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Lakes receive and transform significant amounts of terrestrial carbon and are often considered a source of atmospheric carbon dioxide (CO2). Yet, continuous direct measurements of lake‐atmosphere CO2 exchange with high temporal resolution are sparse. In this study, we measured the CO2 exchange of a mountain lake in the eastern Austrian Alps continuously for one year using the eddy covariance (EC) and the boundary layer model (BLM) approaches. Results from both the EC and the BLM methods indicated the lake to be a small source of atmospheric CO2 with highest emissions in fall. EC flux measurements were affected by low‐frequency contributions especially during low wind conditions. The CO2 concentration gradient at the air‐water interface decreased during night‐time due to an increase in atmospheric CO2 above the lake, likely caused by cold and CO2‐rich air draining from the surrounding land. Consequently, BLM fluxes were lower during night‐time than during daytime. This diel pattern was lacking in the EC flux measurements because the EC instruments deployed at the shore of the lake did not capture low nocturnal lake CO2 fluxes due to the local wind regime. Overall, this study illustrates the effect of the surrounding landscape on lake‐atmosphere flux measurements. We conclude that estimating CO2 evasion from lakes situated in complex topography needs to explicitly account for biases in EC flux measurements caused by low‐frequency contributions and local wind regimes. Plain Language Summary: Lakes and rivers are an important link in the global carbon cycle transporting carbon from the land to the oceans. However, part of the carbon entering the water is stored in lake sediments or released from the water into the atmosphere as carbon dioxide (CO2). Therefore, lakes are considered important sources of atmospheric CO2, yet continuous direct measurements of this water‐air CO2 exchange are sparse. In this study, we used two different methods to measure the CO2 exchange of a small lake in the Austrian Alps for an entire year. We found that during the ice covered period, in spring, and in summer the CO2 exchange between lake and air was small. In fall, the lake released CO2 at higher rates. Overall, the lake was only a small source of CO2. We also found significant differences in the results of the two measuring methods. Those results demonstrated that measurements of lake‐atmosphere CO2 exchange are complex and—especially if the lake is small and situated in a mountainous landscape—the surrounding land can influence the measurements. Key Points: We estimated the carbon dioxide (CO2) exchange of a small mountain lake using the eddy covariance and the boundary layer model approachesCO2 fluxes were small and variable, and the variation in local atmospheric CO2 concentration was an important driver of lake CO2 exchangeThe study demonstrates that the influence of the surrounding land has to be considered in lake‐atmosphere flux measurements [ABSTRACT FROM AUTHOR]

An overview of acetaldehyde exchange above a managed temperate mountain grassland in Austria over four growing seasons is presented. The meadow acted as a net source of acetaldehyde in all 4 years, emitting between 7 and 28 mgCm-2 over the whole growing period. The cutting of the meadow resulted in huge acetaldehyde emission bursts of up to 16.5 nmolm-2 s-1 on the day of harvesting or 1 day later. During undisturbed conditions both periods with net uptake and net emissions of acetaldehyde were observed. The bidirectional nature of acetaldehyde fluxes was also reflected by clear diurnal cycles during certain time periods, indicating strong deposition processes before the first cut and emission towards the end of the growing season. The analysis of acetaldehyde compensation points revealed a complex relationship between ambient acetaldehyde mixing ratios and respective fluxes, significantly influenced by multiple environmental parameters and variable throughout the year. As a major finding of this study, we identified both a positive and negative correlation between concentration and flux on a daily scale, where soil temperature and soil water content were the most significant factors in determining the direction of the slope. In turn, this bidirectional relationship on a daily scale resulted in compensation points between 0.40 and 0.54 ppbv, which could be well explained by collected ancillary data. We conclude that in order to model acetaldehyde fluxes at the site in Neustift on a daily scale over longer time periods, it is crucial to know the type of relationship, i.e., the direction of the slope, between mixing ratios and fluxes on a given day. [ABSTRACT FROM AUTHOR]

