Anne Ojala Timo Vesala, Jussi Huotari, Elina Peltomaa, Jukka Pumpanen, Pertti HariÜllar 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

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

Study site: Lake Valkea-Kotinen

Pristine boreal lake Surface area 0.041 km2

- Max depth 6.5 m, mean depth

2.5 m

high content of DOC

pH < 6

- inorganic carbon mostly in the

form of CO2

Headwater lake

- Catchment area 0.30 km2

- Mainly old-growth forest

A true reference lake !

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) CH4 production

Autumn turn over

2003 2004

Carbon gas measurements

Continuous CO2 flux

measurements since

summer 2002

Eddy Covariance (EC)

Chamber measurements Measurements based on

water column CO2

concentrations Throughout the growing

seasons Striking improvement in

temporal resolution

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

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12Month

Pre

cipi

tati

on (

mm

)

2003

2004

2005

10

11

12

13

14

15

16

17

18

April June August October

Month

mg

l-1

2003

2004

2005

Results:CO2 in the water column

CO2 concentration (mmol m-3)

Stratification in summer Autumn turn over Higher in 2004

2003 2004

Comparison of different methods

August 2003

-0.5

0

0.5

1

1.5

0 4 8 12 16 20 24

EC

C&C

Chamber

Month

µm

ol m

-2 s

-1

*C&C = estimate based on CO2 concentration difference between the surface water and atmosphere

Results:CO2 fluxes based on EC measurements

Seasonal pattern in CO2 exchange

- fluxes higher during the fall turn-over than during

the spring turn-over

- lowest fluxes in summer

EC

-20

-10

0

10

20

30

40

50

5 6 7 8 9 10-11

Month

CO

2 fl

ux (

mm

ol m

-2d-1

)

2003

2004

2005

Results:Process parameters

Community Respiration (R)

and Primary Production (P) CO2 production to

consumption ratio (R to

P) higher in 2004 than

in 2003 Carry-over effect in

2005? R:P vs CO2 flux

EC

-20

-10

0

10

20

30

40

50

5 6 7 8 9 10-11

Month

CO

2 fl

ux (

mm

ol m

-2d-1

)

2003

2004

2005

R/PT

0

2

4

6

8

10

4-5 6 7 8 9 10-11Month

2003

2004

2005

36

CO2 probes

Surface water CO2 concentration: seasonal

dynamics

0

50

100

150

200

250

300

350

400

4-2005 7-2005 10-2005 1-2006 4-2006 7-2006 10-2006

CO

2 (µ

mo

l l-1

)

0

0.5

1.5

Eq

0 calc

0

20

40

60

80

100

120

21.05 23.05 25.05 27.05 29.05 31.05 02.06 04.06

Date in 2006

CO

2 (µ

mo

l l-1

)

0

0.5

1.5

Eq

0 calc

Determination of CO2 exchange in water

0 ),(

)(bh

ua FFdhdt

thCdtg

g = CO2 exchange rate

C = CO2 concentration

Fa= CO2 flux from lake to the atmosphere

Fu = CO2 flux from deep to surface

Photosynthesis-irradiance response curves

6-10 September, 2006 11-15 September, 2006

P-I Response Curve and Photosynthesis Estimate

(submitted to Limnology and Oceanography: Methods)

Fall photosynthetic rate:

0.35 g(CO2)m-2day-1

Daily respiration: 1.00 g(CO2)m-2day-1

NEE -0.65 g(CO2)m-2day-

1

Lake appeared as a small source of CO2

Size-fractionated PP at surface

12.5. 19.5. 23.5. 22.6. 27.6. 21.7. 25.7. 25.8. 29.8. 22.9. 26.9. 20.10.24.10.

0

10

20

30

40

50

60

70

80

90

100

0 m

Pe

rcen

tage

va

lue 0,2-2 µm

2-20 µm 20-50 µm > 50 µm

Date

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

Size-fractionated PP at 1,5 meter

12.5. 19.5. 23.5. 22.6. 27.6. 21.7. 25.7. 25.8. 29.8. 22.9. 26.9. 20.10.24.10.

0

10

20

30

40

50

60

70

80

90

100

Date

1,5 m

Pe

rce

nta

ge

va

lue 0,2-2 µm

2-20 µm 20-50 µm > 50 µm

Picoplankton (0.2-2 µm) dominated (57-62 %) production in autumn

zero

Long-term changes in photosynthetic biomass

0

2

4

6

8

10

12

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Vuosi

Bio

ma

ss

a m

g C

m-3

kokonaisbiomassa trendi

0

2

4

6

8

10

12

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Vuosi

Bio

ma

ss

a m

g C

m-3

G. semen -biomassa trendi

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)

CO2 fluxes from a larger lake:

Lake Pääjärvi and an extreme weather event

0

10

20

30

40

50

60

04 May 22 May 12 Jun 27 Jun 07 Jul 30 Jul 21 Aug 01 Sep 20 Sep 05 Oct 23 Oct 22 Nov

2004

Pre

cip

itat

ion

(mm

)

Summer time precipitation doubled >> a clear peak in CO2

flux- Not due to in-lake biological processes, but imported from the catchment area

FC = chamber measurementΔCO2 = concentration gradient method

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 CO2 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 CO2