Dissolved Organic Carbon accounts for many different functions in the boreal forest ecosystem. It is the main vehicle for organic carbon transport from the litterfall to the forest soil profile and together with water drainage it can be transported to streams. In boreal forests, the DOC transport have gain attention because of recently documented rise in concentration. Several models have been proposed, first to gain understanding in the main cauces of this increase in concentration, and then to simulate the transport of DOC in the landscape. An exploratory work was made to identify the extent of physical control and hydrological pathways for DOC discharge and the long-term biological control over DOC production, transport in the soil profile and discharge in 2 different situations. A 22-year dataset from the Krycklan Catchment site was used. Meteorological data was used as driving variables to calibrate DOC concentration and runoff in a small catchment (Site C7). The CoupModel was set up to represent the described vegetation and documented soil characterization and then calibrated to fit the measured variables. A stepwise calibration process was preferred to promote the understanding of the different components of the landscape in the organic carbon cycle. Results point to soil heat and water transfer processes as the most relevant group to explain both water runnof and DOC discharge, with increasing relevance in the deeper layers, explaining up to 97% of short-term variability in DOC discharge for the 27-35 cm layer. Soil organic carbon pools showed to have relevance in organic carbon stock balance along the soil profile. Conclusions state that, In concordance with other authors, there is a hydrological primary control over DOC discharge, but that soil organics and especially vegetation perform a relevant role in long-term balance of the organic carbon cycle. Further studies with this model could include time-series of atmospheric deposition of Sulphur and nitrogen and running the model in cascade.

Hydrological drought at high latitudes represents a rising environmental hazard induced byglobal climate change. Yet, we still know little about how drought events influence thebiogeochemistry of boreal streams. Here, I used 15 years of data from eight streams withinthe Krycklan Catchment to test how interannual variability in summer low flows influencesstream water chemistry. My analysis focused several key biogeochemical indicators in thesestreams, including concentrations of dissolved organic carbon (DOC), dissolved organicnitrogen (DON), nitrate (NO3) and ammonium (NH4), as well as the total C/N and NH4/NO3ratios. Overall, results revealed widespread declines in summer average DOC concentrationsand C/N ratios with greater drought severity. These responses likely reflect shifts in thebiogeochemical properties of soils that feed streams during high- versus low-flow summers.By comparison, nitrogen-based parameters were less clearly influenced by drought, exceptfor in mire-dominated headwaters, where NH4 and DON both increased during the lowestflow periods. Overall, the strong effects of flow variability drove a high degree of interannualsynchrony for DOC and C/N across all sites in the drainage system. This synchrony was morevariable overall for nitrogen-based parameters, with several sites having unique year-to-yearchanges in concentrations and ratios. However, strong temporal coherence for NH4 acrossforested streams suggest other broad-scale factors (e.g., related to forest processes) mayregulate interannual patterns for this nutrient. Collectively, results provide insight into howincreases in drought frequency and severity may alter boreal streams and rivers in the future.

Iron (Fe) speciation is important for many biogeochemical processes. The high abundance and limited solubility of Fe(III) are responsible for the widespread occurrence of Fe(III) minerals in the environment. Co-precipitation and adsorption onto mineral surfaces limits the free concentrations of compounds such as arsenate (As(V)), Fe(III) and, phosphate (P(V)). Mineral dissolution, on the other hand, might lead to elevated concentrations of these compounds. Fe speciation is strongly affected by natural organic matter (NOM), which suppresses hydrolysis of Fe(III) via complexation. It limits the formation of Fe(III) minerals and Fe(III) co-precipitation. This thesis is focused on interactions between Fe(III) and NOM as well as their impact on other elements (i.e. As(V) and P(V)). X-ray absorption spectroscopy (XAS) was used to obtain molecular scale information on Fe and As speciation. This was complemented with infrared spectroscopy, as well as traditional wet-chemical analysis, such as pH and total concentration determinations. Natural stream waters, soil solutions, ground water and soil samples from the Krycklan Catchment, in northern Sweden, were analyzed together with model compounds with different types of NOM. A protocol based on ion exchange resins was developed to concentrate Fe from dilute natural waters prior to XAS measurements. Iron speciation varied between the stream waters and was strongly affected by the surrounding landscape. Stream waters originating from forested or mixed sites contained both Fe(II, III)-NOM complexes and precipitated Fe(III) (hydr)oxides. The distribution between these two pools was influenced by pH, total concentrations and, properties of NOM. In contrast, stream waters from wetland sites and soil solutions from a forested site only contained organically complexed Fe. Furthermore, the soil solutions contained a significant fraction Fe(II)-NOM complexes. The soil samples were dominated by organically complexed Fe and a biotite-like phase. Two pools of Fe were also identified in the ternary systems with As(V) or P(V) mixed with Fe(III) and NOM: all Fe(III) was complexed with NOM at low total concentrations of Fe(III), As(V) and/or P(V). Hence, Fe(III) complexation by NOM reduced Fe(III)-As(V)/P(V) interactions at low Fe(III) concentrations, which led to higher bioavailability. Exceeding the Fe(III)-NOM complex equilibrium resulted in the occurrence of Fe(III)-As(V)/P(V) (co-)-precipitates.

