ABSTRACT Using daily precipitation dataset (>100 years) from 3 meteorological stations in ... more ABSTRACT Using daily precipitation dataset (>100 years) from 3 meteorological stations in southern, central and northern areas, we evaluated long-term variations in characteristics of extreme precipitation (intensity, frequency and duration) in Finland. For intensity, two indices of AP95p as Annual Precipitation computing from days with precipitation >95th percentile of wet days (>1mm), and RX1day as one-day maximum precipitation were evaluated. For frequency and duration analyses, N95p (number of precipitation events exceeding long-term 95th percentile of days with precipitation >0.00 mm) and LCWD (longest consecutive wet days) were defined, respectively. Mann-Kendall non-parametric test for determining significant trends and Fourier series fitting for showing possible cyclic patterns were used. The AP95p index significantly (p<0.05) increased in southern, central and northern Finland by 0.24, 1.17 and 1.31 (mm/year) during the last century, respectively, while no clear trends were found in RX1day. The N95p increased (0.03 and 0.02 day/year) in southern and northern, while decreased (0.03 day/year) in central Finland. No significant trends were determined in LCWD index over Finland. Estimated time cycles for a low-high mode of AP95p were 89, 222 and 232 years in sequence over southern, central and northern Finland. Time cycles for RX1day ranged from 3 to 21 years over Finland. For N95p, time cycle was 94 years in central, but about 3 to 4 years in other areas of Finland. Time cycle for LCWD was 4, 6 and 12 years in central, southern and northern regions. Results clearly indicate significant increasing trends in extreme precipitation intensity in Finland during the last century. The frequency of extreme precipitation increased in northern and southern Finland, while it decreased in central Finland. Besides, extreme precipitation duration showed cyclic patterns following in Finland. Extreme precipitation characteristics as design parameters for drainage and flood systems are changing and need to be re-evaluated more precisely.
Interest in climate change effects on groundwater has increased dramatically during the last deca... more Interest in climate change effects on groundwater has increased dramatically during the last decade. The mechanisms of climate-related groundwater depletion have been thoroughly reviewed, but the influence of global warming on groundwater dependent ecosystems (GDEs) remains poorly known. Here we report long-term water temperature trends in 66 northern European cold-water springs. A vast majority of the springs (82%) exhibited a significant increase of water temperature during 1968-2012. Mean spring water temperatures were closely related to regional air temperature and global radiative forcing of the corresponding year. Based on three alternative climate scenarios representing low (RCP2.6), intermediate (RCP6) and high emission scenarios (RCP8.5), we estimate that increase of mean spring water temperature in the region is likely to range from 0.67 °C (RCP2.6) to 5.94 °C (RCP8.5) by 2086. According to the worst-case scenario, water temperature of these originally cold-water ecosystems (regional mean in the late 1970s: 4.7 °C) may exceed 12 °C by the end of this century. We used bryophyte and macroinvertebrate species data from Finnish springs and spring-fed streams to assess ecological impacts of the predicted warming. An increase in spring water temperature by several degrees will likely have substantial biodiversity impacts, causing regional extinction of native, cold-stenothermal spring specialists, whereas species diversity of headwater generalists is likely to increase. Even a slight (by 1 °C) increase of water temperature may eliminate endemic spring species, thus altering bryophyte and macroinvertebrate assemblages of spring-fed streams. Climate change-induced warming of northern regions may thus alter species composition of the spring biota and cause regional homogenization of biodiversity in headwater ecosystems. This article is protected by copyright. All rights reserved.
ABSTRACT Erosion from peat extraction areas is known to cause siltation of water courses and poor... more ABSTRACT Erosion from peat extraction areas is known to cause siltation of water courses and poor water quality. However, the main soil parameters affecting peat erosion and suspended sediment (SS) yields from different catchments are not well understood. This paper used peat properties (degree of humification, peat type, ash content, porosity, moisture content, bulk density, and shear strength) and novel erosion threshold measurements from intact soil cores to explain peat erodibility and spatial variations in SS concentrations (SSC) and SS loads (SSL) at 20 Finnish peat extraction sites. The erosion threshold measurements suggested that critical shear stresses for particle entrainment decrease with increasing degree of humification (von Post scale) and are significantly lower in well-decomposed peat than in slightly or moderately decomposed peat. Two critical shear stresses were obtained from moderately decomposed peat samples, indicating a degree of surface armoring by coarse peat fibers. Monitored long-term average SSC was highest at study sites with well-decomposed peat, while very fine-grained mineral subsoil explained some of the highest long-term SSC in areas where drainage ditches penetrated below the upper peat layer. Average SSL (kg d-1) at the study sites was best explained (R2 = 0.89) by average discharge and surface peat decomposition level. Overall, this study provides new knowledge on peat erosion and critical shear stresses that can be used in water conservation and sediment management practices for cutover peatlands and other similar land uses.
