Current water quality guidelines across North America and Europe do not protect lakes from salinization.

Human-induced salinization caused by the use of road deicing salts, agricultural practices, mining operations, and climate change is a major threat to the biodiversity and functioning of freshwater ecosystems. Yet, it is unclear if freshwater ecosystems are protected from salinization by current water quality guidelines. Leveraging an experimental network of land-based and in-lake mesocosms across North America and Europe, we tested how salinization-indicated as elevated chloride (Cl-) concentration-will affect lake food webs and if two of the lowest Cl- thresholds found globally are sufficient to protect these food webs. Our results indicated that salinization will cause substantial zooplankton mortality at the lowest Cl- thresholds established in Canada (120 mg Cl-/L) and the United States (230 mg Cl-/L) and throughout Europe where Cl- thresholds are generally higher. For instance, at 73% of our study sites, Cl- concentrations that caused a ≥50% reduction in cladoceran abundance were at or below Cl- thresholds in Canada, in the United States, and throughout Europe. Similar trends occurred for copepod and rotifer zooplankton. The loss of zooplankton triggered a cascading effect causing an increase in phytoplankton biomass at 47% of study sites. Such changes in lake food webs could alter nutrient cycling and water clarity and trigger declines in fish production. Current Cl- thresholds across North America and Europe clearly do not adequately protect lake food webs. Water quality guidelines should be developed where they do not exist, and there is an urgent need to reassess existing guidelines to protect lake ecosystems from human-induced salinization.

A drained nutrient-poor peatland forest in boreal Sweden constitutes a net carbon sink after integrating terrestrial and aquatic fluxes.

Northern peatlands provide a globally important carbon (C) store. Since the beginning of the 20th century, however, large areas of natural peatlands have been drained for biomass production across Fennoscandia. Today, drained peatland forests constitute a common feature of the managed boreal landscape, yet their ecosystem C balance and associated climate impact are not well understood, particularly within the nutrient-poor boreal region. In this study, we estimated the net ecosystem carbon balance (NECB) from a nutrient-poor drained peatland forest and an adjacent natural mire in northern Sweden by integrating terrestrial carbon dioxide (CO2 ) and methane (CH4 ) fluxes with aquatic losses of dissolved organic C (DOC) and inorganic C based on eddy covariance and stream discharge measurements, respectively, over two hydrological years. Since the forest included a dense spruce-birch area and a sparse pine area, we were able to further evaluate the effect of contrasting forest structure on the NECB and component fluxes. We found that the drained peatland forest was a net C sink with a 2-year mean NECB of -115 ± 5 g C m-2 year-1 while the adjacent mire was close to C neutral with 14.6 ± 1.7 g C m-2 year-1 . The NECB of the drained peatland forest was dominated by the net CO2 exchange (net ecosystem exchange [NEE]), whereas NEE and DOC export fluxes contributed equally to the mire NECB. We further found that the C sink strength in the sparse pine forest area (-153 ± 8 g C m-2 year-1 ) was about 1.5 times as high as in the dense spruce-birch forest area (-95 ± 8 g C m-2 year-1 ) due to enhanced C uptake by ground vegetation and lower DOC export. Our study suggests that historically drained peatland forests in nutrient-poor boreal regions may provide a significant net ecosystem C sink and associated climate benefits.

Landscape-variability of the carbon balance across managed boreal forests.