Inland waters are important systems for transforming, storing and transporting carbon along the aquatic continuum, but also by emitting carbon dioxide (CO2) and methane (CH4) to the atmosphere. In light of the last decades observed increase in dissolved organic carbon (DOC) in many inland waters across the northern hemisphere, a logical question arise whether other aquatic carbon species display similar trends. This study examined the measured concentrations of dissolved inorganic carbon (DIC) in a boreal catchment over a 14-year period. The objectives were to determine changes in DIC concentration over time and try to explain the causes for the observed changes. Data from 15 mostly forested sub-catchments were analyzed, both over the full time period, and grouped by season. Over the full 14-year period, only two of the sites exhibited significant trends in DIC concentration, both being negative. However, by seasonally grouping the data distinct patterns for the different seasons emerged. The autumn and winter data displayed no significant trends, whereas the spring flood data showed significant negative trends for almost all sites (14 out of 15). The summer data showed significant negative trends for seven sites, and positive for one site. The DIC concentration data were expectedly positively correlated with pH across most sites (13 out of 15). The correlation with DOC was negative for most sites (11 out of 15), possibly indicating different origins of the different carbon species. The DIC concentration was also negatively correlated with discharge for most sites (13 out of 15), suggesting a diluting effect with increased discharge. In conclusion, significant negative trends were observed during the spring flood and summer periods. Although the cause of these trends will require further investigation, the correlation analysis showed that the DIC concentration was closely related to the catchment hydrology. This suggests changes in terrestrial source areas where DIC is mobilized during spring and summer, and that these changes might continue during altered hydrometeorological conditions. The differences in DIC trends between sub-catchments further show the variability of the boreal landscape and highlight the need for local-scale process understanding when scaling to larger landscape units. We further conclude that trends in DIC concentration do not follow observed DOC changes over time, suggesting that DIC and DOC exports are mechanistically decoupled.

Compiling comprehensive understanding of all the available natural resources is an important task which should be carried out as it holds a crucial role for the next generation’s lives. In particular, groundwater is considered as one of the vital resources in providing essential drinking water. Krycklan catchment is a well-monitored catchment in Sweden that is characterized with almost 30% of the world’s forest cover and it has a range of data sets stored from 1920. A numerical model with several observational constrains is used in this study to investigate the groundwater flow circulation. The numerical model is developed with Visual MODFLOW Flex 6.1 software to investigate the temporal effects of the climate condition on the groundwater flow of the Krycklan catchment through a transient-state condition. Daily precipitation and daily evapotranspiration data along with stream data are used to represent the climatic boundary conditions. The impact of climatic condition on groundwater flow was investigated using two different metrices: groundwater level, and groundwater flow travel time reaching the stream network. The results clearly indicated the variability in groundwater level due to the impact of climatic condition in which the winter and summer months have the highest and lowest groundwater levels, respectively. In addition, the particles tracing results show that physical characteristics of the stream channel substantially influence the shallow groundwater travel time.