ABSTRACT In this study, we analysed stable isotopes and water quality of runoff water collected d... more ABSTRACT In this study, we analysed stable isotopes and water quality of runoff water collected daily from two different peatland drainage areas with automated samplers from March 2012 to October 2012, located in Northern Finland. In addition we collected weekly snow samples for stable isotope analysis. Our primary aim was to find out how different land use types, i) peat extraction area and ii) peatland forestry, are affecting the flow paths and runoff water quality during the snow melt period. Results show that there is a clear difference in δO18 signal between these systems. The peatland forestry area is located at groundwater dominated area which can be seen as a flat line when δO18 values of all samples are plotted. Samples taken at the peat extraction area show a clear response to the snowmelt event. Most likely this difference is caused by different soil frost conditions. Quantity of the groundwater at the forestry area prevents the soil from freezing during winter, therefore water originating from melting snow is able to infiltrate to the peat soil and push pre-event water into the drainage system. This observation is also visible in water quality of runoff water as high peak in colour during the snow melt period. Contrary, the peat extraction area behaves in opposite way. Melting water from snow is not able to infiltrate to ditches but instead will rapidly move on the frozen soil surface as a Hortonian overland flow. Because the soil is frozen, moving water is not able to leach humic substances from soil layers or erode particulate matter from the soil surface. These observations can be used to develop water quality protection policies for drained peatland areas. In Northern areas, where freezing of soil during winter is common it is not crucial to emphasize water protection during spring snowmelt, as frozen soil helps to maintain the runoff water quality at reasonable levels. In the areas where ground frost is rarer the impact of purifying runoff water in spring thaw will be more beneficial for the receiving water bodies.
ABSTRACT Construction of dams affects river flow regimes and changes downstream ecosystems in man... more ABSTRACT Construction of dams affects river flow regimes and changes downstream ecosystems in many ways. The impacts of dams depend on local climate, dam purpose and operation policy. This study analysed the impacts of dam construction on river flow regimes and developed a new method to evaluate the effects on river flow. By using three main flow characteristics of river regime (magnitude, timing and intra-annual alteration), a new combined river impact (RI) index to assess construction impacts based on monthly flow data was developed. A preliminary evaluation of the method was made using the river Ebro as a case. A classification was developed where the impact was assessed using a scale from low to drastic. The method was further tested to quantify the effect of dams in 15 rivers located in different regions. The method showed good potential to classify different dam types of variable sizes constructed for different purposes. The RI index method also proved suitable for assessing the environmental impacts of dam construction and could be extended by adding other characteristics of flow regime to evaluate the effects of climate change or changes in land use on river flow regimes.
ABSTRACT Treatment wetlands are considered best management practice as they can remove nitrogen, ... more ABSTRACT Treatment wetlands are considered best management practice as they can remove nitrogen, phosphorus and suspended solids from peat extraction or forestry runoff. Treatment wetlands are established on intact or formerly drained peatlands after restoration of the site hydrology by ditch blocking or other methods. However, drainage changes the physical properties of peat, so the hydraulic and hydrological conditions in peatlands may change. Consequently, treatment wetlands constructed on drained areas may have short residence times, resulting in poor purification results. This study determined the hydrological and hydraulic characteristics of treatment wetlands constructed on drained peatlands by studying peat hydraulic conductivity and water distribution in 20 different wetlands in Finland. Four wetlands were studied in more detail using tracer tests, water balance and continuous groundwater measurements. More than 50% of the drained treatment wetlands studied had a lower active flow depth layer (only 10 or 20 cm) with higher hydraulic conductivity (K) than normally found in pristine peatlands. At 25% of the sites, K was high at all depths investigated (down to 60–70 cm). At several sites, the water was unequally distributed across the wetland surface, indicating that only part of the wetland was being used for purification. The residence time, as determined by tracer tests, varied significantly between sites. These results indicate that past drainage still influences the hydraulics of treatment wetlands.
