The role of lakes for carbon cycling in boreal catchments

Lakes are an important component of ecosystem carbon cycle through both organic carbon sequestration and carbon dioxide and methane emissions, although they cover only a small fraction of the Earth’s surface area. Lake sediments are considered to be one of the rather permanent sinks of carbon in boreal regions and furthermore, freshwater ecosystems process large amounts of carbon originating from terrestrial sources. These carbon fluxes are highly uncertain especially in the changing climate.The present study provides a large-scale view on carbon sources and fluxes in boreal lakes situated in different landscapes. We present carbon concentrations in water, pools in lake sediments, and carbon gas (CO2 and CH4) fluxes from lakes. The study is based on spatially extensive and randomly selected Nordic Lake Survey (NLS) database with 874 lakes. The large database allows the identification of the various factors (lake size, climate, and catchment land use) determining lake water carbon concentrations, pools and gas fluxes in different types of lakes along a latitudinal gradient from 60oN to 69oN.Lakes in different landscapes vary in their carbon quantity and quality. Carbon (C) content (total organic and inorganic carbon) in lakes is highest in agriculture and peatland dominated areas. In peatland rich areas organic carbon dominated in lakes but in agricultural areas both organic and inorganic C concentrations were high. Total inorganic carbon in the lake water was strongly dependent on the bedrock and soil quality in the catchment, especially in areas where human influence in the catchment is low. In inhabited areas both agriculture and habitation in the catchment increase lake TIC concentrations, since in the disturbed soils both weathering and leaching are presumably more efficient than in pristine areas.TOC concentrations in lakes were related to either catchment sources, mainly peatlands, or to retention in the upper watercourses. Retention as a regulator of the TOC concentrations dominated in southern Finland, whereas the peatland sources were important in northern Finland. The homogeneous land use in the north and the restricted catchment sources of TOC contribute to the close relationship between peatlands and the TOC concentrations in the northern lakes. In southern Finland the more favorable climate for degradation and the multiple sources of TOC in the mixed land use highlight the importance of retention.Carbon processing was intensive in the small lakes. Both CO2 emission and the Holocene C pool in sediments per square meter of the lake area were highest in the smallest lakes. However, because the total area of the small lakes on the areal level is limited, the large lakes are important units in C processing in the landscape. Both CO2 and CH4 concentrations and emissions were high in eutrophic lakes. High availability of nutrients and the fresh organic matter enhance degradation in these lakes. Eutrophic lakes are often small and shallow, enabling high contact between the water column and the sediment. At the landscape level, the lakes in agricultural areas are often eutrophic due to fertile soils and fertilization of the catchments, and therefore they also showed the highest CO2 and CH4 concentrations. Export from the catchments and in-lake degradation were suggested to be equally important sources of CO2 and CH4 in fall when the lake water column was intensively mixed and the transport of substances from the catchment was high due to the rainy season. In the stagnant periods, especially in the winter, in-lake degradation as a gas source was highlighted due to minimal mixing and limited transport of C from the catchment.The strong relationship between the annual CO2 level of lakes and the annual precipitation suggests that climate change can have a major impact on C cycling in the catchments. Increase in precipitation enhances DOC export from the catchments and leads to increasing greenhouse gas emissions from lakes. The total annual CO2 emission from Finnish lakes was estimated to be 1400 Gg C a-1. The total lake sediment C pool in Finland was estimated to be 0.62 Pg, giving an annual sink in Finnish lakes of 65 Gg C a-1

Geographic and tourist position of Ternopil region as a factor of tourism development

The quantity of carbon dioxide (CO2) emissions from inland waters into the atmosphere varies, depending on spatial and temporal variations in the partial pressure of CO2 (pCO2) in waters. Using 22,664 water samples from 851 boreal lakes and 64 boreal streams, taken from different water depths and during different months we found large spatial and temporal variations in pCO2, ranging from below atmospheric equilibrium to values greater than 20,000 μatm with a median value of 1048 μatm for lakes (n = 11,538 samples) and 1176 μatm for streams (n = 11,126). During the spring water mixing period in April/May, distributions of pCO2 were not significantly different between stream and lake ecosystems (P > 0.05), suggesting that pCO2 in spring is determined by processes that are common to lakes and streams. During other seasons of the year, however, pCO2 differed significantly between lake and stream ecosystems (P < 0.0001). The variable that best explained the differences in seasonal pCO2 variations between lakes and streams was the temperature difference between bottom and surface waters. Even small temperature differences resulted in a decline of pCO2 in lake surface waters. Minimum pCO2 values in lake surface waters were reached in July. Towards autumn pCO2 strongly increased again in lake surface waters reaching values close to the ones found in stream surface waters. Although pCO2 strongly increased in the upper water column towards autumn, pCO2 in lake bottom waters still exceeded the pCO2 in surface waters of lakes and streams. We conclude that throughout the year CO2 is concentrated in bottom waters of boreal lakes, although these lakes are typically shallow with short water retention times. Highly varying amounts of this CO2 reaches surface waters and evades to the atmosphere. Our findings have important implications for up-scaling CO2 fluxes from single lake and stream measurements to regional and global annual fluxes

