Strong Warming Rates in the Surface and Bottom Layers of a Boreal Lake: Results From Approximately Six Decades of Measurements (1964–2020)

High-latitude lakes are warming faster than the global average with deep implications for life on Earth. Using an approximately six-decade long in situ data set, we explored the changes in lake surface-water temperature (LST), lake deep-water temperature (LDT), lake depth-weighted mean water temperature (LDMT), and ice-free days in Lake Kallavesi, a boreal lake in central Finland, when the lake was stratified (June–August). Our results suggest that the LST is warming faster than the local air temperature (AT). As for the LST, fast warming was also observed in the LDT and LDMT, but at rates slower than those in the LST. The number of ice-free days also shows an upward trend, with a rate of about 7 days per decade during the study period. The corresponding local AT is the main driver of the LST, followed by the ice-free days and annual mean AT. Air temperature and ice-free days also mainly contribute to the changes in the LDMT. The LDT is affected more by the North Atlantic Oscillation signals in this freshwater lake. The AT in the prior months does not affect the LDT in Lake Kallavesi although the AT during the prior season, that is, spring, is the main driver of summer LDT. This highlights the local AT impact on the LDT at time scales longer than a month. The warming rates in the lake water are at a minimum in June because the lake is not yet strongly stratified in this month when compared to July and August. These findings improve our knowledge of long-term changes in the lake water temperature in a high-latitude lake, a region with severe environmental consequences due to fast changes in the AT and lake ice phenology

Maaperärakenteiden maatutkaus aapasuoalueen hydrologian selvittämiseksi

Diplomityön tarkoituksena oli hankkia maatutkaluotauksen avulla tietoa Pudasjärven kunnan alueella sijaitsevan aapasuoalueen maaperästä ja sen rakenteesta. Tutkimusalue sijaitsee Viinivaaran pohjavesiharjun välittömässä läheisyydessä. Se koostuu kahdesta eri suosta, Mesisuosta ja Sarvisuosta. Lähtökohtainen tarkoitus maaperää koskevan uuden tiedon hankkimiselle oli alueen hydrologisten mekanismien ymmärtämisessä. Ennakkotietojen perusteella monet suoalueella esiintyvistä hydrologisista ilmiöistä ovat pohjaveden ja sen virtauksen aiheuttamia. Maatutkaluotaus on geofysikaalinen menetelmä, joka perustuu maaperän tutkaamiseen sähkömagneettisten aaltojen avulla. Maaperän rakenteessa esiintyvät muutokset, kuten eri maalajien muodostamat rajapinnat näkyvät tutkan avulla, mikäli niissä tapahtuu sähköisten ominaisuuksien muutoksia. Maatutkaukset toteutettiin maaliskuussa 2015. Tutkauksista 42 km tehtiin 50 MHz:n ja 100 MHz:n antenniyhdistelmää moottorikelkalla vetäen. Vaikeakulkuinen maasto tutkattiin vetämällä 50 MHz:n ja 250 MHz:n yhdistelmää suksin tai lumikengin. Tällä yhdistelmällä tutkauslinjaa kertyi 21 km, eli yhteensä linjaa oli 63 km. Ennakkotietoina tutkimusalueesta olivat GTK:n vuonna 1996 toteutettujen tutkimusten kairaustulokset ja Oulun yliopiston tekemistä hydrologiaan liittyvistä tutkimuksista saadut tulokset. GTK:n tutkimusten avulla oli määritetty soiden turvevarantoja sekä turvekerroksen alapuolisen mineraalimaalajin ominaisuuksia. Oulun yliopiston tutkimuksissa oli tutkittu alueen hydrologiaa isotooppitutkimusten ja lämpökamerakuvausten avulla. Hydrologisisten tutkimusten, karttatietojen ja maastokäyntien perusteella alueelta pystyttiin määrittämään sijainnit, joilla esiintyi lähteitä. Merkittävimmät lähteet sijaitsevat Viinivaaran pohjoisreunalla, Niemenmaan harjanteen itäpuolella ja Mesisaaren lounaispuolella. Karttatietojen perusteella voitiin olettaa, että pohjaveden pääsääntöinen virtaussuunta tutkimusalueella on etelästä pohjoiseen. Saatujen tutkaustulosten perusteella pystyttiin määrittämään kattavasti tutkimusalueen turvepaksuuksia ja turvekerroksen alapuolisten mineraalimaalajikerrosten ominaisuuksia. Näiden kahden huomattiin olevan kytköksissä toisiinsa, ja turvekerroksen paksuuden muutokset pystyttiin yleisesti selittämään sen alapuolisissa kerroksissa tapahtuvien rakenteellisten muutosten avulla. Turvekerroksen alapuolisista rakenteellisista tekijöistä nousivat vahvimmin esiin alueella esiintyvät moreenimuodostumat. Maaperän ominaisuuksista saadut tulokset liitettiin alueen hydrologiaan tarkastelemalla sellaisia alueita, joilla esiintyi ennakkotietojen perusteella lähteitä. Näiden alueiden maaperän ominaisuuksista pyrittiin löytämään sellaisia tekijöitä, jotka selittäisivät lähteiden sijainteja ja toimintamekanismeja. Tulosten avulla havaittiin selkeitä yhteyksiä lähdealueiden maaperän rakenteellisten ominaisuuksien ja lähteiden sijaintien välillä. Viinivaaran pohjoisreunassa sijaitsevien lähteiden muodostumiseen vaikuttaa tulosten mukaan turvekerroksen paksuuden äkillinen kasvu. Turvekerros muodostaa pohjaveden etenemiselle esteen, jolloin sen virtaussuunta kääntyy osittain kohti maanpintaa. Suoalueen keskiosissa sijaitsevien lähteiden sijainnit voidaan perustella suoalueella ilmenevien moreenimuodostumien avulla. Pohjaveden pääsääntöisen virtaussuunnan vastaiset moreenimuodostumat estävät matalan hydraulisen johtavuutensa takia virtausta, jolloin pohjavesi pyrkii virtaamaan kohti maanpintaa, purkautuen siellä lähteiden muodossa.The purpose of this thesis was to gather information on string bog subsurface and its properties in area located in Pudasjärvi municipality. The area was studied using ground penetrating radar. The study area was located next to Viinivaara groundwater esker. The area consists of two separate peatlands Sarvisuo and Mesisuo. The reason to gather new information of subsurface layers was to understand the hydrological mechanisms of the peatland. This was due to the fact that many hydrological phenomena present in the area were considered groundwater dominant. Ground penetrating radar is a geophysical method. Its operation is based on probing the ground with electromagnetic waves. It can be used to locate structural changes e.g. soil interfaces in subsurface. Their appearance in the radar is based on the differences in electromagnetic properties of different soils. The measurements of this thesis were done during March 2015. Most part (42 km) of the measurements was done by pulling the radar system with a snowmobile. The combination of used antennae was 100 MHz and 50 MHz. Measurements in rough terrain were done by pulling the antennae with skis. The antenna combination in this case was 250 MHz and the 50 MHz. The length of the measurement line done by skis was 21 km long. In total this makes 63 km of measurements. Prior information on the studied area consists of results of researches done by Geological Survey of Finland and University of Oulu. Studies by Geological Survey of Finland determined the properties of the peat and mineral soil beneath. The research done by the university was about the hydrological aspects of the area. The studies included stable water isotope analysis and determination of surface water temperatures with thermal camera. By using results from the hydrological studies in addition to information gained from the maps and field studies, it was possible to determine the locations of springs in the area. The springs were located in the north edge of the Viinivaara esker, east of Niemenmaa ridge and south-west from Mesisaari moraine formation located in the center of the Mesisuo peatland. The main direction of the groundwater flow in the study area was determined from the cartographic information to be from south to north. The thickness of the peat layer and the properties of soil layer beneath it were determined extensively. The changes in peat layer thickness were generally explainable with structural changes in the soil layer below. The most significant structural changes were found to be the till formations below the peat. The results on subsurface and its properties were integrated with the prior knowledge on hydrology by focusing on the areas where springs were present. Locations and mechanisms of springs were explained with the subsurface properties of the areas. The results indicated clear connections between the spring locations and the structural properties of the subsurface. Sudden thickening of the peat layer appears to be the cause for the location of the Viinivaara esker spring. The thick peat layer formed a block for the groundwater flow and the flow direction partially changed towards the ground surface. The locations of the other springs near the center of the study area can be explained with the locations and properties of the till formations. These thicker till formation hill beneath peat were situated parallel with the groundwater flow direction and blocked the flow. The low hydraulic conductivity of till forces the water to flow vertically towards the peatland surface

