Carbon gas fl uxes are monitored on plot and landscape level in the Zackenberg valley using two measurement techni-ques:
• Automatic chamber measurements of CH4 and CO2 exchange on plot scale in a fen site.
• Eddy covariance measurements of CO2 and H2O exchange on landscape scale in heath and fen sites.
Automatic chamber measurements The CH4 exchange has been monitored in six automatic chambers in a wet fen area since 2006 (Klitgaard et al. 2007). During 2011-2012, the automatic chamber system has been expanded to include four new chambers, giving a total of ten chambers.
The temporal variation in CH4 produc-tion is mainly associated with tempera-ture, water table depth and substrate qua-lity and availabiqua-lity. It has also been found from this site that frost action resulting in accumulated CH4 gas being squeezed out from the soil matrix can be of high importance for the annual CH4 exchange (Mastepanov et al. 2008).
In 2012, CH4 fl ux measurements began 25 June and lasted until 29 October (fi gure 2.16). On 16 July, water accidently entered
2012
May Jun Jul Aug Sep Oct Nov
5 cm 10 cm 30 cm 50 cm
Figure 2.15 Soil moisture content throughout the fi eld season 2012 at the three automatic weather stations M2, M3 and M4.
the measuring equipment, which had to be turned off and dried out until 23 July.
Apart from that the equipment performed well during 2012. At the start of the mea-surement period, CH4 fl uxes were low (< 1 mg CH4 m-2 h-1), but in mid-July emis-sions increased rapidly and reached peak emissions around 27 July (approximately 2.2 mg CH4 m-2 h-1). The early summer and mid-summer peak emissions were similar to those measured in 2009 and 2010, whereas earlier years have shown both higher (2006-2007) and lower (2008) peak values. The seasonal development of CH4 exchange resembles previous years, with an early season peak followed by a smooth decrease in CH4 emissions.
In late August, CH4 fl uxes reached below 1 mg CH4 m-2 h-1. Fluxes generally continued to decrease until 22 October, when the trend reversed and fl uxes began to increase. It is most likely that the slightly higher emissions during the last few days in October compared to earlier autumn periods can be related to frost action giving rise to increased CH4 emissions.
Eddy covariance measurements The land-atmosphere exchange of CO2 is measured using the eddy covariance technique at two sites in Zackenberg: one located in a Cassiope heath site where mea-surements have been carried out since 2000, and one located in a wet fen area where measurements have been carried out since 2007. The heath site instrumentation con-sists of a 3D sonic anemometer (Gill R3) and a closed-path CO2 and H2O gas analyser (Licor-7000). See Klitgaard and Rasch 2008, and Rasch and Caning 2003 for further de-tails on the heath site instrumentation. The fen site instrumentation was upgraded du-ring 2011 to include a 3D sonic anemometer
(Gill HS) and an enclosed-path CO2 and H2O gas analyser (Licor-7200), see Jensen (2012) for more details.
The temporal variations in the mean daily net ecosystem exchange of CO2 (NEE) and air temperature during 2012 for the heath and fen sites are shown in fi gu-res 2.17 and 2.18, and tables 2.10 and 2.11.
NEE refers to the sum of all CO2 exchange processes; including photosynthetic CO2 uptake by plants, plant respiration and mi-crobial decomposition. The CO2 exchange is controlled by climatic conditions, mainly temperature and photosynthetic active radiation (PAR), along with amount of biomass and soil moisture content. The sign convention used in fi gures and tables is the standard for micrometeorological measurements; fl uxes directed from the surface to the atmosphere are positive whereas fl uxes directed from the atmos-phere to the surface are negative.
Heath site
Eddy covariance CO2 fl ux measurements at the heath site in 2012 were initiated 26 April and lasted until 29 October (fi gure
Methane emission (mg CH4 m-2 h-1)
DOY
160 180 200 220 240 260 280 300 320
Figure 2.16 Daily methane (CH4) emissions during 2012 measured at the fen site. Values are mean of six chambers (replicates).
Net ecosystem exchange (g C m–2 d–1) Air temperature (°C)
NEE Tair
100 120 140 160 180 200 220 240 260 280 300 320
Figure 2.17 Daily net ecosystem exchange (NEE) and air temperature (Tair) measured at the heath site in 2012.
2.17). When measurements began, the heath was covered by approximately 1 m of snow. The area was not snow-free until late June. During the cold, snow covered period in late April and May, CO2 fl uxes were low, generally below 0.1 g C m-2 d-1. In late June, when air temperature was close to 10 °C, CO2 fl uxes increased and a maximum springtime daily emission of 0.7 g C m-2 d-1 was detected 25 June. As the vegetation developed during July, the photosynthetic uptake of CO2 started, and 11 July the heath ecosystem switched from being a source to a sink of atmospheric
CO2 on a daily basis. This is the latest start of the net uptake period on record, likely related to the late timing of snowmelt.
The period with net CO2 uptake lasted for 42 days, which is among the shortest net uptake periods measured so far. The onset of the uptake period varies from year to year due to timing of snowmelt.