Regional Variability and Drivers of Below Ice CO2 in Boreal and Subarctic Lakes

Northern lakes are ice-covered for considerable portions of the year, where carbon dioxide (CO2) can accumulate below ice, subsequently leading to high CO2 emissions at ice-melt. Current knowledge on the regional control and variability of below ice partial pressure of carbon dioxide (pCO(2)) is lacking, creating a gap in our understanding of how ice cover dynamics affect the CO2 accumulation below ice and therefore CO2 emissions from inland waters during the ice-melt period. To narrow this gap, we identified the drivers of below ice pCO(2) variation across 506 Swedish and Finnish lakes using water chemistry, lake morphometry, catchment characteristics, lake position, and climate variables. We found that lake depth and trophic status were the most important variables explaining variations in below ice pCO(2) across the 506 lakes(.) Together, lake morphometry and water chemistry explained 53% of the site-to-site variation in below ice pCO(2). Regional climate (including ice cover duration) and latitude only explained 7% of the variation in below ice pCO(2). Thus, our results suggest that on a regional scale a shortening of the ice cover period on lakes may not directly affect the accumulation of CO2 below ice but rather indirectly through increased mobility of nutrients and carbon loading to lakes. Thus, given that climate-induced changes are most evident in northern ecosystems, adequately predicting the consequences of a changing climate on future CO2 emission estimates from northern lakes involves monitoring changes not only to ice cover but also to changes in the trophic status of lakes.Peer reviewe

Lakes as nitrous oxide sources in the boreal landscape

Abstract Estimates of regional and global freshwater N2O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N2O fluxes the focus predominantly being on summer time measurements from emission hot spots, agricultural streams. Here we present four-season data of N2O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry and land use cover. Nitrate was the key driver for N2O dynamics, explaining as much as 78% of the variation of the seasonal mean N2O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples 71% were oversaturated with N2O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N2O measurements in annual emission estimates. Including winter data resulted in four-fold annual N2O emission estimates compared to summer only measurements. Nutrient rich calcareous and large humic lakes had the highest annual N2O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 Gg and 29 Gg N2O-N y-1, respectively. The Global Warming Potential (GWP) of N2O cannot be neglected in the boreal landscape, being 35% of that of diffusive CH4 emission in Finnish lakes.peerReviewe

Towards a more comprehensive understanding of lacustrine greenhouse gas dynamics — two-year measurements of concentrations and fluxes of CO2, CH4 and N2O in a typical boreal lake surrounded by managed forests

Peer reviewe

Methane and carbon dioxide fluxes over a lake : comparison between eddy covariance, floating chambers and boundary layer method

Freshwaters bring a notable contribution to the global carbon budget by emitting both carbon dioxide (CO2) and methane (CH4) to the atmosphere. Global estimates of freshwater emissions traditionally use a wind-speed-based gas transfer velocity, k CC (introduced by Cole and Caraco, 1998), for calculating diffusive flux with the boundary layer method (BLM). We compared CH4 and CO2 fluxes from BLM with k CC and two other gas transfer velocities (k TE and k HE), which include the effects of water-side cooling to the gas transfer besides shear-induced turbulence, with simultaneous eddy covariance (EC) and floating chamber (FC) fluxes during a 16-day measurement campaign in September 2014 at Lake Kuivajarvi in Finland. The measurements included both lake stratification and water column mixing periods. Results show that BLM fluxes were mainly lower than EC, with the more recent model k TE giving the best fit with EC fluxes, whereas FC measurements resulted in higher fluxes than simultaneous EC measurements. We highly recommend using up-to-date gas transfer models, instead of kCC, for better flux estimates. BLM CO2 flux measurements had clear differences between daytime and night-time fluxes with all gas transfer models during both stratified and mixing periods, whereas EC measurements did not show a diurnal behaviour in CO2 flux. CH4 flux had higher values in daytime than night-time during lake mixing period according to EC measurements, with highest fluxes detected just before sunset. In addition, we found clear differences in daytime and night-time concentration difference between the air and surface water for both CH4 and CO2. This might lead to biased flux estimates, if only daytime values are used in BLM upscaling and flux measurements in general. FC measurements did not detect spatial variation in either CH4 or CO2 flux over Lake Kuivajarvi. EC measurements, on the other hand, did not show any spatial variation in CH4 fluxes but did show a clear difference between CO2 fluxes from shallower and deeper areas. We highlight that while all flux measurement methods have their pros and cons, it is important to carefully think about the chosen method and measurement interval, as well as their effects on the resulting flux.Peer reviewe

Järvien merkitys hiilenkierrossa pohjoisilla valuma-alueilla

Lakes are an important component of ecosystem carbon cycle through both organic carbon sequestration and carbon dioxide and methane emissions, although they cover only a small fraction of the Earth's surface area. Lake sediments are considered to be one of rather perma-nent sinks of carbon in boreal regions and furthermore, freshwater ecosystems process large amounts of carbon originating from terrestrial sources. These carbon fluxes are highly uncer-tain especially in the changing climate. -- The present study provides a large-scale view on carbon sources and fluxes in boreal lakes situated in different landscapes. We present carbon concentrations in water, pools in lake se-diments, and carbon gas (CO2 and CH4) fluxes from lakes. The study is based on spatially extensive and randomly selected Nordic Lake Survey (NLS) database with 874 lakes. The large database allows the identification of the various factors (lake size, climate, and catchment land use) determining lake water carbon concentrations, pools and gas fluxes in different types of lakes along a latitudinal gradient from 60oN to 69oN. Lakes in different landscapes vary in their carbon quantity and quality. Carbon (C) content (total organic and inorganic carbon) in lakes is highest in agriculture and peatland dominated areas. In peatland rich areas organic carbon dominated in lakes but in agricultural areas both organic and inorganic C concentrations were high. Total inorganic carbon in the lake water was strongly dependent on the bedrock and soil quality in the catchment, especially in areas where human influence in the catchment is low. In inhabited areas both agriculture and habitation in the catchment increase lake TIC concentrations, since in the disturbed soils both weathering and leaching are presumably more efficient than in pristine areas. TOC concentrations in lakes were related to either catchment sources, mainly peatlands, or to retention in the upper watercourses. Retention as a regulator of the TOC concentrations dominated in southern Finland, whereas the peatland sources were important in northern Finland. The homogeneous land use in the north and the restricted catchment sources of TOC contribute to the close relationship between peatlands and the TOC concentrations in the northern lakes. In southern Finland the more favorable climate for degradation and the multiple sources of TOC in the mixed land use highlight the importance of retention. Carbon processing was intensive in the small lakes. Both CO2 emission and the Holocene C pool in sediments per square meter of the lake area were highest in the smallest lakes. How-ever, because the total area of the small lakes on the areal level is limited, the large lakes are important units in C processing in the landscape. Both CO2 and CH4 concentrations and emissions were high in eutrophic lakes. High availability of nutrients and the fresh organic matter enhance degradation in these lakes. Eutrophic lakes are often small and shallow, enabling high contact between the water column and the sediment. At the landscape level, the lakes in agricultural areas are often eutrophic due to fertile soils and fertilization of the catchments, and therefore they also showed the highest CO2 and CH4 concentrations. Export from the catchments and in-lake degradation were suggested to be equally important sources of CO2 and CH4 in fall when the lake water column was intensively mixed and the transport of sub-stances from the catchment was high due to the rainy season. In the stagnant periods, especially in the winter, in-lake degradation as a gas source was highlighted due to minimal mixing and limited transport of C from the catchment. The strong relationship between the annual CO2 level of lakes and the annual precipitation suggests that climate change can have a major impact on C cycling in the catchments. Increase in precipitation enhances DOC export from the catchments and leads to increasing greenhouse gas emissions from lakes. The total annual CO2 emission from Finnish lakes was estimated to be 1400 Gg C a-1. The total lake sediment C pool in Finland was estimated to be 0.62 Pg, giving an annual sink in Finnish lakes of 65 Gg C a-1.Hiili esiintyy järvissä sekä orgaanisessa että epäorgaanisessa muodossa. Orgaaninen hiili voi olla peräisin järvessä tapahtuvasta perustuotannosta tai huuhtoutua järveen ympäröivän valuma-alueen kasvillisuudesta tai maaperästä. Epäorgaaninen hiili esiintyy järvessä joko liuenneina karbonaatteina, bikarbonaatteina tai kaasumaisessa muodossa hiilidioksidina ja metaanina. Bikarbonaatti on peräisin maaperässä tapahtuvasta rapautumisesta, mutta hiilidioksidi muodostuu pääosin orgaanisen aineen hajoamisen seurauksena. Hapettomissa oloissa hajoamisen seurauksena muodostuu metaania. Järvet ovat tärkeä osa ekosysteemin hiilenkiertoa, koska maaekosysteemistä huuhtoutunutta orgaanista ainesta hajoaa järvissä muodostaen ilmastoa lämmittäviä metaani- ja hiilidioksidikaasuja. Osa järvessä muodostuvasta ja ympäröivältä valuma-alueelta tulevasta hiilestä varastoituu järvien pohjasedimenttiin. Tässä tutkimuksessa selvitimme järvien roolia hiilikaasujen lähteenä ja hiilen varastona. Lisäksi tutkimme valuma-alueelta järviin tulevan hiilen alkuperää sekä hiilen eri muotojen (orgaaninen, epäorgaaninen) suhteellista osuutta erilaisissa järvissä. Tutkimus perustuu satunnaisotannalla valittuun Pohjoismaiseen järvikartoitusaineistoon, joka käsittää 874 järveä etelärannikolta pohjoisimpaan Suomeen. Laaja aineisto mahdollistaa tulosten yleistämisen Suomen ja koko boreaalisen alueen mittakaavassa. Erityyppisillä alueilla sijaitsevat järvet poikkesivat toisistaan järviveden hiilen määrän suhteen. Hiilen kokonaismäärä vedessä oli suurin maatalous- tai turvemaavaltaisilla alueilla. Turvemaavaltaisilla alueilla orgaaninen hiili oli pääasiallinen hiilen muoto järvissä, mutta maatalousvaltaisilla alueilla sekä orgaanisen että epäorgaanisen hiilen pitoisuudet olivat suuria. Metsäisillä mineraalimailla sijaitsevat järvet olivat tyypillisesti vähähiilisiä. Tutkimustulosten perusteella voitiin havaita että järvien orgaanisen hiilen pitoisuudet määräytyivät järveä ym-päröivältä valuma-alueelta, lähinnä turvemailta, tulevan kuormituksen perusteella etenkin Pohjois-Suomessa. Yläpuolisissa järvissä tapahtuva orgaanisen aineen pidättyminen hajoamisen tai sedimentoitumisen myötä oli etenkin eteläisessä Suomessa keskeinen järvien orgaanisen hiilen määrää säätelevä tekijä, mikä johtui valuma-alueiden pohjoista monipuolisemmasta maankäytöstä (maatalous, asutus) ja orgaanisen aineen hajoamiselle suotuisemmista olosuh-teista. Valuma-alueen kallioperä ja maaperä vaikuttivat järvien epäorgaanisen hiilen pitoisuuksiin. Asutuilla alueilla maanviljelys lisää järvien epäorgaanisen hiilen pitoisuuksia, sillä maanviljelys on myös tyypillisesti keskittynyt hienojakoisille maille, joissa rapautuminen on nopeampaa. Tämän tutkimuksen tulosten perusteella järvien rehevöityminen lisää järvien luontaisia hiilidioksidi- ja metaanipäästöjä. Orgaanisen aineen hajotus tehostuu, kun ravinteita on riittävästi saatavilla ja lisäksi rehevät järvet tuottavat runsaasti helposti hajoavaa orgaanista ainetta. Rehevimmät järvet ovat usein pieniä ja matalia, mikä tehostaa myös pohjalietteestä vapautuvien kaasujen pääsyä ilmakehään. Maatalousalueilla sijaitsevat järvet ovat usein keskimääräistä rehevämpiä pelloilta huuhtoutuvien ravinteiden vuoksi. Verrattaessa hiilen prosessointia erikokoisissa järvissä havaittiin, että orgaanisen aineen hajotus ja sedimentaatio olivat tehokkaimmillaan pienissä järvissä. Hiilidioksidi ja metaanipäästöt järvistä sekä hiilen pysyvä va-rastoituminen järvisedimenttiin olivat pinta-alaan suhteutettuina suurimpia pienissä järvissä. Pienten järvien kokonaispinta-ala Suomessa on kuitenkin suuriin järviin verrattuna pieni, joten kokonaispäästöiksi ja varastoiksi laskettuina myös suuret järvet ovat tärkeitä valuma-alueiden hiilenkierrossa. Eri vuodenaikoina järvien hiilidioksidi ja metaanipitoisuuksien säätelyssä havaittiin eroja. Syksyisin, kun sademäärät usein ovat suuria ja valuma-alueelta veden mukana kulkeutuvia aineita on järvissä paljon, sekä järven sisäisissä prosesseissa syntyvillä että valuma-alueelta kulkeutuvilla hiilidioksidilla ja metaanilla oli merkitystä järvien pitoisuuksiin. Talvella, kun järven ulkopuolelta tuleva kuormitus on pieni, järven oman hajotustoiminnan tuloksena syn-tyvien kaasujen määrä oli ratkaiseva. Vuosittaisen sademäärän ja järvien vuosittaisten hiilidi-oksidipäästöjen väliltä löydettiin voimakas riippuvuus, joka todennäköisesti johtui siitä, että sateisina vuosina valuma-alueilta huuhtoutui järviin enemmän orgaanista ainetta. Tämä tulos viittaa siihen, että jos sademäärä kasvaa ilmastonmuutoksen myötä, myös luonnon lähteistä tulevat hiilidioksidipäästöt kasvavat. Järvisedimetteihin viime jääkauden jälkeen Suomessa varastoitunut hiilimäärän arvioitiin olevan 0.62 Pg, joka on kolmanneksi suurin luonnon hiilivarasto Suomessa soiden ja metsämaiden jälkeen. Suomen järvien vuotuiset hiilidioksidipäästöt arvioitiin 1400 Gg suuruisiksi

