Boreal Stream Network Research Articles

Multiscale Processes Drive Formation of Logjam Habitats and Use by Juvenile Chinook Salmon Across a Boreal Stream Network in Alaska

Multiscale Processes Drive Formation of Logjam Habitats and Use by Juvenile Chinook Salmon Across a Boreal Stream Network in Alaska

Landscape regulation of microbial use of terrestrial carbon in boreal streams

Microbes decomposing leaf litter in aquatic ecosystems are exposed to two major sources of carbon (C), namely, particulate organic C (POC) and dissolved organic C (DOC). The use of DOC relative to POC during litter decomposition likely depends on the availability of DOC, which in turn is regulated by the characteristics of the surrounding landscape, although this extrinsic indirect control of DOC use remains largely unexplored. We have investigated how variations in stream physical and chemical characteristics, distribution of major landscape elements (i.e., forest, mires, and lakes), and riparian vegetation community composition (i.e., relative cover of deciduous vs. coniferous tree species) influence DOC use by leaf-associated microbes (LAM). Specifically, in a boreal stream network of ten first- to third-order streams, we related in-stream characteristics, landscape elements, and riparian vegetation community composition to DOC/POC respiration ratios (i.e., the amount of CO2 produced by LAM respiration of DOC + POC divided by the amount of leaf C mass lost through decomposition). The results showed that DOC/POC respiration ratios were > 1 in most of the study sites, indicating that LAM use a substantial amount of DOC during leaf litter decomposition. This microbial reliance on DOC varied with in-stream DOC and nutrient concentrations, proportional mire and forest cover, and riparian vegetation community composition. In particular, high mire and coniferous cover increased DOC use by LAM. As such, landscape configuration and how it is modified by land use and climate change must be considered in order to understand microbial turnover of terrestrial C in boreal streams.

Differential Trends in Iron Concentrations of Boreal Streams Linked to Catchment Characteristics

AbstractIncreasing iron (Fe) concentrations have been reported for freshwaters across northern Europe over the last decades. This increase, together with elevated concentrations of dissolved organic carbon (DOC), leads to browning of freshwaters, which affects aquatic organisms, ecosystem functioning, biogeochemical cycles, and brings challenges to drinking water production. However, how such increasing trends in stream Fe concentrations reflect the contribution of different catchment sources remains poorly resolved. Here, we explored how catchment characteristics, that is, mires and coniferous soils, regulate spatial and temporal patterns of Fe in a boreal stream network. For this, we determined Fe speciation in riparian and mire soils, and studied temporal Fe dynamics in soil‐water and stream‐water over a span of 18 years. Positive Fe trends were found in the solution of the riparian soil, while no long‐term trend was observed in the mire. These differences were reflected in stream‐water, where three headwater streams dominated by coniferous cover also displayed positive Fe trends, whereas the mire dominated stream showed no trend. Surprisingly, the majority of higher order streams showed declining Fe trends, despite long‐term increases in DOC. In addition, we found that an extreme drought event led to a prolonged release of Fe and DOC from the riparian soils, that could have long‐term effects on stream Fe concentrations. Our results show that riparian forest soils can be major contributors to ongoing increases in freshwater Fe concentrations and that drought can further promote the release of Fe from organic soils.

Co‐occurrence of browning and oligotrophication in a boreal stream network

AbstractThe relative supply of carbon (C), nitrogen (N), and phosphorus (P) to freshwater ecosystems is of fundamental importance to aquatic productivity, nutrient cycling, and food web dynamics. In northern landscapes, ongoing climate change, as well as legacies from atmospheric deposition, have the potential to drive changes in how these elements are recycled on land and exported to streams. While it is well established that dissolved organic carbon (DOC) concentrations have increased in many high latitude streams, the simultaneous trends for N and P and the ratios among these resources, are not well documented. We used data from 13 sites in a boreal stream network to analyze decadal‐scale changes in dissolved inorganic N (DIN), dissolved organic N (DON), and dissolved inorganic P (DIP) concentrations and partition these trends seasonally. We observed widespread declines for DIP and DIN in streams, regardless of catchment characteristics. DIN decline was strongest during the growing season, and together with increases in DOC/DON at several sites, suggests increasing N retention by plants and soil microbes across this landscape. By contrast, declines for DIP occurred primarily during late autumn and winter, indicating that key biogeochemical changes are also occurring during non‐growing season. Linking these trends to increases in DOC concentration in streams revealed changes in the ratio of energy to nutrient supply for the majority of sites, becoming richer in carbon and poorer in limiting nutrients over time. Overall, our observations from this stream network point to ongoing oligotrophication, with possible consequences for aquatic ecosystems in boreal landscapes.