What drives species diversity across landscapes is one of the most fundamental questions in ecology. Further, understanding the mechanisms underlying species diversity patterns is important not only for forming and challenging ecological theories but also essential for appropriate landscape management and effective nature conservation. This thesis focuses on patterns of vascular plant, moss and liverwort species richness and composition in relation to water flow in boreal-forest catchments, focusing mostly on riparian zones (RZs), that is terrestrial areas bordering streams and rivers. I addressed some of the most essential questions related to the ecology of riparian vegetation including the role of stream network position, groundwater (GW) flow paths, substrate availability, upland perturbations, and stream restoration. I also investigated how riparian soil processes and habitat properties relate to these factors in order to provide a holistic understanding of riparian dynamics. The results showed that the species richness and composition of riparian vascular plants, mosses and liverworts are strongly influenced by position along the stream network, GW discharge, presence of variable substrates in RZs, and by stream restoration. Generally, more species were found downstream in the network, at sites with inputs of upland GW, sites with high diversity of substrates (e.g., open mineral soil, rocks, stones, wood and bark), and along streams restored after channelization. This thesis also describes how riparian habitat properties responded to position in the landscape and human impacts, thus providing mechanistic links between plant species diversity and riparian processes across spatial scales. These ecological insights are further implemented into numerous recommendations for freshwater and upland management in boreal Sweden. Given that streams and rivers connect landscape elements both longitudinally and laterally I argue that management plans should be designed for entire catchments instead of individual river segments. Ignoring the connectivity of streams as well as the high connectivity of riparian areas to uplands via GW flows may result in failure of restoration, mitigation and/or protection actions. Further, during forestry operations more emphasis should be placed on GW discharge areas along streams and rivers, because they represent important ecological and biogeochemical hotspots in the landscape. The riparian buffers left along streams in boreal catchments affected by forestry are presently insufficiently wide and often uniform in width. This threatens the assemblages of species in GW discharge hotspots and the ecosystem services they provide. Overall, this thesis describes a holistic picture of riparian diversity patterns and riparian processes in boreal landscapes, acknowledges and elaborates on current ecological theories, presenting new patterns in biodiversity, and offers management guidelines.

This thesis aimed to evaluate the groundwater discharge distribution in the Krycklan catchment by developing a groundwater flow model using COMSOL Multiphysics and assuming a topography- controlled groundwater surface. Previous research has shown that the groundwater surface can be modeled as a subdued replica of the topography in humid climate regions where the permeability of the subsurface is low and where the groundwater surface is shallow. In earlier studies by Mojarrad (2021), it has been shown that the modeled infiltration becomes higher than the observed infiltration when a topography-controlled groundwater surface boundary is used and that a solution to this is to decrease the resolution in recharge areas. This method was therefore used in the thesis; however, the modeled infiltration was unsuccessfully lowered. This is thought to be due to differences between the model and the previous study and which are discussed further in this thesis. The discharge and recharge areas were identified using the direction of the vertical component of Darcy velocity, and the discharge flow distribution was evaluated in ArcMAP. The discharge flow distribution in the landscape was compared to real geographical data of surface water to identify a discharge threshold value for when the water balance is upheld by surface water flow or by evapotranspiration. The evapotranspiration discharge flow distribution was also evaluated, where the highest flow values were found in riparian zones of the landscape. / Syftet med denna avhandling var att utvärdera föredelningen av grundvattnets utflöde i Krycklans avrinningsområde genom att utveckla en grundvattenflödesmodell i COMSOL Multiphysics med antagandet av en topografikontrollerad grundvattenyta. Tidigare forskning har visat att grundvattenytan kan modelleras som en dämpad kopia av topografin i fuktiga klimatområden där permeabiliteten i underytan är låg och där grundvattenytan är grund. En implikation av att använda en topografikontrollerad grundvattenyta har visat sig vara att den modellerade infiltrationen blir högre jämfört med den observerade infiltrationen. Tidigare studier av Krycklans avrinningsområde har visat att genom att minska upplösningen i infiltrationsområden kan den modellerade infiltrationen framgångsrikt sänkas, därför användes denna mesh-utjämningsmetod i avhandlingen. För validering av modellen jämfördes den modellerade infiltrationen med den observerade infiltrationen i Krycklans avrinningsområde. Valideringen visade att mesh-utjämningen misslyckades med att minska infiltrationen, vilket tros bero på skillnader mellan modellen och den tidigare studien och som diskuteras vidare i denna avhandling. Grundvattnets infiltration- och utflödesområden identifierades med hjälp av riktningen på den vertikala komponenten av Darcy-hastighet, och utflödesfördelningen utvärderades i ArcMAP. Utflödesfördelningen i landskapet jämfördes med verkliga geografiska data för ytvatten för att identifiera ett tröskelvärde för utflödet när vattenbalansen upprätthålls av ytvattenflöde eller av evapotranspiration. Fördelningen av utflöden genom evapotranspirations utvärderades också, där de högsta flödesvärdena återfanns i områden nära ytvattenkroppar i landskapet.