexes and precipitated as nanoparticulate iron(oxy)hydroxides which aggregated as the pH increased, with their size eventually exceeding the membrane filters cut-off (0.2 µm). The measured NOM-bound Fe decreased with increasing pH, from 0.38 to 0.16 mmol Fe∙gNOM-1. The high concentrations of NOM-bound Fe emphasize the importance of boreal catchments to Fe export to the oceans. Concentrations of As in the <0.2 µm fraction but larger than what is usually considered "truly dissolved" (<1000 g∙mol-1), decreased from 75% to 26% with increasing pH. The As in this size range was mainly associated with NOM but at pH >4.5 became associated with iron(oxy)hydroxides, and its transport thus became more coupled to that of the iron(oxy)hydroxides downstream in the circumneutral streams.
[show abstract][hide abstract] ABSTRACT: Boreal forests, characterized by distinct winter seasons, store a large proportion of the global terrestrial carbon (C) pool. We studied summer soil C-dynamics in a boreal forest in northern Sweden using a seven-year experimental manipulation of soil frost. We found that winter soil climate conditions play a major role in controlling the dissolution/mineralization of soil organic-C in the following summer season. Intensified soil frost led to significantly higher concentrations of dissolved organic carbon (DOC). Intensified soil frost also led to higher rates of basal heterotrophic CO2 production in surface soil samples. However, frost-induced decline in the in situ soil CO2 concentrations in summer suggests a substantial decline in root and/or plant associated rhizosphere CO2 production, which overrides the effects of increased heterotrophic CO2 production. Thus, colder winter soils, as a result of reduced snow cover, can substantially alter C-dynamics in boreal forests by reducing summer soil CO2 efflux, and increasing DOC losses.
Environmental Research Letters 05/2013; 8(2):024017. · 3.58 Impact Factor
[show abstract][hide abstract] ABSTRACT: Highlights
► Clear-cutting increases snow water equivalents by 27%. ► Harvesting resulted in variations of timing and runoff volume of the spring freshet. ► Meteorological conditions during the melt determine melt water losses. ► Snowmelt represents 31–41% of the annual discharge in a Swedish boreal forest. ► The spring freshet is highly variable in managed and unmanaged boreal forests.
Journal of Hydrology 03/2013; 484:105–114. · 2.96 Impact Factor
[show abstract][hide abstract] ABSTRACT: Evasion of gaseous carbon (C) from streams is often poorly quantified in landscape C budgets. Even though the potential importance of the capillary network of streams as C conduits across the land–water–atmosphere interfaces is sometimes mentioned, low-order streams are often left out of budget estimates due to being poorly characterized in terms of
gas exchange and even areal surface coverage. We show that evasion of C is greater than all the total dissolved C (both organic and inorganic) exported downstream in the waters of a boreal landscape. In this study evasion of carbon dioxide (CO2) from running waters within a 67 km2 boreal catchment was studied. During a 4 year period (2006–2009) 13 streams were sampled on 104 different occasions for dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). From a locally determined model of gas exchange properties, we estimated the daily CO2 evasion with a highresolution (5 * 5 m) grid-based stream evasion model comprising the entire~100 km stream network. Despite the low areal coverage of stream surface, the evasion of CO2 from the stream network constituted 53% (5.0 (�1.8)g C m�2 yr�1) of the entire stream C flux (9.6 (�2.4) g C m�2 yr�1) (lateral as DIC, DOC, and vertical as CO2). In addition, 72% of the total CO2 loss took place already in the first- and second-order streams. This study demonstrates the importance of including CO2 evasion from low-order boreal streams into landscape C budgets as it more than doubled the magnitude of the aquatic conduit for C from this landscape. Neglecting this term will consequently result
in an overestimation of the terrestrial C sink strength in the boreal landscape.
Global Change Biology 01/2013; 19(3):785–797. · 6.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: There is a growing awareness that mid-to-high latitude regions will be strongly affected by climate change. These changes are predicted to be especially pronounced during winter, particularly at higher latitudes. To test how water quality in northern catchments could be affected by warmer winter climates, we assembled long-term data from eight well-studied catchments in Sweden, Scotland, Canada and the USA across a climatic gradient spanning from À2 to +9 C in mean annual temperature and between À11.6 and + 6.1 C in average winter temperature. We used the climatic gradient combined with inter-annual variability among catchments to examine how warmer winters could affect the seasonality (seasonal timing) and synchroneity (coupling) of water and dissolved organic carbon (DOC) fluxes. In general, sites with colder winters (less than À5 C) experienced an export concentrated in spring, whereas sites with warmer winters (>0 C) displayed a more evenly distributed export across all seasons. Catchments with warmer winters also displayed less synchroneity between water and DOC flux during winter compared with colder sites, whereas the opposite was found for the spring. Patterns from the climatic gradient were supported by inter-annual variability at individual sites where both seasonality and synchroneity in the spring were related to the temperature during the preceding winter. Our findings suggest that one likely consequence of warmer winters in northern regions is that the proportion of the annual DOC and water export will increase during winter and decrease during spring and summer. This is of importance as it is the latter seasons during which downstream utilization of both water and DOC often is largest.