Transport of particulate pollutants in fluvial systems can contribute greatly to total loads. Und... more Transport of particulate pollutants in fluvial systems can contribute greatly to total loads. Understanding transport mechanics under different hydrological conditions is key in successful load estimation. This study analysed trace elements and physico-chemical parameters in time-integrated suspended sediment samples, together with dissolved and total concentrations of pollutants, along two agriculture- and peatland-dominated boreal river systems. The samples were taken in a spatially and temporally comprehensive sampling programme during the ice-free seasons of 2010 and 2011. The hydrochemistry and transport of particle-bound elements in the rivers were strongly linked to intense land use and acid sulphate soils in the catchment area, with arable, pasture and peat areas in particular being main diffuse sources. There were significant seasonal and temporal variations in dissolved and particulate fluxes, but spatial variations were small. Continuous measurements of EC, turbidity and discharge proved to be an accurate indicator of dissolved and particulate fluxes. Overall, the results show that transport of particle-bound elements makes a major contribution to total transport fluxes in agriculture-dominated boreal rivers.
ABSTRACT Peat extraction for energy purposes causes major changes in the aquatic and terrestrial ... more ABSTRACT Peat extraction for energy purposes causes major changes in the aquatic and terrestrial environment. According to national strategies for extracting peat in Finland, new peat extraction areas should be established on previously drained peatlands. On such areas it is difficult to find the natural, intact peatland required for treatment wetlands or so-called overland flow areas, which are considered the best available technology for runoff water purification. Therefore treatment wetlands must be constructed on drained peatland. It is known that drainage causes physical, biogeochemical and hydraulic changes in the peat layer, as well as changes in vegetation. It is probable that these changes affect the purification efficiency of wetland treatment systems in many ways. This study evaluated the function and characteristics of drained peatland areas for purification of peat extraction runoff water. Study sites were established on 11 drained wetlands and their purification efficiency was evaluated. Detailed measurements of peat physical properties and hydraulic conductivity, as well as studies on vegetation, were also made in the study areas. The results showed that wetlands constructed on drained peatland areas can purify peat extraction runoff. However, leaching of phosphorus (P) and iron (Fe) was observed in some areas. Leaching is influenced e.g. by pH and the soil P pool. The chemical oxygen demand was also observed to increase in runoff water from the wetland. The results indicated that low (Fe + aluminium (Al) + manganese (Mn))/P ratio (≤ 25) and quite high P content (> 1200 mg/kg) in the surface peat characterised those areas where P leaching was observed. The presence of a dense tree stand in a drained peatland area also seemed to indicate release of nutrients from the area after its rewetting and use as a treatment wetland. Thus, potential nutrient release from a drained peatland area intended for use as a treatment wetland can be assessed by studying the characteristics of the peatland area, especially the peat mineral content, and the vegetation, especially tree stand density in the area. Using the findings obtained, a conceptual decision tree was drawn up in order to help to establish and design wetlands in previously drained peatland areas.
ABSTRACT Peatland drainage can have major ecological and biogeochemical impacts on aquatic enviro... more ABSTRACT Peatland drainage can have major ecological and biogeochemical impacts on aquatic environments in boreal regions. In the Sanginjoki river catchment in North Ostrobothnia, Finland, peatland drainage is suspected to increase the acidity of recipient waters. Possible reasons are acid sulphate soils (ASS) or black schist deposits, which occur especially in lower parts of the river basin, or increased transport of humic substances. It is not known how long acidification has affected water quality and biota in the river. To study historical pH levels, we examined sediment cores from two ponds in the main channel of the Sanginjoki river and one small lake in the Sanginjoki catchment. Diatom and geochemical analysis and 137Cs-derived chronology were combined to estimate historical pH levels and changes in land use practices. The results suggest that changes in land use in the 1970s have affected runoff acidity. During that period, there was a change in diatom communities and sediment geochemistry, indicating that land use in the catchment area, most likely peatland drainage, had probably changed suspended sediment fluxes and pH conditions. The results also showed that changes in pH levels probably did not originate from ASS or black schist horizons, but rather mainly from humic substances leached from the catchment area. However, ASS and black schist deposits may have had an influence on pH levels in lower parts of the Sanginjoki river during dry/wet periods. Thus, the findings suggest that future changes in land use and land management are likely to cause increased acidity in the Sanginjoki catchment. Therefore actual or potential acidification of river water should be included in risk assessments and future land use practices, and should be avoided by not changing natural humic-rich ecosystems.
