Measurements of GHGs in Lake Kuivajärvi, Hyytiälä

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1 Measurements of GHGs in Lake Kuivajärvi, Hyytiälä Anne Ojala 1,2, Jukka Pumpanen 2, Jaana Bäck 2, Jouni J. Heiskanen 3, Miitta Rantakari 1, Heli Miettinen 1, Ivan Mammarella 3 & Timo Vesala 3 1 University Helsinki, Department of Environmental, Lahti, Finland 2 University of Helsinki, Department of Forest Sciences, Helsinki, Finland 3 IDepartment of Physics, Division of Atmospheric Sciences, Helsinki, Finland ICOS Kuopio 2014

2 In Finland on average 10% of the land area covered by lakes Most of the lakes small Strongly affected by the catchment area Started

3 Lake Kuivajärvi Location Lake SMEAR

4 Lake Kuivajärvi Surrounded by managed forest ca. 2 km long max depth ca. 14 m high DOC concentration mg l -1 peatlands turns lakes net heterotorphic affects the physics of the lake seen in GHGs

5 Wind conditions on the lake

6 Lake Kuivajärvi High chlorophyll concentration 3-5 µg l -1, but occasional high peaks Eutrophicated probably due to long existence of the field station PO 4 concentration in the surface 4 µg l -1, but much higher close to the bottom in summer strongly stratified Anoxic hypolimnion during summer stratification No anoxia in winter CH 4, N 2 O Typical Finnish humic lake Not in natural state

7 Stratification Dimictic lake = turns over in spring and autumn Stratifies in summer as well as in winter Eroding thermocline in summer Euphotic zone ca. 4 m deep

8 Lake-SMEAR instrumentation Manual sampling for ph, DOC, O 2, CO 2, CH 4, N 2 O, NH 4+, NO 3-, DON, P, 18 O, 13 C Maximum time interval 1 week Eddy covariance measurements 2 m above water Li-Cor LI-7200 Enclosed CO 2 /H 2 O Analyzer (from June 2011 onwards) Gas fluxes, energy fluxes Metek sonic anemometer Picarro CH 4 analyzer, CRDS (installed in summer 2012) Net radiation and PAR above water Temperature profile in the water column 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 8.0, 10.0, 12.0 m PT 100 CO 2 -concentration profile in the lake 0.5, 1.0, 1.5, 2.5 m Vaisala GMP 343 CO 2 probes + pumps Gas collection unit (silicone) Primary production/community respiration measurements ph, oxygen, conductivity 0.5 m YSI 6600 EDS water quality sensor PAR at 0.5 m and 1.0 m depth under water Li-Cor LI-193SA Underwater PAR sensor

9 Lake-SMEAR instrumentation STREAMS Inflow and outflow continuously monitore Water level monitoring continuously with Levelogger sensors CO 2 -concentration monitoring with Vaisala GMM220 CO 2 sensors All small/temporary streams/brooks sampled manually Intensive sampling campaign in spring 2012 and 2013 Importange of hydrology Spring flood

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11 Model for gas transfer coefficient (k) = ( ) + k depends on wind speed (U) and penetrative convective velocity (w * ) (and thus heat flux)

12 Why to bother? The most commonly used model for k: k 600 = ws 1.7 where ws is the wind speed at 10 m height. Nearly all studies on the role of lakes on catchment/landscape/global scale carbon cycling are based on this simple model NB: EC and chambers are direct ways to measure fluxes No need for gas transfer coefficients

13 Gas transfer velocity: models Has the role of lakes been underestimated so far? Half hour averages of ksite (black, dashed line, cm h -1 ) and kfp (blue line), and the modelled gas transfer velocities with kuw (green), ksr (red), kra (turquoise) and kcc (purple) during days

14 Lake Kuivajärvi non-ec GHG measurements Water column CO 2, CH 4 and N 2 O measurements Once a week during the open water period, once a fortnight when the lake was ice covered Gas chromatography Flux was calculated from the equation: F gas =αk gas (C gas C eq ) (Wanninkhof and Knox 1996) C gas is the concentration in the surface water C eq is the equilibrium concentration k is the gas transfer coefficient α is the chemical enhancement factor (assumed to be 1) Gas transfer coefficient was determined from chamber measurements, not taken from a model

15 CO 2, CH 4 and N 2 O concentrations in the surface water and hypolimnion of Lake Kuivajärvi in 2011 and 2012 Surface Hypolimnion No anoxia in hypolimnion in winter > hypolimnion is not the source of spring CH 4 peak

16 Daily CO 2, CH 4 and N 2 O fluxes (mmol m -2 d -1 ) from Lake Kuivajärvi in 2011 and , ,8 CH4 (mmol m-2 d-1) 450 1,6 1,4 CH 4 (mmol m -2 d -1 ) 1,2 1,0 0,8 0,6 0,4 0,2 N2O (mmol m-2 d-1) CO2 (mmol m-2 d-1) CO 2 (mmol m -2 d -1 ) N 2 O (µmol m -2 d -1 ) 0,

17 Lake physics: Lake Number Year 2011 Year 2012 Lake Number

18 Take home message All three GHGs measured in Lake Kuivajärvi The lake is a source of all of them Vertical GHG fluxes tightly connected to the water column physics k higher than anticipated Lake specific? Signs of lateral fluxes Strong influence of the surrounding catchment on GHGs Runoff DOC

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