[show abstract][hide abstract] ABSTRACT: The boreal landscape is a complex, spatio-temporally varying mosaic of forest and mire landscape el-ements that control surface water hydrology and chemistry. Here, we assess long-term water quality time series from three nested headwater streams draining upland forest (C2), peat/mire (C4) and mixed (C7) (forest and mire) catchments. Acid deposition in this region is low and is further declin-ing. Temporal trends in weather and runoff (1981–2008), dissolved organic carbon concentration [DOC] (1993–2010) and other water quality parameters (1987–2011) were as-sessed. There was no significant annual trend in precipitation or runoff. However, runoff increased in March and declined in May. This suggested an earlier snowmelt regime in recent years. Significant monotonic increasing trends in air temper-ature and length of growing season suggested a decrease in snowfall and less spring runoff. Stream [DOC] was positively correlated with some trace metals (copper, iron and zinc) and negatively with several other chemical parameters (e.g. sul-fate, conductivity, calcium). Both sulfate and conductivity showed declining trends, while a significant increase was ob-served in pH during winter and spring. Calcium and mag-nesium showed monotonic decreasing trends. The declining trajectories of stream base cation and sulfate concentrations during other times of the year were not accompanied by changes in pH and alkalinity. These results indicate subtle effects of recovery from acidification. Water temperature in-creased significantly both annually and in most months. A simultaneous monotonic increase in iron (Fe) and [DOC] in autumn suggests co-transport of Fe-DOC in the form of organometallic complexes. A monotonic increase in UV ab-sorbance in most months without co-occurring changes in DOC trend suggests a shift in DOC quality to a more humic-rich type. The observed increase in soil solution [DOC] and subtle trends in stream [DOC] suggest that climate rather than recovery from acidification is the dominant driver of DOC trends in the Svartberget catchment.
[show abstract][hide abstract] ABSTRACT: The higher mid-latitudes of the northern hemisphere are particularly sensitive to change due to the important role the 0oC isotherm plays in the phase of precipitation and intermediate storage as snow. An international inter-catchment comparison program called North-Watch seeks to improve our understanding of the sensitivity of northern catchments to change by examining their hydrological and biogeochemical variability and response. Here, eight North-Watch catchments located in Sweden (Krycklan), Scotland (Girnock and Strontian), the United States (Sleepers River, Hubbard Brook and HJ Andrews) and Canada (Dorset and Wolf Creek) with 10 continuous years of daily precipitation and runoff data were selected to assess daily to seasonal coupling of precipitation (P) and runoff (Q) using wavelet coherency, and to explore the patterns and scales of variability in streamflow using color maps. Wavelet coherency revealed that P and Q were decoupled in catchments with cold winters, yet were strongly coupled durin
[show abstract][hide abstract] ABSTRACT: The concentrations of uranium and thorium in ten partly nested streams in the boreal forest region were monitored over a two-year period. The investigated catchments ranged from small headwaters (0.1 km2) up to a fourth-order stream (67 km2). Considerable spatiotemporal variations were observed, with little or no correlation between streams. The fluxes of both uranium and thorium varied substantially between the subcatchments, ranging from 1.7 to 30 g km−2 a−1 for uranium and from 3.2 to 24 g km−2 a−1 for thorium. Airborne gamma spectrometry was used to measure the concentrations of uranium and thorium in surface soils throughout the catchment, suggesting that the concentrations of uranium and thorium in mineral soils are similar throughout the catchment. The fluxes of uranium and thorium were compared to a wide range of parameters characterising the investigated catchments and the chemistry of the stream water, e.g. soil concentrations of these elements, pH, TOC (total organic carbon), Al, Si and hydrogen carbonate, but it was concluded that the spatial variabilities in the fluxes of both uranium and thorium mainly were controlled by wetlands. The results indicate that there is a predictable and systematic accumulation of both uranium and thorium in boreal wetlands that is large enough to control the transport of these elements. On the landscape scale approximately 65–80% of uranium and 55–65% of thorium entering a wetland were estimated to be retained in the peat. Overall, accumulation in mires and other types of wetlands was estimated to decrease the fluxes of uranium and thorium from the boreal forest landscape by 30–40%, indicating that wetlands play an important role for the biogeochemical cycling of uranium and thorium in the boreal forest landscape. The atmospheric deposition of uranium and thorium was also quantified, and its contribution to boreal streams was found to be low compared to weathering.
