| 1. |
- Golub, Malgorzata, et al.
(author)
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Diel, seasonal, and inter-annual variation in carbon dioxide effluxes from lakes and reservoirs
- 2023
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In: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 18:3
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Journal article (peer-reviewed)abstract
- Accounting for temporal changes in carbon dioxide (CO2) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO2 based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO2 flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2 emissions had an average of 25% (range 3%-58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO2 flux variability were delineated through mutual information analysis. Sample analysis of CO2 fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO2 fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.
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| 2. |
- MacIntyre, Sally, et al.
(author)
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Turbulence in a small boreal lake: Consequences for air-water gas exchange
- 2021
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In: Limnology and Oceanography. - : WILEY. - 0024-3590 .- 1939-5590. ; 66:3, s. 827-854
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Journal article (peer-reviewed)abstract
- The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air-water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate-forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (epsilon) using an acoustic Doppler velocimeter and a temperature-gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near-surface epsilon varied from 10(-8) to 10(-6) m(2) s(-3) for the 0-4 m s(-1) winds and followed predictions from Monin-Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10(-3) m(2) s(-1) on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L-N) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using epsilon agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s(-1), a condition that can lead to elevated near-surface epsilon and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind-sheltered lakes, and provide new equations to quantify fluxes.
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| 3. |
- Müller, Roger A., et al.
(author)
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Hourly, daily, and seasonal variability in the absorption spectra of chromophoric dissolved organic matter in a eutrophic, humic lake
- 2014
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In: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 119:10, s. 1985-1998
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Journal article (peer-reviewed)abstract
- The short-term (hourly and daily) variation in chromophoric dissolved organic matter (CDOM) in lakes is largely unknown. We assessed the spectral characteristics of light absorption by CDOM in a eutrophic, humic shallow mixed lake of temperate Sweden at a high-frequency (30 min) interval and during a full growing season (May to October). Physical time series, such as solar radiation, temperature, wind, and partial pressures of carbon dioxide in water and air, were measured synchronously. We identified a strong radiation-induced summer CDOM loss (25 to 50%) that developed over 4 months, which was accompanied by strong changes in CDOM absorption spectral shape. The magnitude of the CDOM loss exceeded subhourly to daily variability by an order of magnitude. Applying Fourier analysis, we demonstrate that variation in CDOM remained largely unaffected by rapid shifts in weather, and no apparent response to in-lake dissolved organic carbon production was found. In autumn, CDOM occasionally showed variation at hourly to daily time scales, reaching a maximum daily coefficient of variation of 15%. We suggest that lake-internal effects on CDOM are quenched in humic lake waters by dominating effects associated with imported CDOM and solar exposure. Since humic lake waters belong to one of the most abundant lake types on Earth, our results have important implications for the understanding of global CDOM cycling.
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| 4. |
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| 5. |
- Nilsson, Erik, 1983-, et al.
(author)
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Evaluating humidity and sea salt disturbances on CO2 flux measurements
- 2018
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In: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 35, s. 859-875
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Journal article (peer-reviewed)abstract
- Global oceans are an important sink of atmospheric carbon dioxide (CO2). Therefore, understanding the air-sea flux of CO2 is a vital part in describing the global carbon balance. Eddy covariance (EC) measurements are often used to study CO2 fluxes from both land and ocean. CO2 are usually measured with infrared absorption sensors, which at the same time measure water vapor. Studies have shown that presence of water vapor fluctuations in the sampling air potentially result in erroneous CO2 flux measurements due to cross-sensitivity of the sensor. Here we compare measured CO2 fluxes from both enclosed path Li-Cor 7200 sensors and open-path Li-Cor 7500 instruments from an inland measurement site and a marine site. We also introduce new quality control criteria based upon a Relative Signal Strength Indicator (RSSI). The sampling gas in one of the Li-Cor 7200 instruments was dried by means of a multi-tube diffusion dryer so that the water vapor fluxes were close to zero. With this setup we investigated the effect that cross-sensitivity of the CO2 signal to water vapor can have on the CO2 fluxes. The dryer had no significant effect on the CO2 fluxes. We tested the hypothesis that the cross-sensitivity effect is caused by hygroscopic particles such as sea salt by spraying a saline solution on the windows of the Li-Cor 7200 instruments during the inland field test. Our results confirm earlier findings that sea salt contamination can affect CO2 fluxes significantly and confirm earlier findings, that drying the sampling air for the gas analyzer is an effective method to reduce this signal contamination.
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| 6. |
- Podgrajsek, Eva, et al.