Seasonal variation in the coupling of microbial activity and leaf litter decomposition in a boreal stream network

Abstract Most stream networks are characterised by spatial and temporal variability in the physico‐chemical conditions that regulate microbial processing of particulate organic matter. How these patterns control the turnover of particulate organic matter via altered activity of leaf‐associated microbes has rarely been studied in high‐latitude landscapes, particularly throughout long (i.e., up to 6 months) ice‐ and snow‐covered periods. We investigated development of fungal biomass, enzyme activity, microbial respiration, and birch leaf litter decomposition from autumn to early summer in 11 nested streams in a boreal catchment that encompass a gradient in wetland (mire) cover. We observed relatively low variability in decomposition rates across the network, despite differences in key physical and chemical variables (e.g. temperature, pH, and dissolved organic carbon [DOC] concentrations) over time and space. Microbial enzymatic activity and respiration were positively related to leaf litter decomposition rates during early stages of decomposition (i.e., up to c. 30% loss of initial ash‐free dry mass). Thereafter, variation in microbial activity and respiration was decoupled from leaf litter mass loss, as enzymatic activity and respiration instead became positively related to DOC concentrations and upstream mire (wetland) cover among streams. Our results suggest that leaf‐associated microbes increase their reliance on external sources of energy over time. This switch in resource use was more evident in streams with higher DOC concentration, which in boreal landscapes is largely determined by mire cover. Hence, variation in DOC concentration, linked to landscape configuration, or from intensified land use and climate change, could affect how different carbon sources are used by stream microbial communities, with consequences for overall carbon cycling in boreal headwaters.

Changing Source‐Transport Dynamics Drive Differential Browning Trends in a Boreal Stream Network

AbstractDissolved organic carbon (DOC) concentrations are increasing in freshwaters worldwide, with important implications for aquatic ecology, biogeochemistry, and ecosystem services. While multiple environmental changes may be responsible for these trends, predicting the occurrence and magnitude of “browning” and relating such trends to changes in DOC sources versus hydrologic transport remain key challenges. We analyzed long‐term trends in DOC concentration from the two dominant landscape sources (riparian soils and mire peats) and receiving streams in a boreal catchment to evaluate how browning patterns relate to land cover and hydrology. Increases in stream DOC were widespread but not universal. Browning was most pronounced in small, forested streams, where trends corresponded to twofold to threefold increases in DOC production in riparian soils and increases in annual DOC export from a forested headwater. By contrast, DOC did not change in mire peats or streams draining catchments with high lake or mire cover, nor did we observe trends in DOC export from a mire‐dominated headwater. The distinct long‐term trends in DOC sources also altered concentration‐discharge relationships, with a forested headwater shifting from transport‐limited toward chemostasis, and a mire outlet stream shifting from chemostasis to source‐limitated. Modified DOC supply to headwaters, together with altered seasonal hydrology and differences in the dominant water source along the stream network gave rise to predictable browning trends and consistent concentration‐discharge relationships. Overall, our results show that the sources of DOC to boreal aquatic ecosystems are responding to environmental change in fundamentally different ways, with important consequences for browning along boreal stream networks.

