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deeper than 10–12 m are isolated from the atmosphere and superficial inputs as shown
by the low PM^14 C value of 45 % at 10 m depth.- In the superficial layer, an input of 13 ..± 02 kmold−^1 of DIC is brought by several
small brooks, and an output of 30 ..± 02 kmold−^1 is calculated for the discharge. - CO 2 exchanges with atmosphere are highly variable seasonally; the lake is strongly
under saturated in summer and oversaturated in November. An annual balance is
derived from modeling the global lake functioning, leading to a CO 2 escape of
41 ± kmold−^1.
Budgets for DIC and alkalinity (alk) are proposed. For this latter, we need to distinguish
two kinds of dissolved species:- “conservative” ions (e.g. Na+, K+, Ca^2 +, Mg^2 +, Cl−) brought by streams and other
water venues - “reactive” species (e.g. Fe^2 +, NH 4 +, HP 24 O−), essentially produced within the sedi-
ment and consumed or precipitated at ca. 60 m depth i.e. at the redox interface.
CO 2 behaviour is deduced from both DIC and Alk budgets. The dissolved concentra-
tion of CO 2 in the monimolimnion, associated with those of CH 4 and N 2 , allow assessing
the gas outburst natural hazard, which is very low considering the actual conditions.Keywords
Carbon cycle • Lake modeling • Gas outburst natural hazard • CO 2 and CH 4 emissions11.1 Introduction
Dissolved gases (carbon dioxide and methane) play an impor-
tant role in the carbon cycle of lakes (Cole et al. 1994 ;
Bastviken et al. 2004 , 2008 ). Generally, CO 2 is consumed dur-
ing photosynthesis in the upper layer and produced during
mineralization of organic matter (OM) in the hypolimnion and
in sediments. When the hypolimnion becomes anoxic, pro-
duction of methane occurs especially when the sulfate content
of waters is low. Depending on the water column properties
(height, stratification, eddy diffusion...) a part of the produced
methane is oxidized in CO 2 by bacteria within the lake whereas
the other part may escape toward the atmosphere.
In temperate latitude, the exchange of carbon dioxide
between water and air is generally a gas absorption during
the spring and summer and a large degassing during lake
overturn at the end of fall. Although it may be essential for
the greenhouse effect when combined at regional scale, the
net balance is often difficult to assess.
In crater lakes, possible inputs of gases from mantle still
increase the amount of dissolved gases in the system. For
meromictic lakes, such an increase involves gas concentra-
tions in the monimolimnion close to saturation and a gas out-
burst may occur (for instance in Monoun and Nyos lakes,
Cameroon, in 1984 and 1986 respectively; Sigurdsson et al.
1987 ; Freeth and Kay 1987 ; Schmid et al. 2006 ).
The possibility of such a gas outburst in Lake Pavin,
France, has been already studied (Camus et al. 1993 ;
Aeschbach-Hertig et al. 1999 , 2002 ). However the answer to
a controversial discussion on risk assessment and manage-
ment in Lake Pavin vicinity (Lavina and del Rosso 2006 )
needs more than a simple chemical monitoring. A quantita-
tive understanding of the carbon cycle in the lake is needed.
In this paper, we associate previous data and some new anal-
yses in order to present a carbon cycle with three
objectives:- Distinguish between biogenic and volcanic CO 2
- Predict the evolution of the saturation status of gases in
the monimolimnion - Evaluate the transfers of CH 4 and CO 2 to the
atmosphere.
11.2 Presentation of the Lake
Lac Pavin shows several characteristics that have attracted
the attention of numerous scientists (in the last decades:
Olivier 1952 ; Alvinerie et al. 1966 ; Omaly, 1968 ; Pelletier
1968 ; Meybeck et al. 1975 ; Devaux et al. 1983 ; Amblard and
Restituito 1983 ; Restituito 1984 , 1987 ; Martin 1985 ; Martin
et al. 1992 ; Camus et al. 1993 ; Cossa et al. 1994 ; Michard
et al. 1994 , 2003 ; Schmid 1997 ; Viollier 1995 ; Viollier et al.
1995 , 1997 ; Aeschbach-Hertig et al. 1999 , 2002 ; Albéric
et al. 2000 ; Olive and Boulègue 2004 ; Lehours et al. 2005 ,
2007 ; Schettler et al. 2007 ; Bonhomme 2008 ).D. Jézéquel et al.