Boreal forests are important global carbon (C) sinks and, therefore, considered as a key element in climate change mitigation policies. However, their actual C sink strength is uncertain and under debate, particularly for the actively managed forests in the boreal regions of Fennoscandia. In this study, we use an extensive set of biometric- and chamber-based C flux data collected in 50 forest stands (ranging from 5 to 211 years) over 3 years (2016-2018) with the aim to explore the variations of the annual net ecosystem production (NEP; i.e., the ecosystem C balance) across a 68 km2 managed boreal forest landscape in northern Sweden. Our results demonstrate that net primary production rather than heterotrophic respiration regulated the spatio-temporal variations of NEP across the heterogeneous mosaic of the managed boreal forest landscape. We further find divergent successional patterns of NEP in our managed forests relative to naturally regenerating boreal forests, including (i) a fast recovery of the C sink function within the first decade after harvest due to the rapid establishment of a productive understory layer and (ii) a sustained C sink in old stands (131-211 years). We estimate that the rotation period for optimum C sequestration extends to 138 years, which over multiple rotations results in a long-term C sequestration rate of 86.5 t C ha-1 per rotation. Our study highlights the potential of forest management to maximize C sequestration of boreal forest landscapes and associate climate change mitigation effects by developing strategies that optimize tree biomass production rather than heterotrophic soil C emissions.

On the uncertainty in estimates of the carbon balance recovery time after forest clear-cutting.

An essential metric for describing carbon dynamics in managed forest landscapes is the recovery time of the carbon balance after clear-cutting. Here, we demonstrate how the age-dependent mathematical trajectory is affected by both the selected model and data availability, leading to considerable uncertainty in the modelling of the net ecosystem production (NEP) over stand age. We further show that the initial carbon loss estimates associated with the timing of the source-sink transition (SST) are significant, but may have a limited effect on the total carbon sequestration at the end of the standard (RP, 120 years) or optimal (OCS) rotation periods.

The role of terrestrial productivity and hydrology in regulating aquatic dissolved organic carbon concentrations in boreal catchments.

The past decades have witnessed an increase in dissolved organic carbon (DOC) concentrations in the catchments of the Northern Hemisphere. Increasing terrestrial productivity and changing hydrology may be reasons for the increases in DOC concentration. The aim of this study is to investigate the impacts of increased terrestrial productivity and changed hydrology following climate change on DOC concentrations. We tested and quantified the effects of gross primary production (GPP), ecosystem respiration (RE) and discharge on DOC concentrations in boreal catchments over 3 years. As catchment characteristics can regulate the extent of rising DOC concentrations caused by the regional or global environmental changes, we selected four catchments with different sizes (small, medium and large) and landscapes (forest, mire and forest-mire mixed). We applied multiple models: Wavelet coherence analysis detected the delay-effects of terrestrial productivity and discharge on aquatic DOC variations of boreal catchments; thereafter, the distributed-lag linear models quantified the contributions of each factor on DOC variations. Our results showed that the combined impacts of terrestrial productivity and discharge explained 62% of aquatic DOC variations on average across all sites, whereas discharge, gross primary production (GPP) and RE accounted for 26%, 22% and 3%, respectively. The impact of GPP and discharge on DOC changes was directly related to catchment size: GPP dominated DOC fluctuations in small catchments (<1 km2 ), whereas discharge controlled DOC variations in big catchments (>1 km2 ). The direction of the relation between GPP and discharge on DOC varied. Increasing RE always made a positive contribution to DOC concentration. This study reveals that climate change-induced terrestrial greening and shifting hydrology change the DOC export from terrestrial to aquatic ecosystems. The work improves our mechanistic understanding of surface water DOC regulation in boreal catchments and confirms the importance of DOC fluxes in regulating ecosystem C budgets.

How tree species, tree size, and topographical location influenced tree transpiration in northern boreal forests during the historic 2018 drought.

Trees in northern latitude ecosystems are projected to experience increasing drought stress as a result of rising air temperatures and changes in precipitation patterns in northern latitude ecosystems. However, most drought-related studies on high-latitude boreal forests (>50°N) have been conducted in North America, with few studies quantifying the response in European and Eurasian boreal forests. Here, we tested how daily whole-tree transpiration (Q, Liters day-1 ) and Q normalized for mean daytime vapor pressure deficit (QDZ , Liters day-1 kPa-1 ) were affected by the historic 2018 drought in Europe. More specifically, we examined how tree species, size, and topographic position affected drought response in high-latitude mature boreal forest trees. We monitored 30 Pinus sylvestris (pine) and 30 Picea abies (spruce) trees distributed across a topographic gradient in northern Sweden. In general, pine showed a greater QDZ control compared to spruce during periods of severe drought (standardized precipitation-evapotranspiration index: SPEI < -1.5), suggesting that the latter are more sensitive to drought. Overall, QDZ reductions (using non-drought QDZ as reference) were less pronounced in larger trees during severe drought, but there was a species-specific pattern: QDZ reductions were greater in pine trees at high elevations and greater in spruce trees at lower elevations. Despite lower QDZ during severe drought, drought spells were interspersed with small precipitation events and overcast conditions, and QDZ returned to pre-drought conditions relatively quickly. This study highlights unique species-specific responses to drought, which are additionally driven by a codependent interaction among tree size, relative topographic position, and unique regional climate conditions.