Landscape control on the hydrogeochemistry of As, Co and Pb in a boreal stream network

In a boreal stream network, stream water concentrations of As, Co and Pb (filtered, <0.4μm) of 10 nested streams were studied during two consecutive years in order to evaluate the influence of land cover on the temporal and spatial variability of metal concentrations and speciation. Mean concentrations of Co and Pb showed significant but contrasting relationship to landscape type, while As concentrations were not related to landscape type. Highest concentrations of Pb were found in the wetland dominated streams (>30% wetland), which was suggested to be controlled by atmospheric deposition in combination with high DOC release from the wetlands. For Co, the highest concentrations were found in the forest dominated sites (>98% forest), which were attributed to the weathering of minerogenic sources. Contrasting response to runoff events could also be related to landscape type; during the spring flood, decreasing concentrations of As, Co and Pb were observed in the wetland dominated catchments due to dilution, while increasing concentrations during spring flood were observed in the mixed catchments (2–30% wetland) and to some degree in the forested catchments, probably due to flushing of the organic-rich riparian sources. Further, metal speciation was calculated using the geochemical equilibrium model Visual MINTEQ. This suggests that dissolved inorganic species of As and Co dominated in headwater streams with low pH while DOC had a major influencing role for Pb. In the larger mixed streams where pH was higher and precipitation of e.g. colloidal Fe and Mn (hydr)oxides was favoured, the major influencing factor was instead adsorption to colloidal Fe for As and Pb, while association to organic matter and colloids of e.g. Mn influenced the concentrations of Co. We thus conclude that landscape type and the magnitude of the runoff events are of great importance for the spatial and temporal variations of As, Co and Pb in this boreal stream network. Projected climate change, with increasing runoff, may therefore influence riverine concentrations and fluxes differently, depending on the prevailing landscape type.

Nitrogen export from a boreal stream network following forest harvesting: seasonal nitrate removal and conservative export of organic forms

Abstract. Clear-cutting is today the primary driver of large-scale forest disturbance in boreal regions of Fennoscandia. Among the major environmental concerns of this practice for surface waters is the increased mobilization of nutrients, such as dissolved inorganic nitrogen (DIN) into streams. But while DIN loading to first-order streams following forest harvest has been previously described, the downstream fate and impact of these inputs is not well understood. We evaluated the downstream fate of DIN and dissolved organic nitrogen (DON) inputs in a boreal landscape that has been altered by forest harvests over a 10-year period. The small first-order streams indicated substantial leaching of DIN, primarily as nitrate (NO3−) in response to harvests with NO3− concentrations increasing by ∼ 15-fold. NO3− concentrations at two sampling stations further downstream in the network were strongly seasonal and increased significantly in response to harvesting at the mid-sized stream, but not at the larger stream. DIN removal efficiency, Er, calculated as the percentage of "forestry derived" DIN that was retained within the stream network based on a mass-balance model was highest during the snowmelt season followed by the growing season, but declined continuously throughout the dormant season. In contrast, export of DON from the landscape indicated little removal and was essentially conservative. Overall, net removal of DIN between 2008 and 2011 accounted for ∼ 65 % of the total DIN mass exported from harvested patches distributed across the landscape. These results highlight the capacity of nitrogen-limited boreal stream networks to buffer DIN mobilization that arises from multiple clear-cuts within this landscape. Further, these findings shed light on the potential impact of anticipated measures to increase forest yields of boreal forests, such as increased fertilization and shorter forest rotations, which may increase the pressure on boreal surface waters in the future.

234U/238U in a boreal stream network — Relationship to hydrological events, groundwater and scale

Uranium isotopes (234U/238U) were measured in eight well-studied boreal streams of different sizes and characters and in seven deep groundwater wells (up to 11m deep) adjacent to these streams. The measurements covered an entire spring flood cycle, from winter baseflow conditions, through the peak flow and into the early summer, thereby capturing much of the hydrological dynamics in these systems. To deepen the interpretation of the spatiotemporal variability of the 234U/238U ratio a wide range of hydrochemical and geographical data were included in the analysis. Special attention was given to δ18O, the most widely used isotopic tracer for hydrological pathways and groundwater transit times, because high 234U/238U ratios have often traditionally been attributed to input of deep groundwater and long transit times. The results demonstrated that there was generally a gradual increase in 234U/238U ratios in groundwater with increasing catchment area, which based on δ18O data was caused by deeper groundwater pathways and longer residence times in larger catchments. A similar dependence on the catchment area was observed in the streams, which during baseflow conditions generally displayed higher 234U/238U ratios in the larger streams. This relationship remained as the peak flow arrived, although it coincided with a general decrease in the 234U/238U ratios. Simultaneously, the δ18O signal dropped, indicating a greater influence of meltwater and younger groundwater. When combined, this confirmed that the activation of shallower groundwater pathways in connection with the spring flood also activated shallower sources of uranium. In connection with the spring flood there was a strong, nearly 1:1 relationship between the streams and the adjacent groundwater wells in terms of 234U/238U ratios, suggesting that the Quaternary deposits were the main source for uranium. Despite large differences in uranium concentrations between different landscape types no effects could be seen on the isotope signature, indicating that the 234U/238U ratio is a reliable tracer for the primary source and that the fractionation after mobilisation probably is negligible. All in all, the results demonstrated that 234U/238U ratios and δ18O generally provide a consistent picture of the hydrological and biogeochemical functioning of the investigated catchment.