[show abstract][hide abstract] ABSTRACT: Groundwater flowing from hillslopes through riparian (near-stream) soils
often undergoes chemical transformations that can substantially
influence stream water chemistry. We used landscape analysis to predict
total organic carbon (TOC) concentration profiles and groundwater levels
measured in the riparian zone (RZ) of a 67 km2 catchment in
Sweden. TOC exported laterally from 13 riparian soil profiles was then
estimated based on the riparian flow-concentration integration model
(RIM). Much of the observed spatial variability of riparian TOC
concentrations in this system could be predicted from groundwater levels
and the topographic wetness index (TWI). Organic riparian peat soils in
forested areas emerged as hotspots exporting large amounts of TOC. These
TOC fluxes were subject to considerable temporal variations caused by a
combination of variable flow conditions and changing soil water TOC
concentrations. Mineral riparian gley soils, on the other hand, were
related to rather small TOC export rates and were characterized by
relatively time-invariant TOC concentration profiles. Organic and
mineral soils in RZs constitute a heterogeneous landscape mosaic that
potentially controls much of the spatial variability of stream water
TOC. We developed an empirical regression model based on the TWI to move
beyond the plot scale and to predict spatially variable riparian TOC
concentration profiles for RZs underlain by glacial till.
[show abstract][hide abstract] ABSTRACT: There is no scientific consensus about how dissolved organic carbon
(DOC) in surface waters is regulated. Here we combine recent literature
data from 49 catchments with detailed stream and catchment process
information from nine well established research catchments at mid- to
high latitudes to examine the question of how climate controls stream
water DOC. We show for the first time that mean annual temperature (MAT)
in the range from -3° to +10° C has a strong control over
the regional stream water DOC concentration in catchments, with highest
concentrations in areas ranging between 0° and +3° C MAT.
Although relatively large deviations from this model occur for
individual streams, catchment topography appears to explain much of this
divergence. These findings suggest that the long-term trajectory of
stream water DOC response to climate change may be more predictable than
Geophysical Research Letters 09/2012; 39(18):18404-. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: The spatial distribution of (238)U, (226)Ra, (40)K and the daughters of (232)Th, (228)Ra and (228)Th, were measured in a small mire in northern Sweden. High activity concentrations of (238)U and (232)Th (up to 41 Bq (238)U kg(-1)) were observed in parts of the mire with a historical or current inflow of groundwater from the surrounding till soils, but the activities declined rapidly further out in the mire. Near the outlet and in the central parts of the mire the activity concentrations were low, indicating that uranium and thorium are immobilized rapidly upon their entering the peat. The (226)Ra was found to be more mobile with high activity concentrations further out into the mire (up to 24 Bq kg(-1)), although the central parts and the area near the outlet of the mire still had low activity concentrations. Based on the fluxes to and from the mire, it was estimated that approximately 60-70% of the uranium and thorium entering the mire currently is retained within it. The current accumulation rates were found to be consistent with the historical accumulation, but possibly lower. Since much of the accumulation still is concentrated to the edges of the mire and the activities are low compared to other measurements of these radionuclides in peat, there are no indications that the mire will be saturated with respect to radionuclides like uranium, thorium and radium in the foreseen future. On the contrary, normal peat growth rates for the region suggest that the average activity concentrations of the peat currently may be decreasing, since peat growth may be faster than the accumulation of radionuclides. In order to assess the total potential for accumulation of radionuclides more thoroughly it would, however, be necessary to also investigate the behaviour of other organophilic elements like aluminium, which are likely to compete for binding sites on the organic material. Measurements of the redox potential and other redox indicators demonstrate that uranium possibly could be reduced in parts of the mire. The results of the study suggest that this mire currently is, and historically has been, an important sink for radionuclides and that it most likely will continue to be so for a long time to come.
Journal of environmental radioactivity 07/2012; · 1.47 Impact Factor
[show abstract][hide abstract] ABSTRACT: Riparian soils exert a major control on stream water dissolved organic carbon (DOC) in northern latitudes. As the winter climate
in northern regions is predicted to be particularly affected by climate change, we tested the sensitivity of DOC formation
to winter conditions in riparian soils using an 8year field-scale soil frost manipulation experiment in northern Sweden.
In conjunction with the field experiment, we also carried out a laboratory experiment based on three levels of four winter
climatic factors: frost intensity, soil water content, frost duration and frequency of freeze–thaw cycles. We evaluated changes
in lability of DOC in soil solution from lysimeter samples taken at different depths (10–80cm) as well as from DOC extracted
from soils in the laboratory, using carbon-specific ultraviolet absorbance at 254nm (sUVA254). In the field, significantly more labile DOC was observed during the spring and summer from upper horizons of frost-exposed
soils, when compared to controls. In addition, the amount of labile DOC was positively correlated with frost duration at a
soil depth of 10cm. In the laboratory, frost intensity was the factor that had the greatest positive influence on DOC lability;
it also reduced the C:N ratio which may indicate a microbial origin of the DOC. The laboratory experiment also demonstrated
significant interactions between some of the applied climatic factors, such as frost intensity interacting with water content.