(author)
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Comparison of floating chamber and eddy covariance measurements of lake greenhouse gas fluxes
- 2014
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In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 11, s. 4225-4233
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Journal article (peer-reviewed)abstract
- Fluxes of carbon dioxide (CO2) and methane (CH4) from lakes may have a large impact on the magnitude of the terrestrial carbon sink. Traditionally lake fluxes have been measured using the floating chamber (FC) technique; however, several recent studies use the eddy covariance (EC) method. We present simultaneous flux measurements using both methods at lake Tämnaren in Sweden during field campaigns in 2011 and 2012. Only very few similar studies exist. For CO2 flux, the two methods agree relatively well during some periods, but deviate substantially at other times. The large discrepancies might be caused by heterogeneity of partial pressure of CO2 (pCO2w) in the EC flux footprint. The methods agree better for CH4 fluxes. It is, however, clear that short-term discontinuous FC measurements are likely to miss important high flux events.
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| 7. |
- Podgrajsek, Eva, et al.
(author)
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Diurnal cycle of lake methane flux
- 2014
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 119:3, s. 236-248
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Journal article (peer-reviewed)abstract
- Air-lake methane flux (FCH4) and partial pressure of methane in the atmosphere (pCH4a) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both FCH4 and pCH4a with high values during nighttime (FCH4 ≈ 300 nmol m−2 s−1, pCH4a ≈ 2.5 µatm) and low values during day (FCH4 ≈ 0 nmol m−2 s−1, pCH4a ≈ 2.0 µatm) for a large part of the data set. This diurnal cycle persist in all open water season; however, the magnitude of the diurnal cycle is largest in the spring months. Estimations of buoyancy in the water show that high nighttime fluxes coincide with convective periods. Our interpretation of these results is that the convective mixing enhances the diffusive flux, in analogy to previous studies. We also suggest that the convection may bring methane-rich water from the bottom to the surface and trigger bubble release from the sediment. A diurnal cycle is not observed for all convective occasions, indicating that the presence of convection is not sufficient for enhanced nighttime flux; other factors are also necessary. The observed diurnal cycle of pCH4a is explained with the variation of FCH4 and a changing internal boundary layer above the lake. The presence of a diurnal cycle of FCH4 stresses the importance of making long-term continuous flux measurements. A lack of FCH4measurements during night may significantly bias estimations of total CH4 emissions from lakes to the atmosphere.
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| 8. |
- Podgrajsek, Eva, et al.
(author)
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Enhanced nighttime gas emissions from a lake
- 2016
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In: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315.
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Conference paper (peer-reviewed)abstract
- Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Previous studies have shown that lakes can be important natural sources of atmospheric CH4 and CO2. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange. Most previous lake flux studies are based on chamber measurements, by using the eddy covariance method, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes. Eddy covariance measurements at a Swedish lake show that both methane fluxes (FCH4 ) and carbon dioxide fluxes (FCO2 ) experience high nighttime fluxes for a large part of the data set (largest median FCH4night ≈ 13 nmol m2 s-1 and smallest median FCH4day ≈ 4.0 nmol m-2 s-1, largest median FCO2night ≈ 0.2 μmol m2 s-1 and smallest median FCO2day ≈ 0.02 μmol m-2 s-1, with larger variability during night). For the diel cycle of the CH4 fluxes it is suggested that water side convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to water side convection. If gas fluxes are not measured during nighttime, when water side convection normally is generated, periods of potential high gas flux might be missed and estimations of the total amount of gas released from lakes to the atmosphere will be biased.
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| 9. |
- Podgrajsek, Eva
(author)
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Lake Fluxes of Methane and Carbon Dioxide
- 2015
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Doctoral thesis (other academic/artistic)abstract
- Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Recent studies have shown that lakes, although they cover a small area of the globe, can be very important natural sources of atmospheric CH4 and CO2. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange.By using the eddy covariance method for lake flux studies, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes.Eddy covariance measurements at a Swedish lake revealed a diel cycle in the fluxes of both CH4 and CO2, with higher fluxes during nighttime than daytime. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to waterside convection. For the diel cycle of CH4 flux it was suggested that waterside convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition.Simultaneous flux measurements of CH4 and CO2 have been presented using both the eddy covariance method and the floating chambers method of which the latter is the traditional measuring method for lake fluxes. For CO2 the two methods agreed well during some periods but differed considerably during others. Disagreement between the methods might be due to horizontal heterogeneity in partial pressure of CO2 in the lake. The methods agreed better for the CH4 flux measurements. However, it is clear that due to the discontinuous nature of the floating chambers, this method will likely miss important high flux events.The main conclusions of this thesis are:1) the two gas flux methods are not directly comparable and should be seen as supplementary to each other2) waterside convection enhances the fluxes of both CH4 and CO2 over the water-air surface. If gas flux measurements are not conducted during nighttime, potential high flux periods might be missed and estimates of the total amount of gas released from lakes to the atmosphere may be biased.
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| 10. |
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