Predicting iron transport in boreal agriculture-dominated catchments under a changing climate

Abstract Increases in iron (Fe) concentration have been reported in boreal regions in recent decades, raising concerns about the fate of ecosystems along water courses. In this study, the SWAT (Soil and Water Assessment Tool) model was applied to the river Mustijoki catchment in southern Finland to determine the current state of Fe transport and to evaluate possible effects of ongoing environmental change in this agriculture-dominated catchment. The model was calibrated using five-year discharge, suspended solids, and Fe data, and validated with a three-year dataset of the same parameters. Further, the model was run with spatially downscaled and bias-corrected climate change scenario data to the year 2100 obtained using five different global climate models. The results were divided into 20-year time steps (2020–2039, 2040–2059, 2060–2079, 2080–2099) and compared against a reference modeling period (1997–2016). With present catchment characteristics of the river Mustijoki, Fe transport was shown to be related to soil erosion and suspended solids transport, driven by hydrological conditions. Arable fields, especially with steeper slopes, were identified as the most likely source of Fe loading. Climate change-induced alterations in riverine Fe transport were simulated as concentrations and as annual mass fluxes. High Fe transport season is already shifting from spring snowmelt events to autumn and winter, and this change is likely to increase in coming decades. Based on modeling results, annual peak concentration in the River Mustijoki was projected to decrease by up to 32% (from 6.2 mg L⁻¹ to 4.2 mg L⁻¹ in scenarios RCP4.5 and RCP8.5) in the coming 20-year period, while lowest winter concentration was projected to increase by 126% (from 1.5 mg L⁻¹ in the reference period (1997–2016) to 3.5 mg L⁻¹ in 2080–2099 in scenario RCP8.5. To compensate for these changes in Fe transport dynamics, water protection and land use management planning must be improved

Strong warming rates in the surface and bottom layers of a boreal lake:results from approximately six decades of measurements (1964–2020)

Abstract High-latitude lakes are warming faster than the global average with deep implications for life on Earth. Using an approximately six-decade long in situ data set, we explored the changes in lake surface-water temperature (LST), lake deep-water temperature (LDT), lake depth-weighted mean water temperature (LDMT), and ice-free days in Lake Kallavesi, a boreal lake in central Finland, when the lake was stratified (June–August). Our results suggest that the LST is warming faster than the local air temperature (AT). As for the LST, fast warming was also observed in the LDT and LDMT, but at rates slower than those in the LST. The number of ice-free days also shows an upward trend, with a rate of about 7 days per decade during the study period. The corresponding local AT is the main driver of the LST, followed by the ice-free days and annual mean AT. Air temperature and ice-free days also mainly contribute to the changes in the LDMT. The LDT is affected more by the North Atlantic Oscillation signals in this freshwater lake. The AT in the prior months does not affect the LDT in Lake Kallavesi although the AT during the prior season, that is, spring, is the main driver of summer LDT. This highlights the local AT impact on the LDT at time scales longer than a month. The warming rates in the lake water are at a minimum in June because the lake is not yet strongly stratified in this month when compared to July and August. These findings improve our knowledge of long-term changes in the lake water temperature in a high-latitude lake, a region with severe environmental consequences due to fast changes in the AT and lake ice phenology

Six decades of thermal change in a pristine lake situated north of the Arctic Circle

Abstract The majority of lake temperature studies have investigated climate-induced changes occurring at the lake surface, primarily by analyzing detailed satellite images of surface water temperature. Whilst essential to observe long-term change, satellite images do not provide information on the thermal environment at depth, thus limiting our understanding of lake thermal responses to a warming world. Long-term in situ observational data can fill some of the information gap, with depth-resolved field measurements providing a detailed view of thermal change throughout the water column. However, many previous studies that have investigated multi-decadal changes in lake temperature, both at the surface and at depth, have typically focused on north temperate lakes. Relatively few studies have investigated temperature variations in lakes situated north of the Arctic Circle, which is one of the most rapidly warming regions on Earth. Here, using a 60-year (1961–2020) observational data set of summer water temperature (July–September) from Lake Inari (Finland), we investigate changes in the thermal environment of this pristine lake. Our analysis suggests a statistically significant summer warming trend at the lake surface (+0.25°C decade⁻¹, p-value <0.1), whilst deepwater temperatures remain largely unchanged. This contrasting thermal response of surface and bottom water temperature to climatic warming has likewise resulted in a strengthening of summer stratification in this high latitude lake. Implications of the observed change in both temperature and stratification on the lake ecosystem will likely be extensive, including impacts on aquatic organisms which this lake supports. Our work builds on the ever-growing literature regarding lake thermal responses to climate change