In combination, field and laboratory experiments demonstrate that winter soil conditions have profound effects on DOC-concentration
and quality during subsequent seasons.
–Dissolved organic carbon–Water content–Riparian zone–CCF design
[show abstract][hide abstract] ABSTRACT: We bring together three decades of research from a boreal catchment to facilitate an improved mechanistic understanding of
surface water dissolved organic carbon (DOC) regulation across multiple scales. The Krycklan Catchment Study encompasses 15
monitored nested research catchments, ranging from 3 to 6900ha in size, as well as a set of monitored transects of forested
and wetland soils. We show that in small homogenous catchments, hydrological functioning provides a first order control on
the temporal variability of stream water DOC. In larger, more heterogeneous catchments, stream water DOC dynamics are regulated
by the combined effect of hydrological mechanisms and the proportion of major landscape elements, such as wetland and forested
areas. As a consequence, streams with heterogeneous catchments undergo a temporal switch in the DOC source. In a typical boreal
catchment covered by 10-20% wetlands, DOC originates predominantly from wetland sources during low flow conditions. During
high flow, the major source of DOC is from forested areas of the catchment. We demonstrate that by connecting knowledge about
DOC sources in the landscape with detailed hydrological process understanding, an improved representation of stream water
DOC regulation can be provided. The purpose of this study is to serve as a framework for appreciating the role of regulating
mechanisms, connectivity and scaling for understanding the pattern and dynamics of surface water DOC across complex landscapes.
The results from this study suggest that the sensitivity of stream water DOC in the boreal landscape ultimately depends on
changes within individual landscape elements, the proportion and connectivity of these affected landscape elements, and how
these changes are propagated downstream.
Keywordsdissolved organic carbon–scaling–connectivity–boreal forest–Krycklan catchment–hydrology
[show abstract][hide abstract] ABSTRACT: Scenarios indicate that the air temperature will increase in high latitude regions in coming decades, causing the snow covered
period to shorten, the growing season to lengthen and soil temperatures to change during the winter, spring and early summer.
To evaluate how a warmer climate is likely to alter the snow cover and soil temperature in Scots pine stands of varying ages
in northern Sweden, climate scenarios from the Swedish regional climate modelling programme SWECLIM were used to drive a Soil-Vegetation-Atmosphere
Transfer (SVAT)-model (COUP). Using the two CO2 emission scenarios A and B in the Hadley centres global climate model, HadleyA and HadleyB, SWECLIM predicts that the annual
mean air temperature and precipitation will increase at most 4.8°C and 315mm, respectively, within a century in the study
region. The results of this analysis indicate that a warmer climate will shorten the period of persistent snow pack by 73–93days,
increase the average soil temperature by 0.9–1.5°C at 10cm depth, advance soil warming by 15–19days in spring and cause
more soil freeze–thaw cycles by 31–38%. The results also predict that the large current variations in snow cover due to variations
in tree interception and topography will be enhanced in the coming century, resulting in increased spatial variability in
[show abstract][hide abstract] ABSTRACT: We quantified the utilization of terrestrial organic matter (OM) in the food web of a humic lake by analyzing
the metabolism and the consumers’ stable isotopic (C, H, N) composition in benthic and pelagic habitats.
Terrestrial OM inputs (3 g C m22 d21) to the lake greatly exceeded autochthonous OM production
(3 mg C m22 d21) in the lake. Heterotrophic bacterial growth (19 mg C m22 d21) and community respiration (115 mg C m22 d21) were high relative to algal photosynthesis and were predominantly (. 85%) supported by terrestrial OM in both habitats. Consequently, terrestrial OM fueled most (85%) of the total production at the base of the lake’s food web (i.e., the sum of primary and bacterial production). Despite the uncertainties of quantitatively estimating resource use based on stable isotopes, terrestrial OM clearly also supported around half the zooplankton (47%), macrozoobenthos (63%), and fish (57%) biomass. These results indicate that, although rates of terrestrial OM inputs were around three orders of magnitude greater than that of autochthonous OM production, the use of the two resources by higher trophic levels was roughly equal. The disproportionally low reliance on terrestrial OM at higher trophic levels, compared with its high rates of input and high support of basic biomass production in the lake, suggests that autochthonous resources could not be completely replaced by terrestrial resources and indicates an upper limit to terrestrial support of lake food webs.
Limnology and oceanography 04/2012; 57(4):1042-1048. · 3.41 Impact Factor