Mapping Temporal Dynamics in a Forest Stream Network—Implications for Riparian Forest Management

This study focuses on avoiding negative effects on surface waters using new techniques for identifying wet areas near surface waters. This would aid planning and designing of forest buffer zones and off-road forestry traffic. The temporal variability in the geographical distribution of the stream network renders this type of planning difficult. A field study was performed in the 68 km2 Krycklan Catchment to illustrate the variability of a boreal stream network. The perennial stream length was 140 km while the stream length during high-flow conditions was 630 km. Comparing the field-measured stream network to the network presented on current maps showed that 58% of the perennial and 76% of the fully expanded network was missing on current maps. Similarly, cartographic depth-to-water maps showed that associated wet soils constituted 5% of the productive forest land during baseflow and 25% during high flow. Using a new technique, maps can be generated that indicate full stream networks, as well as seasonally active streams and associated wet soils, thus, forestry planning can be performed more efficiently and impacts on surface waters can be reduced.

Metacommunity structure in a small boreal stream network

Current ecological frameworks emphasize the relative importance of local and regional drivers for structuring species communities. However, most research has been carried out in systems with discrete habitat boundaries and a clear insular structure. Stream networks deviate from the insular structure and can serve as excellent model systems for studying hierarchical community dynamics over different temporal and spatial extents. We used benthic invertebrate data from streams in a small northern Swedish catchment to test whether metacommunity dynamics change between seasons, across spatial hierarchies (i.e. at the whole catchment scale vs. the scales of first-order and second/third-order sites within the catchment) and between stream-order groups. We assessed metacommunity structure as a function of three relevant dispersal dimensions (directional downstream processes, along-stream dispersal and overland dispersal). These dispersal dimensions were related to species groups with relevant dispersal traits (flying capacity, drift propensity) and dispersal capacities (weak vs. strong) to elucidate whether the observed spatial signals were due to dispersal limitation or mass effects. Results showed complex community organization that varied between seasons, with the scale of observation, and with stream order. The importance of spatial factors and specific dispersal dimensions was highly dependent on the time of sampling and the scale of observation. The importance of environmental factors was more consistent in our analyses, but their effect on species community structure peaked at first-order sites. Our analyses of species dispersal traits were not unequivocal, but indicated that both mass effects and dispersal limitation could simultaneously contribute to the spatial signal at the scale of the whole catchment through different dispersal pathways. We conclude that the study of hierarchically organized ecosystems uncovers complex patterns of metacommunity organization that may deviate substantially from those of systems with insular structure and discrete habitat boundaries. Moreover, we show that dispersal constraints imposed by the dendritic structure of stream networks and distinct dispersal mechanisms (e.g. dispersal limitation) may be evident also at very small spatial extents. Thus, even at this small scale, a landscape management approach that takes the dendritic nature of stream networks into account is needed to effectively conserve stream biodiversity.

Sensitivity of pH in a boreal stream network to a potential decrease in base cations caused by forest harvest

Increased forest harvest with more whole-tree utilization can decrease base cations (BC) in soils and stream runoff. This paper analyses how reducing stream BC changes the capacity of a boreal stream network to buffer pH changes. We estimated change in stream pH during spring snowmelt in 60 locations throughout a 68 km2 boreal catchment in northern Sweden with different scenarios of BC removal from stream water ranging from 10 to 50 µequiv.·L–1. The pH decreased in all scenarios, and if BC decreased by 50 µequiv.·L–1, stream length with pH above the acid threshold pH 5 during spring snowmelt decreased from 82% to 44% of the stream network, whereas the stream length with pH above 5.5 decreased from 60% to 10%. The pH sensitivity of different stream reaches to reductions in BC was positively related to the slope of the catchment, forest cover, and forested mires, whereas it was negatively related to the percentage of agricultural fields. Because the long-term effect of different forestry practices on stream BC is unclear, there is all the more reason to evaluate BC sensitivity before, rather than after, eventual problems arise.