Carbon Dioxide in Boreal Surface Waters : A Comparison of Lakes and Streams

The quantity of carbon dioxide (CO2) emissions from inland waters into the atmosphere varies, depending on spatial and temporal variations in the partial pressure of CO2 (pCO2) in waters. Using 22,664 water samples from 851 boreal lakes and 64 boreal streams, taken from different water depths and during different months we found large spatial and temporal variations in pCO2, ranging from below atmospheric equilibrium to values greater than 20,000 μatm with a median value of 1048 μatm for lakes (n = 11,538 samples) and 1176 μatm for streams (n = 11,126). During the spring water mixing period in April/May, distributions of pCO2 were not significantly different between stream and lake ecosystems (P > 0.05), suggesting that pCO2 in spring is determined by processes that are common to lakes and streams. During other seasons of the year, however, pCO2 differed significantly between lake and stream ecosystems (P < 0.0001). The variable that best explained the differences in seasonal pCO2 variations between lakes and streams was the temperature difference between bottom and surface waters. Even small temperature differences resulted in a decline of pCO2 in lake surface waters. Minimum pCO2 values in lake surface waters were reached in July. Towards autumn pCO2 strongly increased again in lake surface waters reaching values close to the ones found in stream surface waters. Although pCO2 strongly increased in the upper water column towards autumn, pCO2 in lake bottom waters still exceeded the pCO2 in surface waters of lakes and streams. We conclude that throughout the year CO2 is concentrated in bottom waters of boreal lakes, although these lakes are typically shallow with short water retention times. Highly varying amounts of this CO2 reaches surface waters and evades to the atmosphere. Our findings have important implications for up-scaling CO2 fluxes from single lake and stream measurements to regional and global annual fluxes

The Yasso07 soil carbon model - Testing against repeated soil carbon inventory

Miitta Rantakari, et al, 'The Yasso07 soil carbon model - Testing against repeated soil carbon inventory', Forest Ecology and Management, Vol 286, pp. 137-147, first published online 11 October 2012. The version of record is available online at doi: http://dx.doi.org/10.1016/j.foreco.2012.08.041 © 2012 Elsevier B. V. All rights reserved.Forest soils store large amounts of carbon (C), and releases of C from this pool may significantly increase the CO 2 concentration in the atmosphere. Organic matter decomposition in soils has been shown to strongly depend on temperature and soil moisture and is, therefore, susceptible to the climate change. Reliable methods are needed to monitor and predict the changes in soil C stocks. In this study, we tested the Yasso07 soil C model by comparing the model predictions to repeated soil C measurements of organic layer and, furthermore, to the estimates of two other C models, namely Yasso and ROMUL. In the model simulations, we used the litter input time series derived from forest biomass estimates based on the national forest inventories. Both the repeated empirical measurements and Yasso07 simulations indicated upland forest soils to be small sinks of C in Southern Finland. The Yasso07 model was able to predict both soil C stock and C accumulation within the error limits of the measured values. Yasso07 and the earlier version, Yasso, predicted very similar soil C stocks close to the measured values, but slightly underestimated C accumulation. The annual soil C changes predicted by the Yasso07 and ROMUL models were reasonably close to each other, even though the models are based on a very different basic structure. However, the differences in the model predictions were at the highest in years with the highest precipitation, indicating that there are still uncertainties in predicting the effects of soil moisture on the soil C stock changes.Peer reviewe