The Net Landscape Carbon Balance-Integrating terrestrial and aquatic carbon fluxes in a managed boreal forest landscape in Sweden.

The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape consists of various sinks and sources of carbon dioxide (CO2 ), methane (CH4 ), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB). In this study, we compiled terrestrial and aquatic fluxes of CO2 , CH4 , DOC, DIC, and harvested C obtained from tall-tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km2 ) in Sweden to estimate the NLCB across the land-water-atmosphere continuum. Our results showed that this managed boreal forest landscape was a net C sink (NLCB = 39 g C m-2  year-1 ) with the landscape-atmosphere CO2 exchange being the dominant component, followed by the C export via harvest and streams. Accounting for the global warming potential of CH4 , the landscape was a GHG sink of 237 g CO2 -eq m-2  year-1 , thus providing a climate-cooling effect. The CH4 flux contribution to the annual GHG budget increased from 0.6% during spring to 3.2% during winter. The aquatic C loss was most significant during spring contributing 8% to the annual NLCB. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the temporal variability of the NLCB whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear-cutting) determined their spatial variability. Our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on tall-tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change.

Forest streams are important sources for nitrous oxide emissions.

Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N2 O). N2 O is a transformation product of nitrogenous compounds in soil, sediment and water. Agricultural areas are considered a particular hotspot for emissions because of the large input of nitrogen (N) fertilizers applied on arable land. However, there is little information on N2 O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N2 O concentration data from low-order streams (~1,000 observations from 172 stream sites) covering a large geographical gradient in Sweden from the temperate to the boreal zone and representing catchments with various degrees of agriculture and forest coverage. Our results showed that agricultural and forest streams had comparable N2 O concentrations of 1.6 ± 2.1 and 1.3 ± 1.8 µg N/L, respectively (mean ± SD) despite higher total N (TN) concentrations in agricultural streams (1,520 ± 1,640 vs. 780 ± 600 µg N/L). Although clear patterns linking N2 O concentrations and environmental variables were difficult to discern, the percent saturation of N2 O in the streams was positively correlated with stream concentration of TN and negatively correlated with pH. We speculate that the apparent contradiction between lower TN concentration but similar N2 O concentrations in forest streams than in agricultural streams is due to the low pH (<6) in forest soils and streams which affects denitrification and yields higher N2 O emissions. An estimate of the N2 O emission from low-order streams at the national scale revealed that ~1.8 × 109  g N2 O-N are emitted annually in Sweden, with forest streams contributing about 80% of the total stream emission. Hence, our results provide evidence that forest streams can act as substantial N2 O sources in the landscape with 800 × 109  g CO2 -eq emitted annually in Sweden, equivalent to 25% of the total N2 O emissions from the Swedish agricultural sector.

Towards ecologically functional riparian zones: A meta-analysis to develop guidelines for protecting ecosystem functions and biodiversity in agricultural landscapes.