Temporal and spatial variability of dissolved inorganic carbon in a boreal stream network: Concentrations and downstream fluxes

Carbon dioxide (CO2) and dissolved inorganic carbon (DIC) concentrations and export were analyzed throughout a 67 km2 boreal stream network in northern Sweden. 700 DIC and CO2 samples from 14 subcatchments were collected in 2006 and 2007. All sites were consistently supersaturated in CO2 with respect to the atmosphere. Temporal variability of DIC and CO2 concentration was best correlated with discharge, with concentrations generally diluting at high discharge. However, the variability in CO2 concentration was also dependent on the specific pH range of the stream, as variability was greatest in acidic headwater streams and lowest in larger circumneutral streams. In the larger ones the increase in the CO2 proportion of DIC at increased discharge counteracts the dilution of CO2. The shift toward proportionally more CO2 of the DIC at higher discharge is caused by decline in pH. Spatial patterns showed that DIC and CO2 concentrations were best correlated with peatland coverage of the subcatchment. The highest concentrations were found in headwater streams draining peatlands. The downstream export of DIC from the catchment outlet constitutes 19% of the total downstream export of carbon (DIC + DOC), or 0.7 (±0.09) g C m−2 yr−1. This study demonstrates the importance of including fluvial fluxes of inorganic carbon in landscape carbon budgets via runoff, and also highlights the need to account for stream evasion of CO2 to the atmosphere in such estimates since it can be larger than the downstream DIC export.

Landscape scale patterns in the character of natural organic matter in a Swedish boreal stream network

Abstract. This paper defines landscape-scale patterns in the character of natural organic matter (NOM) and tests for relationships to catchment soil, vegetation and topography. The drainage network of a boreal catchment, subcatchment size 0.12–78 km2, in Northern Sweden was sampled in August 2002 during a period of stable low water flow. The NOM was characterized with UV/Vis spectroscopy, fluorescence, XAD-8 fractionation (%humic substances), gel permeation chromatography (apparent molecular weight), and elemental composition (C:N). The largest spatial variation was found for C:N, absorbance ratio, and specific visible absorptivity. The lowest variation was in fluorescence index, %humic substances and molecular retention time. The variation in total organic carbon (TOC), iron and aluminium concentration was more than twice that of C:N. Between headwater and downstream sites no significant changes were distinguished in the NOM character. At stream reaches, junctions and lakes little change (&lt;10%) in NOM character was observed. Common factor analysis and partial least squares regression (PLS) revealed that the spatial variation in surface coverage of lakes and mires could explain some of the variation of TOC and NOM character. Our suggestion is that the mosaic of landscape elements (different amounts of water from lakes, forest soil and mires) delivers NOM with varying characteristics to a channel network that mixes conservatively downstream, with possible small changes at some stream reaches, junctions and lakes.

Landscape variations in stream water SO 42− and δ 34S SO4 in a boreal stream network