ABSTRACT Using daily precipitation dataset (>100 years) from 3 meteorological stations in ... more ABSTRACT Using daily precipitation dataset (>100 years) from 3 meteorological stations in southern, central and northern areas, we evaluated long-term variations in characteristics of extreme precipitation (intensity, frequency and duration) in Finland. For intensity, two indices of AP95p as Annual Precipitation computing from days with precipitation >95th percentile of wet days (>1mm), and RX1day as one-day maximum precipitation were evaluated. For frequency and duration analyses, N95p (number of precipitation events exceeding long-term 95th percentile of days with precipitation >0.00 mm) and LCWD (longest consecutive wet days) were defined, respectively. Mann-Kendall non-parametric test for determining significant trends and Fourier series fitting for showing possible cyclic patterns were used. The AP95p index significantly (p<0.05) increased in southern, central and northern Finland by 0.24, 1.17 and 1.31 (mm/year) during the last century, respectively, while no clear trends were found in RX1day. The N95p increased (0.03 and 0.02 day/year) in southern and northern, while decreased (0.03 day/year) in central Finland. No significant trends were determined in LCWD index over Finland. Estimated time cycles for a low-high mode of AP95p were 89, 222 and 232 years in sequence over southern, central and northern Finland. Time cycles for RX1day ranged from 3 to 21 years over Finland. For N95p, time cycle was 94 years in central, but about 3 to 4 years in other areas of Finland. Time cycle for LCWD was 4, 6 and 12 years in central, southern and northern regions. Results clearly indicate significant increasing trends in extreme precipitation intensity in Finland during the last century. The frequency of extreme precipitation increased in northern and southern Finland, while it decreased in central Finland. Besides, extreme precipitation duration showed cyclic patterns following in Finland. Extreme precipitation characteristics as design parameters for drainage and flood systems are changing and need to be re-evaluated more precisely.
Interest in climate change effects on groundwater has increased dramatically during the last deca... more Interest in climate change effects on groundwater has increased dramatically during the last decade. The mechanisms of climate-related groundwater depletion have been thoroughly reviewed, but the influence of global warming on groundwater dependent ecosystems (GDEs) remains poorly known. Here we report long-term water temperature trends in 66 northern European cold-water springs. A vast majority of the springs (82%) exhibited a significant increase of water temperature during 1968-2012. Mean spring water temperatures were closely related to regional air temperature and global radiative forcing of the corresponding year. Based on three alternative climate scenarios representing low (RCP2.6), intermediate (RCP6) and high emission scenarios (RCP8.5), we estimate that increase of mean spring water temperature in the region is likely to range from 0.67 °C (RCP2.6) to 5.94 °C (RCP8.5) by 2086. According to the worst-case scenario, water temperature of these originally cold-water ecosystems (regional mean in the late 1970s: 4.7 °C) may exceed 12 °C by the end of this century. We used bryophyte and macroinvertebrate species data from Finnish springs and spring-fed streams to assess ecological impacts of the predicted warming. An increase in spring water temperature by several degrees will likely have substantial biodiversity impacts, causing regional extinction of native, cold-stenothermal spring specialists, whereas species diversity of headwater generalists is likely to increase. Even a slight (by 1 °C) increase of water temperature may eliminate endemic spring species, thus altering bryophyte and macroinvertebrate assemblages of spring-fed streams. Climate change-induced warming of northern regions may thus alter species composition of the spring biota and cause regional homogenization of biodiversity in headwater ecosystems. This article is protected by copyright. All rights reserved.
ABSTRACT Erosion from peat extraction areas is known to cause siltation of water courses and poor... more ABSTRACT Erosion from peat extraction areas is known to cause siltation of water courses and poor water quality. However, the main soil parameters affecting peat erosion and suspended sediment (SS) yields from different catchments are not well understood. This paper used peat properties (degree of humification, peat type, ash content, porosity, moisture content, bulk density, and shear strength) and novel erosion threshold measurements from intact soil cores to explain peat erodibility and spatial variations in SS concentrations (SSC) and SS loads (SSL) at 20 Finnish peat extraction sites. The erosion threshold measurements suggested that critical shear stresses for particle entrainment decrease with increasing degree of humification (von Post scale) and are significantly lower in well-decomposed peat than in slightly or moderately decomposed peat. Two critical shear stresses were obtained from moderately decomposed peat samples, indicating a degree of surface armoring by coarse peat fibers. Monitored long-term average SSC was highest at study sites with well-decomposed peat, while very fine-grained mineral subsoil explained some of the highest long-term SSC in areas where drainage ditches penetrated below the upper peat layer. Average SSL (kg d-1) at the study sites was best explained (R2 = 0.89) by average discharge and surface peat decomposition level. Overall, this study provides new knowledge on peat erosion and critical shear stresses that can be used in water conservation and sediment management practices for cutover peatlands and other similar land uses.
