194
The increasing slope of the concentration versus depth in
the mesolimnion is mainly due to the low dispersion coeffi-
cient in this layer.
At the mixolimnion-mesolimnion interface, which is also
the redox interface, NH 4 + is oxidized by O 2 into NO 3 − as
suggested by the occurrence of a nitrate peak at the redox-
cline (N 2 may be produced through denitrification process in
the anoxic zone just below the nitrate maximum). [data not
shown]
Fe^2 + is oxidized into Fe(OH) 3 , likely through different
processes depending on the efficiency of the mixing in the
mixolimnion. When O 2 is supplied in depth by a strong mix-
ing event (i.e. years with favourable conditions of mixing,
generally a poor ice cover during winter), dissolved oxygen
reacts with ferrous iron and generates a strong turbidity peak.
In most common conditions of low O 2 supply in depth, both
MnO 2 and NO 3 − may oxidize Fe^2 + in anaerobic conditions.
Phosphate ions are adsorbed on authigenic Fe(OH) 3 (cf.
chapter of this book about iron cycle). High efficiency phos-
phorus trapping with Fe(III) particles can explain the very
high P content in the monimolimnion of the lake (up to 350
μM; Michard et al. 1994 ).11.5 Discussion
11.5.1 Gas Saturation in the MonimolimnionCO 2 concentrations are three times greater than CH 4 concen-
trations. But, as the solubility of CH 4 is much smaller than
the solubility of CO 2 , the partial pressure of methane is far
greater and reaches about 2 bars at 88 m depth whereas the
CO 2 partial pressure is less than 0.3 bar.
Another gas presents a high partial pressure close to 1 bar:
nitrogen (N 2 ) at equilibrium with atmosphere at the surface
and produced at the redoxcline by denitrification process
(Anammox reaction may occur but is not considered as a
significant process due to very low nitrite concentration at
the redoxcline).
Finally, as total pressure of dissolved gases (Fig. 11.10) is
significantly lower than the hydrostatic pressure at the same
depth, in the present status, degassing by formation of gas
bubbles cannot occur spontaneously in the monimolimnion.
However, total pressure in the bottom waters is definitelyLi/Rb0
10
2030
40
50
60
70
80
904681012Depth (m)Fig. 11.9 Lithium to rubidium ratio (September 1993)
0102030405060708090150 200 250 300350 400450 500Na (M)Depth (m)Fig. 11.8 Typical
sodium profile in Lake
Pavin
D. Jézéquel et al.