Anne Ojala Timo Vesala, Jussi Huotari, Elina Peltomaa, Jukka Pumpanen, Pertti Hari Üllar Rannik,...
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Transcript of Anne Ojala Timo Vesala, Jussi Huotari, Elina Peltomaa, Jukka Pumpanen, Pertti Hari Üllar Rannik,...
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