Riparian zones contribute with biodiversity and ecosystem functions of fundamental importance for regulating flow and nutrient transport in waterways. However, agricultural land-use and physical changes made to improve crop productivity and yield have resulted in modified hydrology and displaced natural vegetation. The modification to the hydrology and natural vegetation have affected the biodiversity and many ecosystem functions provided by riparian zones. Here we review the literature to provide state-of-the-art recommendations for riparian zones in agricultural landscapes. We analysed all available publications since 1984 that have quantified services provided by riparian zones and use this information to recommend minimum buffer widths. We also analysed publications that gave buffer width recommendations to sustain different groups of organisms. We found that drainage size matters for nutrient and sediment removal, but also that a 3 m wide buffer zone acts as a basic nutrient filter. However, to maintain a high floral diversity, a 24 m buffer zone is required, while a 144 m buffer is needed to preserve bird diversity. Based on the analysis, we developed the concept of "Ecologically Functional Riparian Zones" (ERZ) and provide a step-by-step framework that managers can use to balance agricultural needs and environmental protection of waterways from negative impacts. By applying ERZ in already existing agricultural areas, we can better meet small targets and move towards the long-term goal of achieving a more functional land management and better environmental status of waterways.

The Role of Spring Flood and Landscape Type in the Terrestrial Export of Polycyclic Aromatic Compounds to Streamwater.

Concentrations of polycyclic aromatic compounds (PACs), including 19 polycyclic aromatic hydrocarbons (PAHs) and 15 PAH-derivatives (oxygenated and nitrogen heterocyclic PAHs), were measured in streams in a remote headwater catchment in northern Europe and in more urbanized, downstream areas. Sampling was conducted during 2014 to 2016 and included the main hydrological seasons (snow-free, snow-covered, and spring flood) at six sampling sites. Levels of the targeted PACs varied substantially over time and space and were up to 110-fold (on average 17-fold) and 7000-fold (on average 670-fold) higher for PAHs and PAH-derivatives, respectively, during spring flood compared with preceding snow-covered and snow-free seasons. Higher levels of ∑PACs were generally found in a headwater stream draining a mire than at an adjacent forested site, with up to 20 times and 150 times higher levels for ∑PAH and ∑PAH-derivatives, respectively. The particle-bound PAC levels were positively correlated to surface runoff in the mire stream (∑PAHs: p = 0.032; ∑PAH-derivatives: p = 0.040) but not in the corresponding forest stream, during snowmelt and winter base flow. In more urbanized downstream areas, particle-bound PACs were instead strongly associated with suspended particulate matter ( p < 0.05; ∑PAHs and ∑PAH-derivatives except one site). Levels of ∑PACs in the streamwater were on average 3-fold higher downstream of the most densely populated area than at the outlet of the headwater catchment. The higher PAC levels in the downstream water compared to the remote headwater were clearer when normalized to SPM amounts (instead of water volume), with a gradual downstream trend between the sites.

Aquatic export of young dissolved and gaseous carbon from a pristine boreal fen: Implications for peat carbon stock stability.

The stability of northern peatland's carbon (C) store under changing climate is of major concern for the global C cycle. The aquatic export of C from boreal peatlands is recognized as both a critical pathway for the remobilization of peat C stocks as well as a major component of the net ecosystem C balance (NECB). Here, we present a full year characterization of radiocarbon content (14 C) of dissolved organic carbon (DOC), carbon dioxide (CO2 ), and methane (CH4 ) exported from a boreal peatland catchment coupled with 14 C characterization of the catchment's peat profile of the same C species. The age of aquatic C in runoff varied little throughout the year and appeared to be sustained by recently fixed C from the atmosphere (<60 years), despite stream DOC, CO2 , and CH4 primarily being sourced from deep peat horizons (2-4 m) near the mire's outlet. In fact, the 14 C content of DOC, CO2 , and CH4 across the entire peat profile was considerably enriched with postbomb C compared with the solid peat material. Overall, our results demonstrate little to no mobilization of ancient C stocks from this boreal peatland and a relatively large resilience of the source of aquatic C export to forecasted hydroclimatic changes.

Atmospheric Transport and Deposition of Bromoanisoles Along a Temperate to Arctic Gradient.

