1. Anne Ojala Timo Vesala, Jussi Huotari, Elina Peltomaa, Jukka Pumpanen, Pertti Hari Üllar Rannik, Tanja Suni, Sampo Smolander, Andrey Sogachev and Samuli Launiainen Department of Ecological and Environmental Sciences & Department of Physical Sciences 13 December 2007 HERC Carbon gas fluxes from a boreal lake

2. Rationale of the study Boreal lakes can act as conduits of terrestrially fixed carbon Importance of catchment area -studies in Amazonas by Richey et al. (2002) -Hanson et al. (2004): 14% of NEE of the surrounding watershed can be mineralized and vented to the atmosphere via the lake Allochthonous vs. autochthonous carbon -DOC -Carbon source for heterotrophs -Microbial food webs Kyoto protocol

3. Study site: Lake Valkea-Kotinen Pristine boreal lake Surface area 0.041 km 2 -Max depth 6.5 m, mean depth 2.5 m high content of DOC pH < 6 -inorganic carbon mostly in the form of CO 2 Headwater lake -Catchment area 0.30 km2 -Mainly old-growth forest A true reference lake !

4. Characteristics of the lake: stratification Temperature (ºC) Short or incomplete spring turn over meromixis Steep summer stratification Complete autumn turn over Oxygen (g m -3 ) Anoxic hypolimnion (below 2.5 – 3 m) CH 4 production Autumn turn over 2003 2004

5. Carbon gas measurements Continuous CO 2 flux measurements since summer 2002 Eddy Covariance (EC) Chamber measurements Measurements based on water column CO 2 concentrations Throughout the growing seasons Striking improvement in temporal resolution

6. Results: Allochthonous carbon Precipitation 30% higher in 2004 than in 2003 peak in July DOC (mg C l -1 ) in surface water Clearly higher after July in 2004 Slow recovery -Carry over to the next growing season

7. Results: CO 2 in the water column CO 2 concentration (mmol m -3 ) Stratification in summer Autumn turn over Higher in 2004 2003 2004

8. Comparison of different methods Month µmol m -2 s -1 *C&C = estimate based on CO 2 concentration difference between the surface water and atmosphere

9. Results: CO 2 fluxes based on EC measurements Seasonal pattern in CO 2 exchange -fluxes higher during the fall turn-over than during the spring turn-over -lowest fluxes in summer

10. Results: Process parameters Community Respiration (R) and Primary Production (P) CO 2 production to consumption ratio (R to P) higher in 2004 than in 2003 Carry-over effect in 2005? R:P vs CO 2 flux

11. CO 2 probes

12. Surface water CO 2 concentration: seasonal dynamics

13. Determination of CO 2 exchange in water g = CO 2 exchange rate C = CO 2 concentration F a = CO 2 flux from lake to the atmosphere F u = CO 2 flux from deep to surface

14. Photosynthesis-irradiance response curves 6-10 September, 200611-15 September, 2006

15. P-I Response Curve and Photosynthesis Estimate (submitted to Limnology and Oceanography: Methods) Fall photosynthetic rate: 0.35 g(CO 2 )m -2 day - 1 Daily respiration: 1.00 g(CO 2 )m -2 day - 1 NEE -0.65 g(CO 2 )m - 2 day -1 Lake appeared as a small source of CO 2

16. Size-fractionated PP at surface Small microplankton (20-50 µm) dominated in spring (37-73 %) and autumn (79-96 %) Peridinium Large microplankton (> 50 µm) dominated (46-72 %) from the end of June till the end of September Gonyostomum

17. Size-fractionated PP at 1,5 meter Picoplankton (0.2-2 µm) dominated (57-62 %) production in autumn zerozero

18. Long-term changes in photosynthetic biomass Total biomass; decreasing trend (k = -1,4 x 10 -2 g m -3 a -1 ) Gonyostomum semen; decreasing trend (k = -2,4 x 10 -2 g m -3 a -1 )

19. CO 2 fluxes from a larger lake: Lake Pääjärvi and an extreme weather event Summer time precipitation doubled >> a clear peak in CO 2 flux - Not due to in-lake biological processes, but imported from the catchment area FC = chamber measurement ΔCO2 = concentration gradient method

20. Remarks Truly interdisciplinary research Techniques already in use in environmental physics and terrestrial ecology introduced to aquatic ecology True integration of studies on soil ecology and limnology Spin-off projects So far created the world’s longest EC data series on CO 2 exchange over a lake Extensive data sets on lake ecosystem ecology New insights on functioning of lake ecosystems on landscape level Not only DOC, but also CO 2