Contribution of methane-carbon to lake food webs - Oeschger ...

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Contribution of methane-carbon to lake food webs - Oeschger ...

Contribution of methane-carbon

to lake food webs

Roger Jones

Jyväskylä University

roger.i.jones@jyu.fi

Bern 2011 1


Biogenic methane in lakes

• Lake sediments are “hot spots” of methane production.

Some is emitted by ebullition, but much is accumulated

in anoxic zones or oxidised at anoxic-oxic interfaces.

• δ 13 C values (-50 to -90‰) differ markedly from organic

matter source signature (-25 to -35‰).

• Methanotrophic bacteria using methane diffusing up from

anoxic sediments can be abundant at oxic-anoxic

interfaces; even further 13 C-depleted (by ~ -16‰).

• Hence even a small contribution of MOB to a consumer

diet will lower consumer δ 13 C , while a large contribution

will result in very low consumer δ 13 C values.

Bern 2011 2


• How and why did we get interested in

methane?

• Some recent results.

1) Benthic food webs – chironomid larvae.

2) Pelagic food webs – zooplankton.

Bern 2011 3


Profundal chironomid larvae in Loch Ness

Loch Ness ‘isotopic food web map’ (Jones & Grey, 2011, FWB)

16

Ferox T rout

14

δ 15 N ( o / oo )

12

10

8

6

4

2

0

Contribution from

biogenic methane?

?

Chironomids

Crustacean

Zooplankton

Algae

Eel

Ammocoetes

Charr

Minnow

T er. Plant

T rout

Pike

Stickleback

Macroinvertebrates

Sediment

T rout

parr

Salmon

Salmon

parr

Brook

Lamprey

-2

-42 -38 -34 -30 -26 -22 -18

δ 13 C ( o / oo )

Bern 2011 4


How much might methane-derived carbon actually

contribute to chironomid diets?

Data from almost 100 lakes (Jones et al., 2008, Ecology)

Corrected larval δ 13 C (‰)

10

0

-10

-20

-30

-40

C. plumosus

C. anthracinus

C. tenuistylis

Other Chironomus types

Sergentia

Procladius

y = -3.37 -10.8/x

0 2 4 6 8 10 12

C

-10

0

10

20

30

40

50

% carbon assimilated from methanotrophic biomass

mgO 2

/L

Bern 2011 5


Methanotrophic bacteria

utilise CH 4 at oxic-anoxic

interfaces. Chironomid

larvae graze bacteria.

Oxic

CH 4 production in

anoxic sediment

and diffusion

through sediment.

Anoxic

Lewandowski et al. (2007) Freshwater Biology

Bern 2011 6


13

C-depleted larvae also showed 15 N-depletion, although

the relationship differed between species. The reasons for

this associated 15 N-depletion are currently uncertain.

20

15

10

δ 15 N (‰)

5

0

Chironomus spp.

C. plumosus

-5

C. anthracinus

Sergentia spp.

Procladius spp.

-10

-60 -50 -40 -30 -20 -10 0

δ 13 C (‰)

Bern 2011 7


Contribution to higher trophic levels?

Ruffe

Perch,

roach,

bream

δ 15 N (‰)

Profundal

chironomids

Littoral prey

Sublittoral chironomids

MOB

CH δ 13 C (‰)

4

c. 20% of C in ruffe in Jyväsjärvi is of methane origin

(Ravinet et al., 2010, Oikos)

Bern 2011 8


Halsjärvi experimental mixing

(Rask et al., 2010, Biogeochemistry)

Thermocline manipulation to

simulate projected future increase in

the open water season lake mean

temperature by the end of the 21st

century due to climate change.

δ 13 C (‰)

Perch

-25.00

-30.00

-35.00

Valkea-Kotinen (reference)

Halsjärvi (mixed)

-40.00

2004 2005 2006 2007

Mixing years

Bern 2011 9


•How and why did we get interested in

methane?

•Some recent results.

1) Benthic food webs – chironomid larvae.

2) Pelagic food webs – zooplankton.

Bern 2011 10


Zooplankton nutrition in humic lakes

(Jones et al., 1999, Oikos)

δ 13 C DOM o / oo

DOM and POM δ 13 C values

constant across Finnish lakes with

wide range of water colour.

-10

-20

-30

δ 13 C zoopl o / oo

-20

-30

-40

-50

δ 13 C POM o / oo

-40

-20

-30

-40

0 100 200 300 400

Water colour (mgPt l -1 )

0 100 200 300 400

Water colour (mgPt l -1 )

Bern 2011 11

-60

But zooplankton δ 13 C values

actually decreased with

increasing water colour.

Contribution from biogenic methane?


Laboratory experiments showed that methanotrophic bacteria

provide a suitable food resource for Daphnia longispina.

(Kankaala et al., 2006, L & O)

algal diet

microbial diets with and

without CH 4 supplement

Bern 2011 12


Whole-lake 13 C-enrichment of DIC in 2005

(Taipale et al., 2008, Ecology)

Bern 2011 13


(A) The measured δ 13 C (‰) of adult female Daphnia, POM, DIC and DOC

and

(B) The δ 15 N (‰) of adult female Daphnia and juveniles and males during

the spring, summer and autumn 2005.

Bern 2011 14


4 putative food sources but only 2 isotopes –

IsoSource modelling of Daphnia diets in 2005

80 %

Algae MOB HB Chlorobium

60 %

40 %

20 %

0 %

Spring Summer Autumn 1 Autumn 2

The mean values (± 99% CI) of estimated seasonal proportions of

dietary food sources in adult Daphnia in Mekkojärvi in 2005.

Bern 2011 15


Conclusions:

• Benthic (profundal) food webs in lakes can

frequently be fuelled by methane.

• Chironomid larvae are widespread, key players

linking bacterial mobilisers of methane to higher

trophic levels.

• In some lakes cladoceran zooplankton utilise

bacterial mobilisers of methane, but the extent of

this pelagic link is still uncertain.

• Need more work to quantify the contribution to

whole-lake production and carbon budgets.

Bern 2011 16


Energy sources

Solar radiation

DOC

CH 4

Energy mobilisers

Phytoplankton

Heterotrophic

bacteria

Methanotrophic

bacteria

Phagotrophic

microorganisms

Energy dissipators

Metazoan

zooplankton

Benthic

invertebrates

Fish

(Jones & Grey, 2011, Freshwater Biology)

Bern 2011 17


Likely occurrence of 13 C-depleted consumers under

different lake mixing regimes. (Jones & Grey, 2011, Freshwater Biology)

Bern 2011 18


Thanks to very many people, but especially to

Jon Grey

Sami Taipale

Paula Kankaala

Bern 2011 19

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