Bromoanisoles (BAs) arise from O-methylation of bromophenols, produced by marine algae and invertebrates. BAs undergo sea-air exchange and are transported over the oceans. Here we report 2,4-DiBA and 2,4,6-TriBA in air and deposition on the Swedish west coast (Råö) and the interior of arctic Finland (Pallas). Results are discussed in perspective with previous measurements in the northern Baltic region in 2011-2013. BAs in air decreased from south to north in the order Råö > northern Baltic > Pallas. Geometric mean concentrations at Pallas increased significantly (p < 0.05) between 2002 and 2015 for 2,4-DiBA but not for 2,4,6-TriBA. The logarithm of BA partial pressures correlated significantly to reciprocal air temperature at the coastal station Råö and over the Baltic, but only weakly (2,4-DiBA) or not significantly (2,4,6-TriBA) at inland Pallas. Deposition fluxes of BAs were similar at both sites despite lower air concentrations at Pallas, due to greater precipitation scavenging at lower temperatures. Proportions of the two BAs in air and deposition were related to Henry's law partitioning and source regions. Precipitation concentrations were 10-40% of those in surface water of Bothnian Bay, northern Baltic Sea. BAs deposited in the bay catchment likely enter rivers and provide an unexpected source to northern estuaries. BAs may be precursors to higher molecular weight compounds identified by others in Swedish inland lakes.

Modeling the isotopic evolution of snowpack and snowmelt: Testing a spatially distributed parsimonious approach.

Use of stable water isotopes has become increasingly popular in quantifying water flow paths and travel times in hydrological systems using tracer-aided modeling. In snow-influenced catchments, snowmelt produces a traceable isotopic signal, which differs from original snowfall isotopic composition because of isotopic fractionation in the snowpack. These fractionation processes in snow are relatively well understood, but representing their spatiotemporal variability in tracer-aided studies remains a challenge. We present a novel, parsimonious modeling method to account for the snowpack isotope fractionation and estimate isotope ratios in snowmelt water in a fully spatially distributed manner. Our model introduces two calibration parameters that alone account for the isotopic fractionation caused by sublimation from interception and ground snow storage, and snowmelt fractionation progressively enriching the snowmelt runoff. The isotope routines are linked to a generic process-based snow interception-accumulation-melt model facilitating simulation of spatially distributed snowmelt runoff. We use a synthetic modeling experiment to demonstrate the functionality of the model algorithms in different landscape locations and under different canopy characteristics. We also provide a proof-of-concept model test and successfully reproduce isotopic ratios in snowmelt runoff sampled with snowmelt lysimeters in two long-term experimental catchment with contrasting winter conditions. To our knowledge, the method is the first such tool to allow estimation of the spatially distributed nature of isotopic fractionation in snowpacks and the resulting isotope ratios in snowmelt runoff. The method can thus provide a useful tool for tracer-aided modeling to better understand the integrated nature of flow, mixing, and transport processes in snow-influenced catchments.

Long-term declines in stream and river inorganic nitrogen (N) export correspond to forest change.

Human activities have exerted a powerful influence on the biogeochemical cycles of nitrogen (N) and carbon (C) and drive changes that can be a challenge to predict given the influence of multiple environmental stressors. This study focused on understanding how land management and climate change have together influenced terrestrial N storage and watershed inorganic N export across boreal and sub-arctic landscapes in northern Sweden. Using long-term discharge and nutrient concentration data that have been collected continuously for over three decades, we calculated the hydrologic inorganic N export from nine watersheds in this region. We found a consistent decline in inorganic N export from 1985 to 2011 over the entire region from both small and large watersheds, despite the absence of any long-term trend in river discharge during this period. The steepest declines in inorganic N export were observed during the growing season, consistent with the hypothesis that observed changes are biologically mediated and are not the result of changes in long-term hydrology. Concurrent with the decrease in inorganic N export, we report sustained increases in terrestrial N accumulation in forest biomass and soils across northern Sweden. Given the close communication of nutrient and energy stores between plants, soils, and waters, our results indicate a regional tightening of the N cycle in an already N-limited environment as a result of changes in forest management and climate-mediated growth increases. Our results are consistent with declining inorganic N efflux previously reported from small headwater streams in other ecosystems and shed new light on the mechanisms controlling these patterns by identifying corresponding shifts in the terrestrial N balance, which have been altered by a combination of management activities and climate change.

