No abstract

Alpine ecosystems are, similar to arctic ecosystems, characterized by a very long snow season. Previous studies investigating arctic or alpine ecosystems have shown that winter CO 2 effluxes can dominate the annual balance and that the timing and duration of the snow cover plays a crucial role for plant growth and phenology and might also influence the growing season ecosystem CO 2 strength and dynamics. The objective of this study was to analyze seasonal and annual CO 2 balances of a grassland site at an elevation of 2440 m a.s.l in the Swiss central Alps. We continuously measured the NEP using the eddy covariance method from June 2013 to October 2014, covering two growing seasons and one winter. We analyzed the influence of snow melt date on the CO 2 exchange dynamics at this site, because snow melt differed about 24 days between the 2 years. To this end, we employed a process-based ecosystem carbon cycling model to disentangle the co-occurring effects of growing season length, environmental conditions during the growing season, and physiological/structural properties of the canopy on the ecosystem carbon balance. During the measurement period, the site was a net sink for CO 2 although winter efflux contributed significantly to the total balance. The cumulative growing season NEP as well as mean and maximum daily CO 2 uptake rates was lower during the year with the later snow melt, and the results indicated that the differences were mainly due to differing growing season lengths.

The world's streams, rivers, and lakes contribute substantially to the global carbon (C) cycle through the emission of carbon dioxide (CO 2 ) across the air-water interface. Current estimates quantified that 1-3.8 Pg C per year are released by inland waters to the atmosphere (

In complex, sloping terrain, horizontal measurements of net radiation are not reflective of the radiative energy available for the conductive and convective heat exchange of the underlying surface. Using data from a grassland site on a mountain slope characterised by spatial heterogeneity in inclination and aspect, we tested the hypothesis that a correction of the horizontal net radiation measurements which accounts for the individual footprint contributions of the various surfaces to the measured sensible and latent heat eddy covariance fluxes will yield more realistic slope-parallel net radiation estimates compared to a correction based on the average inclination and aspect of the footprint. Our main result is that both approaches led to clear, but very similar improvements in the phase between available energy and the sum of the latent and sensible heat fluxes. As a consequence the variance in the sum of latent and sensible heat flux explained by available radiation improved by >10 %, while energy balance closure improved only slightly. This is shown to be mainly due to the average inclination and aspect corresponding largely with the inclination and aspect of the main flux source area in combination with a limited sensitivity of the slope correction to small angular differences in, particularly, inclination and aspect. We conclude with a discussion of limitations of the present approach and future research directions.

<p>Lakes receive large amounts of carbon (C) from the surrounding catchment and, together with the connecting streams, play an important and active role in the global C cycle. The received C can either be lost through the outflow and eventually transported to the ocean, or transformed and stored in sediments or outgassed to the atmosphere. Globally, lakes are estimated to emit 0.3 – 0.64 Pg C m-2 in form of CO<sub>2</sub> annually.  Although subalpine and alpine lakes were observed to be supersaturated with CO<sub>2</sub>, long-term measurements of lake-atmosphere CO<sub>2</sub> exchange are sparse. Several methods to quantify water-atmosphere gas exchange exist, like chambers, eddy covariance (EC), mass-balance or gradient based methods including boundary layer models (BLM), each having its own advantages and disadvantages. However, quantifying CO<sub>2</sub> exchange in aquatic ecosystems has often proved to be challenging. Here, both the BLM and the EC methods were used to estimate the air-water CO<sub>2</sub> exchange of Lake Lunz, a small lake situated in complex mountainous topography of the Austrian Alps. The results indicated that the lake was a small source of CO<sub>2</sub>. Fluxes were affected by the thermo-topographic flow regime of the field site and its surroundings which drove the local wind pattern but also determined the local atmospheric CO<sub>2</sub> concentration.  During most nights, a significant increase in atmospheric CO<sub>2</sub> was observed which decreased the differential CO2 concentration at the air-water interface and therefore led to decreased nocturnal CO<sub>2</sub> efflux. This diurnal pattern, however, was obscured in the EC measurements, because the method itself highly depends on the local wind regime. Because lakes are an integral part of mountain ranges which are characterized by catchments with complex topography, our findings are most likely of broader impact.</p>