Despite reduced anthropogenic deposition during the last decades, deposition sulphate may still play an important role in the biogeochemical cycles of S and many catchments may act as net sources of S that may remain for several decades. The aim of this study is to elucidate the temporal and spatial dynamics of both SO 4 2− and δ 34S SO4 in stream water from catchments with varying percentage of wetland and forest coverage and to determine their relative importance for catchment losses of S. Stream water samples were collected from 15 subcatchments ranging in size from 3 to 6780 ha, in a boreal stream network, northern Sweden. In forested catchments (<2% wetland cover) S–SO 4 2− concentrations in stream water averaged 1.7 mg L −1 whereas in wetland dominated catchments (>30% wetland cover) the concentrations averaged 0.3 mg L −1. A significant negative relationship was observed between S–SO 4 2− and percentage wetland coverage ( r 2 = 0.77, p < 0.001) and the annual export of stream water SO 4 2− and wetland coverage ( r 2 = 0.76, p < 0.001). The percentage forest coverage was on the other hand positively related to stream water SO 4 2− concentrations and the annual export of stream water SO 4 2− ( r 2 = 0.77 and r 2 = 0.79, respectively). The annual average δ 34S SO4 value in wetland dominated streams was +7.6‰ and in streams of forested catchments +6.7‰. At spring flood the δ 34S SO4 values decreased in all streams by 1‰ to 5‰. The δ 34S SO4 values in all streams were higher than the δ 34S SO4 value of +4.7‰ in precipitation (snow). The export of S ranged from 0.5 kg S ha −1 yr −1 (wetland headwater stream) to 3.8 kg S ha −1 yr −1 (forested headwater stream). With an average S deposition in open field of 1.3 kg S ha −1 yr −1 (2002–2006) the mass balance results in a net export of S from all catchments, except in catchments with >30% wetland. The high temporal and spatial resolution of this study demonstrates that the reducing environments of wetlands play a key role for the biogeochemistry of S in boreal landscapes and are net sinks of S. Forested areas, on the other hand were net sources of S.

Spatial heterogeneity of the spring flood acid pulse in a boreal stream network

Spatial and temporal patterns in streamwater acidity are ecologically important, but difficult to measure in parallel. Here we present the spatial distribution of streamwater chemistry relevant to acidity from 60 stream sites distributed throughout a 67 km 2 boreal catchment, sampled during a period of winter baseflow (high pH) and during a spring flood episode (low pH). Sites were grouped based on pH level and pH change from winter baseflow to spring flood. The site attributes of each pH group were then assessed in terms of both stream chemistry and subcatchment landscape characteristics. Winter baseflow pH was high throughout most of the stream network (median pH 6.4), but during the spring flood episode stream sites experienced declines in pH ranging from 0–1.6 pH units, resulting in pH ranging from 4.3–6.3. Spring flood pH was highest in larger, lower altitude catchments underlain by fine sorted sediments, and lowest in small, higher altitude catchments with a mixture of peat wetlands and forested till. Wetland-dominated headwater catchments had low but stable pH, while the spring flood pH drop was largest in a group of catchments of intermediate size which contained well-developed coniferous forest and a moderate proportion of peat wetlands. There was a trend with distance downstream of higher pH, acid neutralizing capacity (ANC) and base cation concentrations together with lower dissolved organic carbon (DOC, strongly negatively correlated with pH). This apparent scale-dependence of stream chemistry could be explained by a number of environmental factors which vary predictably with altitude, catchment area and distance downstream—most notably, a shift in surficial sediment type from unsorted till and peat wetlands to fine sorted sediments at lower altitudes in this catchment. As a result of the combination of spatial heterogeneity in landscape characteristics and scale-related processes, boreal catchments like this one can be expected to experience high spatial variability both in terms of chemistry at any given point in time, and in the change experienced during high discharge episodes. Although chemistry patterns showed associations with landscape characteristics, considerable additional variability remained, suggesting that the modeling of dynamic stream chemistry from map parameters will continue to present a challenge.

Hydrogeochemistry of Fe and Mn in small boreal streams: The role of seasonality, landscape type and scale