Persistent Organic Pollutants in Streamwater: Influence of Hydrological Conditions and Landscape Type.

Concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), and hexachlorobenzene (HCB) in streamwater were measured in a remote catchment in northern Sweden and downstream to the Baltic Sea. Sampling took place at seven sites during two years and under different hydrological conditions: during the snow-free, snow-covered, and spring-flood seasons. Concentrations varied substantially between seasons and were up to 20 times higher during the spring flood compared to the preceding snow-covered period. The increase in concentrations with runoff was due to higher levels of particle-associated contaminants, while the dissolved concentrations remained stable. Particulate-contaminant concentrations were positively correlated primarily to suspended particulate matter (SPM) at sites in areas with a high land-cover fraction of sorted sediment. When upstream sampling locations were compared, a mire-dominated stream had higher concentrations and a lower retention of atmospherically deposited contaminants than a forest stream of the same catchment size. Contaminant concentrations (normalized to volume) did not increase consistently downstream despite the presence of several point sources. However, when normalized to the amount of SPM, concentrations were on average >20 times higher at the outlet in the Baltic Sea compared to the outlet from the remote catchment without point sources.

Local and regional processes determine plant species richness in a river-network metacommunity.

River systems form dendritic ecological networks that influence the spatial structure of riverine communities. Few empirical studies have evaluated how regional, dispersal-related processes and local habitat factors interact to govern network patterns of species composition. We explore such interactions in a boreal watershed and show that riparian plant species richness increases strongly with drainage size, i.e., with downstream position in the network. Assemblage composition was nested, with new species successively added downstream. These spatial patterns in species composition were related to a combination of local and regional processes. Breadth in local habitat conditions increased downstream in the network, resulting in higher habitat heterogeneity and reduced niche overlap among species, which together with similar trends in disturbance, allows more species to coexist. Riparian edaphic conditions were also increasingly favorable to more species within the regional pool along larger streams, with greater nitrogen availability (manifested as lower C:N) and more rapid mineralization of C and N (as indicated by ratios of stable isotopes) observed with downstream position in the network. The number of species with the capacity for water dispersal increased with stream size, providing a mechanistic link between plant traits and the downstream accumulation of species as more propagules arrive from upstream sites. Similarity in species composition between sites was related to both geographical and environmental distance. Our results provide the first empirical evidence that position in the river network drives spatial patterns in riparian plant diversity and composition by the joint influence of local (disturbance, habitat conditions, and habitat breadth) and regional (dispersal) forces.

Mass Balance of Perfluorinated Alkyl Acids in a Pristine Boreal Catchment.

Mass balances of ten individual perfluorinated alkyl acids (PFAAs) in two nested pristine catchments in Northern Sweden with different sizes and hydrological functions were assembled for 2011-2012. Concentrations of PFAAs in rain and snowmelt, as well as in streamwater at the outlet of the two watersheds were measured and used to calculate PFAA atmospheric inputs to and riverine outputs from the catchments. The results generally showed a great excess of PFAA inputs for both catchments over the whole study year. However, during the spring flood period, the inputs and outputs were within a factor of 2 for several PFAAs and the streamwater showed PFAA patterns resembling the patterns in rain (as opposed to snowmelt), suggesting that snowmelt water infiltrating the ground had displaced water from the previous summer. Comparison of PFAA mass balances between the two catchments further suggested that atmospheric inputs of short-chain (replacement) perfluoroalkyl carboxylic acids had increased in the years before sampling, while inputs of the legacy perfluorooctane sulfonic acid had decreased. Overall, the mass balances indicate that a considerable portion of the PFAAs deposited from the atmosphere are stored in soil and may be released to surface and marine water environments in the future.

Groundwater discharge creates hotspots of riparian plant species richness in a boreal forest stream network.

