200
Qevap. is estimated to 87 .Ls−^1 from δ^18 O (Assayag et al. 2008 )
Qbrooks determination was about 25 Ls−^1
Qmineral is estimated to 14 .Ls−^1 from δ^18 O data (revisited from
Assayag et al. 2008 )
Qrain is estimated to 18 Ls−^1 from mean meteorological data
and lake surface
As the lake level is stable, these data lead to
QL 53 =±31 45.. 78 sm−−^13 () 2700 ± 500 d^1 , close to the previ-
ous estimation from Camus et al. ( 1993 ).
11.5.3.5 Mineralization of POC
in the Hypolimnion
In opposition to deeper layers, the layers above 50 m depth
present element concentrations that are variable with the sea-
sons and a steady state cannot be assumed. However, chemi-
cal composition is about the same at a given period of the
year and an average budget can be estimated.
Between 20 and 50 m depth, DIC increases and [O 2 ]
decreases; this partly corresponds to oxidation of POC pro-
duced by photosynthesis in superficial layers.
On the contrary of O 2 that varies with the season, the profile
of DIC is almost constant during the year. Actually, DIC increase
is related with POC oxidation and with an upward flux.
In a δ^13 CDIC vs. (1/DIC) plot (Fig. 11.13), diffusive mixing
and oxidation reaction are shown as straight lines. Actual
data points are between these 2 lines. Owing to the estimated
flux at 55 m, we can estimate the production of DIC from
POC at 200 − 500 mold .−^1
11.5.3.6 The Photic Layer
The photic layer in Lake Pavin extends from surface to about
20 m depth (Secchi values ca. 4–10 m depth), and sharp
peaks of [O 2 ] are observed near the thermocline. The diffu-
sion of produced oxygen is limited by the low Kz value
around the thermocline, i.e. near 10 m depth. Thus photosyn-
thesis can occur in layers rather isolated from the surface
where PMC can be as low as 45 %.
The amount of produced POC is rather difficult to derive
from our data. From the amount of POC mineralized in the
deeper layers, it cannot be less than about 3 kmold−^1.11.5.3.7 Input and Output of Rivers
at the Surface
Small brooks are the tributaries of the lake. Their discharges
have been measured all along the year and an average total
discharge can be estimated as 2160 md^31 − (May 2006–Aug.
2007). The mean concentration of DIC and alkalinity is
05 .. 8 molm−^3 The net input of both DIC and Alk is therefore
1300 mold−^1.
From a metrological station installed on the lake, the rain-
water input is 1550 md^31 −. The mean concentration of DIC
and alkalinity are respectively ca. 0.05 and 0 .. 012 molm−^3
The net input of both DIC and Alk are respectively ~100
and 20 mol d–1.
The superficial lake outlet carries away ca. 6000 md^31 − of
water, i.e. 3000 mold−^1 of DIC and alkalinity. The main spe-
cies of DIC is HCO 3 −.
The budget of DIC and Alk is summarized in Fig. 11.15.11.6 Conclusions
The monimolimnion of the lake Pavin contains high concen-
trations of greenhouse carbon gases (CO 2 and CH 4 ), up to
respectively 14 and 4 mmolL−^1. The total pressure of gases isFig. 11.15 Tentative
Carbon cycle. Fluxes are
in kmold−^1 (see Table
11.1 for associated
processes and
uncertainties). Blue:
DIC; red: CH 4 ; black:
POC; green: Alk
D. Jézéquel et al.