Stream water from a stream network of 15 small boreal catchments (0.03–67km2) in northern Sweden was analyzed for unfiltered (total) and filtered (<0.4μm) concentrations of iron (Fetot and Fe<0.4) and manganese (Mntot and Mn<0.4). The purpose was to investigate the temporal and spatial dynamics of Fe, Mn and dissolved organic carbon (DOC) as influenced by snow melt driven spring floods and landscape properties, in particular the proportion of wetland area. During spring flood, concentrations of Fetot, Fe<0.4, Mntot, Mn<0.4 and DOC increased in streams with forested catchments (<2% wetland area). In catchments with high coverage of wetlands (>30% wetland area) the opposite behavior was observed. The hydrogeochemistry of Fe was highly dependent on wetlands as shown by the strong positive correlation of the Fetot/Altot ratio with wetland coverage (r2=0.89, p<0.001). Furthermore, PCA analysis showed that at base flow Fetot and Fe<0.4 were positively associated with wetlands and DOC, whereas they were not associated during peak flow at spring flood. The temporal variation of Fe was likely related to varying hydrological pathways. At peak discharge Fetot was associated with variables like silt coverage, which highlights the importance of particulates during high discharge events. For Mn there was no significant correlation with wetlands, instead, PCA analysis showed that during spring flood Mn was apparently more dependent on the supply of minerogenic particulates from silt deposits on the stream banks of some of the streams. The influence of minerogenic particulates on the concentration of, in particular, Mn was greatest in the larger, lower gradient streams, characterized by silt deposits in the near-stream zone. In the small forested streams underlain by till, DOC was of greater importance for the observed concentrations, as indicated by the positive correlation of both Fetot and Fe<0.4 with DOC (r2=0.77 and r2=0.76, p<0.001) at the smallest headwater forest site. In conclusion, wetland area and DOC were important for Fe concentrations in this boreal stream network, whereas silt deposits strongly influenced Mn concentrations. This study highlights the importance of studying stream water chemistry from a landscape perspective in order to address future environmental issues concerning mobility of Fe, Mn and associated trace metals.

Impact of changing DOC concentrations on the potential distribution of acid sensitive biota in a boreal stream network

Abstract. DOC concentrations have increased in many surface waters in Europe and North America over the past few decades. As DOC exudes a strong influence on pH this DOC increase could have detrimental effects on acid sensitive biota in many streams and lakes. To investigate the potential implications of changes in the DOC concentration on stream water biota, we have used a mesoscale boreal stream network in northern Sweden as a case study. The network was sampled for stream water chemistry at 60 locations during both winter base flow and spring flood periods, representing the extremes experienced annually in these streams both in terms of discharge and acidity. The effect of changing DOC on pH was modeled for all sampling locations using an organic acid model, with input DOC concentrations for different scenarios adjusted by between −30% and +50% from measured present concentrations. The resulting effect on pH was then used to quantify the proportion of stream length in the catchment with pH below the acid thresholds of pH 5.5 and pH 5.0. The results suggest that a change in stream water DOC during base flow would have only a limited effect on pH and hence on the stream length with pH below the acid thresholds. During the spring flood on the other hand a change in DOC would strongly influence pH and the stream length with pH below the acid thresholds. For example an increase in DOC concentration of 30% at all sites would increase the proportion of stream length with pH below 5.5 from 37% to 65%, and the proportion of stream length with pH below 5.0 would increase from 18% to 27%. The results suggest that in high DOC waters, even a marginal change in the DOC concentration could impact acid sensitive biota in a large portion of the aquatic landscape.

Landscape‐scale variability of acidity and dissolved organic carbon during spring flood in a boreal stream network

Acidity is well known to influence stream biota, but the less well‐studied spatial and temporal distributions of acidity are likely to play a larger ecological role than average values. We present data on spatial variability of chemical parameters contributing to acidity during winter baseflow and spring flood periods in Krycklan, a fourth‐order boreal stream network in northern Sweden. Fifteen stream sites were monitored in subcatchments spanning 3 orders of magnitude in size and representing a wide range of percent wetland. At baseflow, pH ranged from 3.9 to 6.5 at the different sites. Baseflow dissolved organic carbon (DOC) concentration varied by an order of magnitude and was positively correlated with subcatchment percent wetland, resulting in high spatial variability in dissociated organic acids (OA−). During spring flood, DOC and OA− increased in forested sites and decreased in wetland sites, resulting in reduced spatial variability in their concentrations. In contrast, base cations and strong acid anions diluted throughout the stream network, resulting in decreased acid neutralizing capacity (ANC) at all sites. The spatial variability of base cations increased slightly with high flow. As a result of the changes in OA− and ANC, pH dropped at all but the most acidic site, giving a slightly narrowed pH range during spring flood (4.2–6.1). The transition from winter to spring flood stream chemistry could largely be explained by: (1) a shift from mineral to upper riparian organic soil flow paths in forested catchments and (2) dilution of peat water with snowmelt in wetland catchments.