178
of dissolved compounds is considered using a one-
dimensional vertical model.
10.1 Physical Presentation of the Lake
In Lake Pavin, only the mixolimnion (from 0 to 60 m) is
affected by seasonal overturns, as opposed to the monimolim-
nion (layer in the 60–92 m depth range) (Martin 1985 ). As
well as many other crater lakes, the lake bottom is exposed to
a heat flux of geothermal origin. Therefore the water tempera-
ture of the monimolimnion exceeds by about 1.8 °C that of
the lower part of the mixolimnion. Warm water being less
dense than cold water, this heating may lead to instability of
the water column. But the concentration of dissolved sub-
stances in the monimolimnion is substantially greater than the
concentration found in the mixolimnion (the specific conduc-
tivity at 25 °C between these two compartments varies from
50 to nearly 500 μS cm−1), thus ensuring monimolimnion sta-
bility. As a consequence, the mixolimnion – monimolimnion
interface is characterized by steep gradients of dissolved sub-
stances, dissolved gases (especially CH 4 and CO 2 ) and by
complete and permanent anoxia (Michard et al. 1994 ).
Both the heat and dissolved compounds of the monimo-
limnion actually diffuse in the direction of the mixolimnion
over time. The combined diffusion of matter and heat from
the monimolimnion, although relatively weak, ensures the
stability of the water column below depths of 30 m (Assayag
et al. 2008 ). This stability differs from 1 year to the next. In
2006 for example, heat and dissolved substances diffuse
more from the monimolimnion to the mixolimnion, the dif-
fuse front reaching 30 m depth, whereas in 2007, the gradi-
ents are steeper between the two lake parts. The position
and intensity of temperature gradient and dissolved species
gradient (chemocline) may vary slightly along the year,
depending on mixing variability in the mixolimnion.
Therefore, stability of the water column in the lower part of
the mixolimnion varies at the inter- and intra- annual time
scale, but the meromictic characteristics of the lake remains
(see Fig. 10.1).10.2 Factors Leading to and Maintaining
MeromixisUnderstanding meromixis and the factors that lead to this
particular regime is a major challenge at present because
many inland water bodies are moving towards this state in
the current context of climate change and increasing anthro-
pogenic inputs of nutrients (Hakala 2004 ). In fact, the warm-
ing of the surface of continental waters causes stronger
density gradients which make it difficult for the lake bottom
to be mixed.
Both the origins and mechanisms involved in the stability
of the Lake Pavin meromixis are poorly known. Hutchinson
( 1957 ) already mentioned different origins for meromixis in
general: (i) ectogenic: in the case of a low salinity water
inflow into the mixolimnion; (ii) crenogenic: inflow of highly-
saline water into the monimolimnion due to volcanic activity;
or (iii) biogenic: biological activity increases dissolved matterFig. 10.1 Comparison of the
evolution of temperature
profiles (in °C) between 2006
(blue) and 2007 (black). Each
profile is separated from the
next by 1 or 2 months
C. Bonhomme et al.