Thickness of peat influences the leaching of substances and greenhouse gas emissions from a cultivated organic soil

Abstract The off-site effects of agricultural organic soils include the leaching of N, P, and organic carbon (OC) to watercourses and CO₂, CH₄, and N₂O emissions into the atmosphere. The aim of this study was to quantify how the thickness of organic layers affects these loads. A 19.56-ha experimental field drained by subsurface pipes was established in Ruukki, northwestern Finland. Three plots had a 60–80 cm-thick sedge peat layer and three others had a thickness of 20 cm or less. The drainage pipes lie in mineral soil that, in this field, contains sulfidic material. This study documents the experimental settings and reports on the leaching of substances in the first two years, as well as CO₂, CH₄ and N₂O emissions during eight weeks in one summer. Total N (TN) and OC loads were higher from the thicker peat plots. The mean TN loads during a hydrological year were 15.4 and 9.2 kg ha⁻¹ from the thicker and thinner peat plots, respectively, with organic N representing 36% of TN load. Total P (TP) load averaged 0.27 kg ha⁻¹ yr⁻¹. Dissolved P load represented 63 and 36% of TP in the thicker peat area and only 23 and 13% in the thinner peat area, and was thus increased upon peat thickness. These N and P loads through the subsurface drainage system represented roughly 83% of TN and 64% of TP loads from this field. There were no clear differences in greenhouse gas emissions among the plots during the eight-week monitoring period. Slowly oxidizing sulfide in the subsoil resulted in annual leaching of 147 kg S ha⁻¹, almost ten times that of non-sulfidic soils. Our first results emphasize the effect of the peat thickness on the leaching of substances and warn about considering all organic soils as a single group in environmental assessments

Spatially varying peatland initiation, Holocene development, carbon accumulation patterns and radiative forcing within a subarctic fen

Abstract High latitude peatlands act as globally important carbon (C) sinks and are in constant interaction with the atmosphere. Their C storage formed during the Holocene. In the course of time, the aggregate effect of the C fluxes on radiative forcing (RF) typically changes from warming to cooling, but the timing of this shift varies among different peatlands. Here we investigated Holocene peatland development, including vegetation history, vertical peat growth and the lateral expansion of a patterned subarctic fen in northern Finland by means of multiple sampling points. We modelled the Holocene RF by combining knowledge on past vegetation communities based on plant macrofossil stratigraphies and present in situ C flux measurements. The peatland initiated at ca. 9500 calibrated years Before Present (cal yr BP), and its lateral expansion was greatest between ca. 9000 and 7000 cal yr BP. After the early expansion, vertical peat growth proceeded very differently in different parts of the peatland, regulated by internal and external factors. The pronounced surface microtopography, with high strings and wet flarks, started to form only after ca. 1000 cal yr BP. C accumulation within the peatland recorded a high degree of spatial variability throughout its history, including the recent past. We applied two flux scenarios with different interpretation of the initial peatland development phases to estimate the RF induced by C fluxes of the fen. After ca. 4000 cal yr BP, at the latest, the peatland RF has been negative (cooling), mainly driven by C uptake and biomass production, while methane emissions had a lesser role in the total RF. Interestingly, these scenarios suggest that the greatest cooling effect took place around ca. 1000 cal yr BP, after which the surface microtopography established. The study demonstrated that despite the high spatial heterogeneity and idiosyncratic behaviour of the peatland, the RF of the studied fen followed the general development pattern of more southern peatlands. Holocene climate variations and warm phases did not seem to induce any distinctive and consistent peatland-scale patterns in C accumulation, whereas our data suggests that the changes in vegetation related to autogenic succession were reflected in the C accumulation patterns and RF more clearly