The end of the uptake period is more stab-le as it is governed by fading solar radia-tion. The accumulated CO2 uptake during the uptake period in 2012, –28.9 g C m-2, was close to the mean of all measured years (–27.4 g C m-2). Also, the maximum
DOY
Net ecosystem exchange (g C m–2 d–1) Air temperature (°C)
NEE Tair
230 240 250 260 270 280 290 300 310
0
−1 0 1 2
–20 –10 0 10 Figure 2.18 Daily net 20
ecosystem exchange (NEE) and air temperature (Tair) measured at the fen site during the fall of 2012.
Table 2.10. Summary of the CO2 exchanges 2003-2012 at the heath site. Please note that the measurement period varies from year to year.
Table 2.11. Summary of the CO2 exchanges 2007-2012 at the fen site. Please note that the measurement period varies from year to year.
Year 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
Measurements start 6 Jun 3 Jun 21 May 28 May 27 May 30 Mar 16 May 5 May 3 May 26 Apr Measurements end 30 Aug 28 Aug 25 Aug 27 Aug 28 Oct 28 Oct 22 Oct 31 Oct 16 Aug 29 Oct Start of net uptake period 29 Jun 23 Jun 8 Jun 8 Jul 16 Jun 6 Jul 13 Jun 1 Jul 26 Jun 11 Jul End of net uptake period 15 Aug 16 Aug 16 Aug 23 Aug 19 Aug 20 Aug 15 Aug 14 Aug 15 Aug 22 Aug NEE for measuring period
(g C m-2) –13.8 –13.2 –37.9 –24.9 –28.2 –11.2 –11.1 5.0 –23.0 –4.6
NEE for net uptake period
(g C m-2) –26.7 –24.6 –38.1 –28.9 –37.8 –32.0 –23.1 –26.8 –31.5 –28.9
Max. daily accumulation
(g C m-2 d-1) –1.26 –1.14 –1.40 –1.11 –1.32 –1.30 –0.97 –1.14 –0.97 –1.11
Year 2007 2008 2009 2010 2011 2012
Measurements start 20 Sep 10 Apr 31 Jul 9 May 7 May 29 Aug
Measurements end 19 Oct 30 Aug 13 Oct 1 Nov 25 Oct 26 Oct
Start of net uptake period – 10 Jul – – 26 Jun –
End of net uptake period – 22 Aug 16 Aug 16 Aug 15 Aug –
NEE for measuring period (g C m-2) 9.8 –65.8 3.5 –73.5 –80.5 37.1
NEE for net uptake period (g C m-2) – –94.6 – – –129.9 –
Max. daily accumulation (g C m-2 d-1) – –4.03 – –5.15 –4.49 –
diurnal CO2 uptake (–1.11 g C m-2 d-1, measured 25 July) was close to the mean (–1.14 g C m-2 d-1).
By 22 August, ecosystem respiration exceeded gross primary production and the heath ecosystem returned to being a net source of atmospheric CO2. In the beginning of this period, soil temperatures remained comparably high, allowing decomposition processes to continue at a decent rate. Highest autumn daily emission was measured 24 August (0.45 g C m-2 d-1).
When air temperature fell below 0 °C in late September daily NEE decreased, and at the end of the measurement period daily NEE was close to zero. During the entire mea-surement period (186 days), the net CO2 balance amounted to –4.6 g C m-2. Taking the rest of the year into account when no measurements were conducted and when low but consistent CO2 emissions can be expected, the CO2 exchange for entire 2012 between the Zackenberg heath and the atmosphere was likely a zero-sum game.
Fen site
The eddy covariance equipment at the fen site was upgraded during 2011. Due to technical problems with storing data on the loggers, data logging setup was changed in August 2012. In this report, only data from 29 August until 26 October is presented (fi gure 2.18).
When measurements began in late August, the fen had already switched from being a net sink to a net source for atmos-pheric CO2 on a daily basis. Generally, during autumn 2012, the CO2 fl uxes were
about twice as high at the fen compared to the heath site. Maximum daily emission (1.3 g C m-2 d-1) was detected 18 September, coinciding with an increase in air tempera-ture. During the measurement period (57 days) the fen emitted 37.1 g C m-2. The growing season’s daily uptake rates as well as the shoulder season’s daily emissions are generally higher at the fen site compared to the heath site. This is because of denser vegetation with higher leaf area index at the fen site, allowing for higher CO2 uptake per area unit.
2.6 Geomorphology
Coastal geomorphology
In 2008, the cliff top along the northern site of the active delta lobe was measured, while the shoreline was measured in 2010, 2011 and 2012.
The shoreline at the river delta showed a rapid decrease from 2008 towards 2010.
Most of the protruding glacial cliff was eroded and a small island remained on the delta plain. The small island eroded in 2012. The shorelines from 2010 and 2012 are almost similar. The shoreline from 2011 was recorded after the fi rst snow and it was not possible to see the high-water mark on the beach. From 2011 to 2012, the shoreline at the delta mouth was eroded several metres. This extensive erosion event in 2012 was caused by the fl ood from the glacial lake outburst in the begin-ning of August.
N
0 100 200 m
Figure 2.19 Delta- and coastal cliff line measured by DGPS in 2008 (yellow line), 17 October 2010 (blue line), 12 October 2011 (purple line) and on 28 September 2012 (black line) on an aerial photo from 8 August 2000.