Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

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static thermometers line (Bonhomme et al. 2011 ). Temperature
profiles obtained with the Seabird profiler (Fig. 11.14) revealed
some anomalies in the hypolimnion but at various depths
according to the period: they were located more often in the
50–55 m depth zone (July 2004, April-July 2007) but also ca.
34–39 m in April 2010 or 33–36 m in June 2010. For these last
periods, T anomalies where well marked and reached up to
−° 02 .C (April 2010). A cold intrusion where also deduced
from thermometers line data at ca. 53 m depth in 2007
(Bonhomme et al. 2011 ). On the other hand, any T anomalies
where observed at other periods (e.g. Oct. 2010 – Fig. 11.14b -,
almost none in 2006 and 2009), suggesting that this inflow may
be intermittent. Concerning the depth of these anomalies, it is
important to remember that measurements where usually per-
formed in the central zone of the lake, and that observed depth
should be considered as the resulting effect of the isodensity and
plume propagation rather than the real depth of the input, which
is still to precise both in depth and direction.

Any anomalies were visible from conductivity measure-

ments at the resolution of the probe ( 25 .μScm−^1 ). This could

be explained as this water, supposed chemically very near

from surface streams, represents an important supply for the

lake: waters from water column and this inflow are then very

difficult to distinguish with this parameter.

In the absence of any identified geochemical tracer for the

sublacustrine input in the mixolimnion, one can only deduce

the corresponding inflow from other inputs/outputs values,

leading to a less accurate determination. Moreover, the

calculated value may correspond to the difference between

the real input and a hypothetical sublacustrine output. Water

budget is equilibrated as following:

QQ^53 =+output QQevap.−−brooks QQmineralr− ain^

Qoutput is estimated to about 67 Ls−^1 from the monitoring of

the outlet;

0

10

20

30

40

50

60

3.9 3.95 44 .054.1 4.15 4.2

Depth(m)

0

10

20

30

40

50

60

70

80

90

3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1

Depth (m)

2010

a

b

Fig. 11.14 (a) and (b):
Temperature profiles in
2007 (0–65 m depth)
and 2010

11 Carbon Cycle in a Meromictic Crater Lake: Lake Pavin, France

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