196
surface waters are either very under saturated or highly over
saturated, the results presented on Table 11.1 are significant.
In addition, exchange of CO 2 was directly measured in
June 2010, June 2011 and December 2012 by the floating
chamber method. In June 2010, flux mean values were
−±13 016..mmolmd−−^21 in the central zone, and about
− 8 mmolmd−−^21 near the shore (negative values are for a
CO 2 uptake by superficial waters). In June 2011 fluxes were
almost zero but in December 2012 the mean value in the cen-
tral zone was 76 .. 82 ± 5 mmolmd−−^21. These values can be
compared to calculated values from pH and alkalinity mea-
surements in the surface water (in situ measurements for pH,
Gran titration for alk after sampling) with the following
relation:
FkCO^22 =×()CO__solubility−CO^20 Z=^
where FCO2 is the carbon dioxide flux (molm d−−^21 ), k (md–1)
is the gas exchange velocity. CO 2 _Z= 0 and CO 2 _solubil-
ity are given in molm−^3. CO 20 _Z= is calculated from alka-
linity (444–460 μM), the water pH (8.05–8.15 range) and the
Ka1 and Ka2 dissociation constants at 14 15−°C (Alk, pH and
T data from June 2010).
Calculated fluxes are in good agreement with measured
ones (June 2010) if km= 1 d−^1 for the stations near the shore,
and km= 17. d−^1 for the stations in the central zone. These k
values correspond to classical values for lakes (Cole and
Caraco 1998 ).
The surface waters are strongly under saturated with
respect to atmospheric CO 2 at least from mid May to the end
of September, due to the photosynthesis uptake. On the con-
trary, they are strongly over saturated after the mixolimnion
overturn, which occurs partially in November to December,
and more after the end of ice melting. With only three mea-
sured flux values the annual budget is impossible to calcu-
late. However from the model of the lake presented by Lopes
et al. ( 2011 ), a value of 41 ± kmol.d−^1 can be derived, corre-
sponding to the main carbon output from the lake. Similar
values were obtained in Lake Kivu (Borges et al. 2014 ).11.5.3 Tentative Quantitative C CycleThe quantitative carbon behaviour in the lake will be
described from the bottom to the surface.11.5.3.1 The Bottom Layer (70–92 m Depth)
In this layer DIC, CH 4 and Alk present concentration gradi-
ents of respectively 0.15, 0.10 and 00 .m 5 olm−^4. The vertical
dispersion coefficient (Kz) was estimated to about
00 .m 5 21 day− (Aeschbach-Hertig et al. 1999 ). Thus the flux
of these species can be estimated to 7.5, 5 and 1 mmolm day−−^21
respectively, i.e. 1500, 1000 and ≈ 500 molday−^1 for the
whole monimolimnion.
These values are in rough agreement with the POC total
input to the bottom of the lake (≈ 3200 mold. −^1 for the whole
monimolimnion). Only a small part of POC is buried.
Uncertainties on both eddy diffusion coefficient and sedi-
ment traps efficiency may explain the difference.11.5.3.2 Input of Mineral Water at 65–70 m
Depth
The minimum of PMC and the maximum of the Li/Rb ratio
at 65–70 m depth suggest the inflow of a^14 C depleted and
lithium rich mineral water. A simple model considering aTable 11.1 Fluxes of Alk, DIC, CH 4 and C burial in the global carbon budget of the Lake Pavin
Depth (m) Process DIC fluxes (kmold−^1 ) Alkalinity fluxes (kmold−^1 )
0 m Output (surface outlet) −± 30 .. 03 −± 30 .. 03
0 m Exchange with atmosphere −± 41 0
0 m Input by rain 0100 ..± 04 0 .. 020 ± 0004
8–10 m Input by brooks 13 ..± 02 13 ..± 02
10–15 m Photosynthesis −± 35 .. 05 0150 ..± 1
20–50 m Aerobic respiration 03 ..± 02 − 00. 2
53 m Fresh water input 16 ..± 02 14 ..± 02
58–60 m Fe(II), NH 4 + and CH 4 oxidation 10 ..± 02 −± 03 .. 02
68 m Mineral water input 26 ..± 03 06 ..± 02
75–92 m DIC flux from the sediment 15 ..± 02 05 ..± 01
CH 4 flux from the sediment 10 ..± 02
92 m C burial in the sediment −±^07 ..^02 -
Negative values correspond either to a subtraction of C species from the dissolved phase or to a carbon escape from the lake (water column)
D. Jézéquel et al.