Up to now the limited knowledge about the exchange of volatile organic compounds (VOCs) between the biosphere and the atmosphere is one of the factors which hinders more accurate climate predictions. Complete long-term flux data sets of several VOCs to quantify the annual exchange and validate recent VOC models are basically not available. In combination with long-term VOC flux measurements the application of gap-filling routines is inevitable in order to replace missing data and make an important step towards a better understanding of the VOC ecosystem-atmosphere exchange on longer time scales. We performed VOC flux measurements above a mountain meadow in Austria during two complete growing seasons (from snowmelt in spring to snow reestablishment in late autumn) and used this data set to test the performance of four different gap-filling routines, mean diurnal variation (MDV), mean gliding window (MGW), look up tables (LUT) and linear interpolation (LIP), in terms of their ability to replace missing flux data in order to obtain reliable VOC sums. According to our findings the MDV routine was outstanding with regard to the minimization of the gap-filling error for both years and all quantified VOCs. The other gap-filling routines, which performed gap-filling on 24 h average values, introduced considerably larger uncertainties. The error which was introduced by the application of the different filling routines increased linearly with the number of data gaps. Although average VOC fluxes measured during the winter period (complete snow coverage) were close to zero, these were highly variable and the filling of the winter period resulted in considerably higher uncertainties compared to the application of gap-filling during the measurement period. The annual patterns of the overall cumulative fluxes for the quantified VOCs showed a completely different behavior in 2009, which was an exceptional year due to the occurrence of a severe hailstorm, compared to 2011. Methanol was the compound which contributed with 381.5mgCm-2 and 449.9mgCm-2 most to the cumulative VOC carbon emissions in 2009 and 2011, respectively. In contrast to methanol emissions, however, considerable amounts of monoterpenes (-327.3mgCm-2) were deposited to the mountain meadow in consequence to the hailstorm in 2009. Other quantified VOCs had considerably lower influences on the annual patterns. [ABSTRACT FROM AUTHOR]

The broad-band normalised difference vegetation index (NDVI) and the simple ratio (SR) were calculated from measurements of reflectance of photosynthetically active and short-wave radiation at two temperate mountain grasslands in Austria and related to the net ecosystem CO2 exchange (NEE) measured concurrently by means of the eddy covariance method. There was no significant statistical difference between the relationships of midday mean NEE with narrowand broad-band NDVI and SR, measured during and calculated for that same time window, respectively. The skill of broad-band NDVI and SR in predicting CO2 fluxes was higher for metrics dominated by gross photosynthesis and lowest for ecosystem respiration, with NEE in between. A method based on a simple light response model whose parameters were parameterised based on broad-band NDVI allowed to improve predictions of daily NEE and is suggested to hold promise for filling gaps in the NEE time series. Relationships of CO2 flux metrics with broad-band NDVI and SR however generally differed between the two studied grassland sites indicting an influence of additional factors not yet accounted for. [ABSTRACT FROM AUTHOR]

Grasslands comprise natural tropical savannah over managed temperate fields to tundra and cover over a quarter of the Earth's land surface. Plant growth, maintenance and decay result in volatile organic compound (VOCs) emissions to the atmosphere. Furthermore, biogenic VOCs (BVOCs) are emitted due to various environmental stresses including cutting and drying during harvesting. Fluxes of BVOCs were measured with a proton-transfer-reaction - mass-spectrometer (PTR-MS) over temperate mountain grassland in Stubai Valley (Tyrol, Austria) over one growing season (2008). VOC fluxes were calculated from the disjunct PTR-MS data using the virtual disjunct eddy covariance method and the gap filling method. The two independent methods obtained methanol fluxes following a regression line of y=0.94x-0.06 (correlation factor: R²=0.94). Methanol showed strong daytime emissions throughout the growing season. With maximal values of 9.7 nmol m-2 s-1 the methanol fluxes from growing grassland were considerably higher at the beginning of the growing season in June compared to those measured during October (2.5 nmol m-2 s-1). During the growth only methanol emissions were observed. The cutting and drying of the grass increased the emissions of methanol, up to 30 nmol m-2 s-2. In addition, emissions of acetaldehyde, up to 10 nmol m-2 s-1, and hexenal (leaf aldehyde) were detected during harvesting. [ABSTRACT FROM AUTHOR]

In order to estimate the air-surface mercury exchange of grasslands in temperate climate regions, fluxes of gaseous elemental mercury (GEM) were measured at two sites in Switzerland and one in Austria during summer 2006. Two classic micrometeorological methods (aerodynamic and modified Bowen ratio) have been applied to estimate net GEM exchange rates and to determine the response of the GEM flux to changes in environmental conditions (e.g. heavy rain, summer ozone) on an ecosystem-scale. Both methods proved to be appropriate to estimate fluxes on time scales of a few hours and longer. Average dry deposition rates up to 4.3 ng m-2 h-1 and mean deposition velocities up to 0.10 cm s-1 were measured, which indicates that during the active vegetation period temperate grasslands are a small net sink for atmospheric mercury. With increasing ozone concentrations depletion of GEM was observed, but could not be quantified from the flux signal. Night-time deposition fluxes of GEM were measured and seem to be the result of mercury co-deposition with condensing water. Effects of grass cuts could also be observed, but were of minor magnitude. [ABSTRACT FROM AUTHOR]