ABSTRACT In this study, we analysed stable isotopes and water quality of runoff water collected d... more ABSTRACT In this study, we analysed stable isotopes and water quality of runoff water collected daily from two different peatland drainage areas with automated samplers from March 2012 to October 2012, located in Northern Finland. In addition we collected weekly snow samples for stable isotope analysis. Our primary aim was to find out how different land use types, i) peat extraction area and ii) peatland forestry, are affecting the flow paths and runoff water quality during the snow melt period. Results show that there is a clear difference in δO18 signal between these systems. The peatland forestry area is located at groundwater dominated area which can be seen as a flat line when δO18 values of all samples are plotted. Samples taken at the peat extraction area show a clear response to the snowmelt event. Most likely this difference is caused by different soil frost conditions. Quantity of the groundwater at the forestry area prevents the soil from freezing during winter, therefore water originating from melting snow is able to infiltrate to the peat soil and push pre-event water into the drainage system. This observation is also visible in water quality of runoff water as high peak in colour during the snow melt period. Contrary, the peat extraction area behaves in opposite way. Melting water from snow is not able to infiltrate to ditches but instead will rapidly move on the frozen soil surface as a Hortonian overland flow. Because the soil is frozen, moving water is not able to leach humic substances from soil layers or erode particulate matter from the soil surface. These observations can be used to develop water quality protection policies for drained peatland areas. In Northern areas, where freezing of soil during winter is common it is not crucial to emphasize water protection during spring snowmelt, as frozen soil helps to maintain the runoff water quality at reasonable levels. In the areas where ground frost is rarer the impact of purifying runoff water in spring thaw will be more beneficial for the receiving water bodies.
ABSTRACT Construction of dams affects river flow regimes and changes downstream ecosystems in man... more ABSTRACT Construction of dams affects river flow regimes and changes downstream ecosystems in many ways. The impacts of dams depend on local climate, dam purpose and operation policy. This study analysed the impacts of dam construction on river flow regimes and developed a new method to evaluate the effects on river flow. By using three main flow characteristics of river regime (magnitude, timing and intra-annual alteration), a new combined river impact (RI) index to assess construction impacts based on monthly flow data was developed. A preliminary evaluation of the method was made using the river Ebro as a case. A classification was developed where the impact was assessed using a scale from low to drastic. The method was further tested to quantify the effect of dams in 15 rivers located in different regions. The method showed good potential to classify different dam types of variable sizes constructed for different purposes. The RI index method also proved suitable for assessing the environmental impacts of dam construction and could be extended by adding other characteristics of flow regime to evaluate the effects of climate change or changes in land use on river flow regimes.
ABSTRACT Treatment wetlands are considered best management practice as they can remove nitrogen, ... more ABSTRACT Treatment wetlands are considered best management practice as they can remove nitrogen, phosphorus and suspended solids from peat extraction or forestry runoff. Treatment wetlands are established on intact or formerly drained peatlands after restoration of the site hydrology by ditch blocking or other methods. However, drainage changes the physical properties of peat, so the hydraulic and hydrological conditions in peatlands may change. Consequently, treatment wetlands constructed on drained areas may have short residence times, resulting in poor purification results. This study determined the hydrological and hydraulic characteristics of treatment wetlands constructed on drained peatlands by studying peat hydraulic conductivity and water distribution in 20 different wetlands in Finland. Four wetlands were studied in more detail using tracer tests, water balance and continuous groundwater measurements. More than 50% of the drained treatment wetlands studied had a lower active flow depth layer (only 10 or 20 cm) with higher hydraulic conductivity (K) than normally found in pristine peatlands. At 25% of the sites, K was high at all depths investigated (down to 60–70 cm). At several sites, the water was unequally distributed across the wetland surface, indicating that only part of the wetland was being used for purification. The residence time, as determined by tracer tests, varied significantly between sites. These results indicate that past drainage still influences the hydraulics of treatment wetlands.
Transport of particulate pollutants in fluvial systems can contribute greatly to total loads. Und... more Transport of particulate pollutants in fluvial systems can contribute greatly to total loads. Understanding transport mechanics under different hydrological conditions is key in successful load estimation. This study analysed trace elements and physico-chemical parameters in time-integrated suspended sediment samples, together with dissolved and total concentrations of pollutants, along two agriculture- and peatland-dominated boreal river systems. The samples were taken in a spatially and temporally comprehensive sampling programme during the ice-free seasons of 2010 and 2011. The hydrochemistry and transport of particle-bound elements in the rivers were strongly linked to intense land use and acid sulphate soils in the catchment area, with arable, pasture and peat areas in particular being main diffuse sources. There were significant seasonal and temporal variations in dissolved and particulate fluxes, but spatial variations were small. Continuous measurements of EC, turbidity and discharge proved to be an accurate indicator of dissolved and particulate fluxes. Overall, the results show that transport of particle-bound elements makes a major contribution to total transport fluxes in agriculture-dominated boreal rivers.
ABSTRACT Peat extraction for energy purposes causes major changes in the aquatic and terrestrial ... more ABSTRACT Peat extraction for energy purposes causes major changes in the aquatic and terrestrial environment. According to national strategies for extracting peat in Finland, new peat extraction areas should be established on previously drained peatlands. On such areas it is difficult to find the natural, intact peatland required for treatment wetlands or so-called overland flow areas, which are considered the best available technology for runoff water purification. Therefore treatment wetlands must be constructed on drained peatland. It is known that drainage causes physical, biogeochemical and hydraulic changes in the peat layer, as well as changes in vegetation. It is probable that these changes affect the purification efficiency of wetland treatment systems in many ways. This study evaluated the function and characteristics of drained peatland areas for purification of peat extraction runoff water. Study sites were established on 11 drained wetlands and their purification efficiency was evaluated. Detailed measurements of peat physical properties and hydraulic conductivity, as well as studies on vegetation, were also made in the study areas. The results showed that wetlands constructed on drained peatland areas can purify peat extraction runoff. However, leaching of phosphorus (P) and iron (Fe) was observed in some areas. Leaching is influenced e.g. by pH and the soil P pool. The chemical oxygen demand was also observed to increase in runoff water from the wetland. The results indicated that low (Fe + aluminium (Al) + manganese (Mn))/P ratio (≤ 25) and quite high P content (> 1200 mg/kg) in the surface peat characterised those areas where P leaching was observed. The presence of a dense tree stand in a drained peatland area also seemed to indicate release of nutrients from the area after its rewetting and use as a treatment wetland. Thus, potential nutrient release from a drained peatland area intended for use as a treatment wetland can be assessed by studying the characteristics of the peatland area, especially the peat mineral content, and the vegetation, especially tree stand density in the area. Using the findings obtained, a conceptual decision tree was drawn up in order to help to establish and design wetlands in previously drained peatland areas.
ABSTRACT Peatland drainage can have major ecological and biogeochemical impacts on aquatic enviro... more ABSTRACT Peatland drainage can have major ecological and biogeochemical impacts on aquatic environments in boreal regions. In the Sanginjoki river catchment in North Ostrobothnia, Finland, peatland drainage is suspected to increase the acidity of recipient waters. Possible reasons are acid sulphate soils (ASS) or black schist deposits, which occur especially in lower parts of the river basin, or increased transport of humic substances. It is not known how long acidification has affected water quality and biota in the river. To study historical pH levels, we examined sediment cores from two ponds in the main channel of the Sanginjoki river and one small lake in the Sanginjoki catchment. Diatom and geochemical analysis and 137Cs-derived chronology were combined to estimate historical pH levels and changes in land use practices. The results suggest that changes in land use in the 1970s have affected runoff acidity. During that period, there was a change in diatom communities and sediment geochemistry, indicating that land use in the catchment area, most likely peatland drainage, had probably changed suspended sediment fluxes and pH conditions. The results also showed that changes in pH levels probably did not originate from ASS or black schist horizons, but rather mainly from humic substances leached from the catchment area. However, ASS and black schist deposits may have had an influence on pH levels in lower parts of the Sanginjoki river during dry/wet periods. Thus, the findings suggest that future changes in land use and land management are likely to cause increased acidity in the Sanginjoki catchment. Therefore actual or potential acidification of river water should be included in risk assessments and future land use practices, and should be avoided by not changing natural humic-rich ecosystems.
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