Riparian vegetation research has traditionally focused on channel-related processes because riparian areas are situated on the edge of aquatic ecosystems and are therefore greatly affected by the flow regime of streams and rivers. However, due to their low topographic position in the landscape, riparian areas receive significant inputs of water and nutrients from uplands. These inputs may be important for riparian vegetation, but their role for riparian plant diversity is poorly known. We studied the relationship between the influx of groundwater (GW) from upland areas and riparian plant diversity and composition along a stream size gradient, ranging from small basins lacking permanent streams to a seventh-order river in northern Sweden. We selected riparian sites with and without GW discharge using a hydrological model describing GW flow accumulation to test the hypothesis that riparian sites with GW discharge harbor plant communities with higher species richness. We further investigated several environmental factors to detect habitat differences between sites differing in GW discharge conditions. Vascular plant species richness was between 15% and 20% higher, depending on the spatial scale sampled, at riparian sites with GW discharge in comparison to non-discharge sites, a pattern that was consistent across all stream sizes. The elevated species richness was best explained by higher soil pH and higher nitrogen availability (manifested as lower soil C/N ratio), conditions which were positively correlated with GW discharge. Base cations and possibly nitrogen transported by groundwater may therefore act as a terrestrial subsidy of riparian vegetation. The stable isotopes 15N and 13C were depleted in soils from GW discharge compared to non-discharge sites, suggesting that GW inputs might also affect nitrogen and carbon dynamics in riparian soils. Despite the fact that many flows of water and nutrients reaching streams are filtered through riparian zones, the importance of these flows for riparian vegetation has not been appreciated. Our results demonstrated strong relationships between GW discharge, plant species richness and environmental conditions across the entire stream size gradient, suggesting that both river hydrology and upland inputs should be considered to fully understand riparian vegetation dynamics.

Metal transport in the boreal landscape-the role of wetlands and the affinity for organic matter.

Stream water concentrations of 13 major and trace elements (Al, Ba, Ca, Cr, Cu, La, Mg, Na, Ni, Si, Sr, U, Y) were used to estimate fluxes from 15 boreal catchments. All elements displayed a significant negative correlation to the wetland coverage, but the influence of wetlands was stronger for organophilic metals; 73% of the spatial differences in the normalized element fluxes could be explained based only on the wetland coverage and the affinity for organic matter, which was quantified using thermodynamic modeling. When the analysis was restrained to the smaller streams (<10 km(2)) the explanatory power increased to 88%. The results suggest that wetlands may decrease the fluxes of metals from boreal forests to downstream recipients by up to 40% at otherwise similar runoff. We suggest that the decrease in element fluxes is caused by a combination of low weathering in peat soils and accumulation of organophilic metals in peat. The model could not explain the spatial patterns for some metals with low affinity for organic matter, some redox-sensitive metals, and some metals with exceptionally high atmospheric deposition, but the results still demonstrate that wetlands play an important role for the biogeochemical cycling of many metals in the boreal landscape.

Impact of forestry on total and methyl-mercury in surface waters: distinguishing effects of logging and site preparation.

Forestry operations can increase the export of mercury (both total and methyl) to surface waters. However, little is known about the relative contribution of different forestry practices. We address this question using a paired-catchment study that distinguishes the effects of site preparation from the antecedent logging. Runoff water from three catchments, two harvested and one untreated control, was sampled biweekly during one year prior to logging, two years after logging, and three years after site preparation. The logging alone did not significantly increase the concentrations of either total or methyl-mercury in runoff, but export increased by 50-70% in one of the harvested catchments as a consequence of increased runoff volume. The combined effects of logging and site preparation increased total and methyl-mercury concentrations by 30-50% relative to preharvest conditions in both treated catchments. The more pronounced concentration effect after site preparation compared to logging could be related to site preparation being conducted during summer. This caused more soil disturbance than logging, which was done during winter with snow covering the ground. The results suggest that the cumulative impact of forest harvest on catchment mercury outputs depends on when and how forestry operations are implemented.