Using a six year data set of eddy covariance flux measurements of sensible and latent heat, soil heat flux, net radiation, above-ground phytomass and meteorological driving forces energy partitioning was investigated at a temperate mountain grassland managed as a hay meadow in the Stubai Valley (Austria). The main findings of the study were: i) Energy partitioning was dominated by latent heat, followed by sensible heat and the soil heat flux; ii) When compared to standard environmental forcings, the amount of green plant matter, which due to three cuts varied considerably during the vegetation period, explained similar, and partially larger, fractions of the variability in energy partitioning; ii) There were little, if any, indications of water stress effects on energy partitioning, despite reductions in soil water availability in combination with high evaporative demand, e.g. during the summer drought of 2003. [ABSTRACT FROM AUTHOR]

Abstract: A process-based spatial simulation model was used to estimate gross primary production, ecosystem respiration, net ecosystem CO2 exchange and water use by the vegetation in Stubai Valley, Austria at landscape scale. The simulations were run for individual years from early spring to late fall, providing estimates in grasslands for carbon gain, biomass and leaf area development, allocation of photoproducts to the below ground ecosystem compartment, and water use. In the case of evergreen coniferous forests, gas exchange is estimated, but spatial simulation of growth over the single annual cycles is not included. Spatial parameterization of the model is derived for forest LAI based on remote sensing, for soil characteristics by generalization from spatial surveys and for climate drivers from observations at monitoring stations along the elevation gradient and from modelling of incident radiation in complex terrain. Validation of the model was carried out at point scale, and was based on comparison of model output at selected locations with observations along elevation gradients in Stubai Valley and Berchtesgaden National Park, Germany as well as with known trends in ecosystem response documented in the literature. The utility of the model for describing long-term changes in carbon and water balances at landscape scale is demonstrated in the context of land use change that occurred between 1861 and 2002 in Stubai Valley. During this period, coniferous forest increased in extent by ca. 11% of the vegetated area of 1861, primarily in the subalpine zone. Managed grassland decreased by 46%, while abandoned grassland and natural alpine mats increased by 14 and 11%, respectively. At point scale, the formulated model predicts higher canopy conductance in 1861 due to lower atmospheric CO2 concentration which opens stomata. As a result, water use at point scale decreased by ca. 8% in 2002 in the valley bottoms versus 10% at tree line. At landscape level, the decrease in water use by vegetation in 2002 was predicted to be twice as high (ca. 17%) due to increase in subalpine forest, reduction of managed grassland in the valley and on slopes, as well as abandonment of grassland which results in natural succession. Net ecosystem CO2 exchange (NEE) was predicted to increase (become more negative) at point scale depending on vegetation type by 10 to 20% in 2002 due to increasing atmospheric CO2 concentration. However, due to the shift from grassland to forest and natural vegetation, landscape level CO2 exchange did not change. As a result of land use change, the export of carbon in harvested biomass in 2002 was estimated at only 30% of that in 1861. While the need for further validation of model assumptions is recognized, especially changes in ecosystem behavior with changing atmospheric CO2 concentration, the model analysis indicates a long-term reduction in water use by vegetation and a shift in ecosystem services. The results provide a case study, where land use change may compensate or override the influences of increasing atmospheric CO2 concentration, maintaining a relatively constant NEE in present time period simulations as compared to 1861, as well as reducing export of carbon from the alpine landscape of Stubai Valley. Use of the model in evaluation of scenarios of future land use change and in relation to vulnerability of ecosystem services are discussed. [Copyright &y& Elsevier]

Metabolic theory and body size constraints on biomass production and decomposition suggest that differences in the intrinsic potential net ecosystem production (NEPPOT ) should be small among contrasting C3 grasslands and therefore unable to explain the wide range in the annual apparent net ecosystem production (NEPAPP ) reported by previous studies. We estimated NEPPOT for nine C3 grasslands under contrasting climate and management regimes using multiyear eddy covariance data. NEPPOT converged within a narrow range, suggesting little difference in the net carbon dioxide uptake capacity among C3 grasslands. Our results indicate a unique feature of C3 grasslands compared with other terrestrial ecosystems and suggest a state of stability in NEPPOT due to tightly coupled production and respiration processes. Consequently, the annual NEPAPP of C3 grasslands is primarily a function of seasonal and short-term environmental and management constraints, and therefore especially susceptible to changes in future climate patterns and associated adaptation of management practices., (© 2013 Blackwell Publishing Ltd/CNRS.)