Comparison of measurements and water boundary layer model for methane and carbon dioxide fluxes over a boreal lake

Freshwaters are a source of carbon to the atmosphere in the form of methane (CH4) and carbon dioxide (CO2). Global estimates of the freshwater contribution to the carbon budget are often based on a water boundary layer model (BLM) with gas transfer coefficient k calculated depending solely on wind speed. According to comparison studies, this model gives underestimated emissions and should not be used for more reliable results. A widely used flux measurement method over lakes is the floating chamber (FC) method. FC measures surface flux from a very small area of the lake, so it may not be representative of the whole ecosystem. Measurements are relatively cheap and easy, but also laborious and sporadic. Instead of measuring just a specific point on the lake, eddy covariance (EC) technique provides continuous flux measurements over a much larger source area (footprint). EC systems have been widely used over land areas, but are now growing their popularity in the lake community as well. The aim of this study was to compare EC, FC and BLM methods for CO2 and CH4 fluxes over a boreal lake. The measurements were made at a small dimictic Lake Kuivajärvi in Hyytiälä (Juupajoki, Southern Finland) during an intensive field campaign in September 2014. Manual FC measurements were done at four measurement spots in the EC footprint area 2-3 times a day for catching spatial and temporal variability. Gas transfer velocity for BLM was calculated according to three different parametrizations. Results indicate that BLM fluxes calculated based on water convection and wind driven turbulent gas exchange compare quite well with EC measurements while the model based solely on wind speed is a clear underestimate. FC measurements show about 1.7 times larger flux values than EC. The comparison is more clear for CH4 than CO2 fluxes. The greatest values of CH4 fluxes were measured near the shore, while CO2 flux did not show any spatial variability. After the lake started its autumn mixing, CH4 flux showed a diurnal variation with highest values measured during daytime. There was no diurnal variation before mixing. CO2 flux on the other hand showed diurnal variation only when calculated according to the BLM method.Sisävedet (järvet, joet ja purot) ovat kasvihuonekaasujen, erityisesti metaanin (CH4) ja hiilidioksidin (CO2) lähteitä. Globaaleissa hiilitaselaskuissa makeiden vesien osuus on usein arvioitu vesirajakerrosmallin mukaan käyttäen kaasunvaihtokertoimen k laskemisessa pelkästään tuulen nopeutta kaasunvaihtoa ajavana tekijänä. Aiempien mittausmenetelmiä vertailevien tutkimusten mukaan tämä malli aliarvioi kasvihuonekaasupäästöjä, eikä sen käyttö ole suositeltavaa luotettavien tulosten saamiseksi. Laajasti järvillä käytetty vuomittausmenetelmä on kammiomenetelmä. Kammioilla saadaan vuomittauksia vain hyvin pieneltä pinta-alalta, joten ne eivät välttämättä kuvaa kattavasti tutkittavaa ekosysteemiä. Kammiomittaukset ovat edullisia ja yksinkertaisia toteuttaa, mutta ne ovat myös työläitä ja mittaukset ovat ajallisesti ja paikallisesti hajanaisia. Tietyn kohdan vuon mittaamisen sijaan nopeat pyörrekovarianssimittaukset (suora vuomittaus, eddy covariance) kattavat suuremman lähdealueen. Pyörrekovarianssimenetelmää on laajasti käytetty maa-alueiden vuomittauksissa, mutta niiden suosio järvitutkimuksen parissa on nykyään nousussa. Tämän tutkimuksen tavoitteena oli verrata pyörrekovarianssi-, kammio- ja rajakerrosmallimenetelmiä CH4- ja CO2-vuomittauksissa sekä tutkia niiden ajallista ja paikallista vaihtelua. Mittaukset suoritettiin intensiivisen mittauskampanjan aikana Kuivajärvellä Hyytiälässä (Juupajoki, Etelä-Suomi) syyskuussa 2014. Manuaalisia kammiomittauksia tehtiin neljällä eri mittauspaikalla pyörrekovarianssin lähdealueella 2-3 kertaa päivässä, jotta saatiin tutkittua kaasunvaihdon ajallista ja paikallista vaihtelua. Kaasunvaihtokerroin rajakerrosmallimenetelmää varten laskettiin kolmen eri parametrisoinnin mukaan. Tulokset osoittavat, että rajakerrosmalli korreloi paremmin pyörrekovarianssimittausten kanssa, kun kaasunvaihtokertoimen mallintamisessa käytettiin tuulen lisäksi vesipatsaan konvektion aiheuttamaa turbulenssia. Pelkkää tuulennopeutta käyttävä malli aliarvioi kaasupäästöjä selvästi verrattuna pyörrekovarianssimittauksiin. Kammioilla mitattiin noin 1,7 kertaa suurempia vuoarvoja kuin pyörrekovarianssilla. Vertailu kuitenkin toimii paremmin metaani- kuin hiilidioksidivuolle. Metaanivuota tarkastellessa huomattiin, että suurimmat vuoarvot on mitattu läheltä rantaa, kun taas hiilidioksidivuossa ei havaittu paikallista vaihtelua. Ajallisesti metaanivuo oli suurin päiväsaikaan, kun järven syyssekoitus oli alkanut. Ennen sekoitusta ei havaittu vuorokausivaihtelua. Hiilidioksidivuossa ajallista vaihtelua havaittiin vain, kun vuo oli laskettu vesirajakerrosmallin mukaan

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

Influences of light and humidity on carbonyl sulfide-based estimates of photosynthesis

Understanding climate controls on gross primary productivity (GPP) is crucial for accurate projections of the future land carbon cycle. Major uncertainties exist due to the challenge in separating GPP and respiration from observations of the carbon dioxide (CO2) flux. Carbonyl sulfide (COS) has a dominant vegetative sink, and plant COS uptake is used to infer GPP through the leaf relative uptake (LRU) ratio of COS to CO2 fluxes. However, little is known about variations of LRU under changing environmental conditions and in different phenological stages. We present COS and CO2 fluxes and LRU of Scots pine branches measured in a boreal forest in Finland during the spring recovery and summer. We find that the diurnal dynamics of COS uptake is mainly controlled by stomatal conductance, but the leaf internal conductance could significantly limit the COS uptake during the daytime and early in the season. LRU varies with light due to the differential light responses of COS and CO2 uptake, and with vapor pressure deficit (VPD) in the peak growing season, indicating a humidity-induced stomatal control. Our COS-based GPP estimates show that it is essential to incorporate the variability of LRU with environmental variables for accurate estimation of GPP on ecosystem, regional, and global scales.Peer reviewe

Carbon dioxide and methane fluxes over a boreal river measured with eddy covariance

Non peer reviewe

Reviews and Syntheses: Carbonyl Sulfide as a Multi-scale Tracer for Carbon and Water Cycles

Abstract. For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO2 during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO, where CA is the more important. The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude. Here we review the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. OCS uptake is correlated well to plant carbon uptake, especially at the regional scale. OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. More work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain