179concentration on the lake bottom. In Lake Pavin, each of
these three individual origins could play a role: (i) a low-
salinity spring in the mixolimnion may stabilize the interface
between mixo- and monimolimnion by increasing the salinity
gradient; (ii) a mineral spring in the monimolimnion may
similarly strengthen the gradient at the interface; and (iii) bio-
logical activity may also increase the particle content, and
therefore the density, in the monimolimnion.
10.2.1 Presence of a Sublacustrine Spring
in the Lower Part of the Mixolimnion
The hypothese of a sublacustrine spring was formulated a
long time ago by geochemical studies. The presence of a
mineralized spring within the monimolimnion was first pre-
dicted in 1975 as an explanation of the observed gradients at
the mixo–monimolimnion interface (Meybeck et al. 1975 ).
10 years later, Martin ( 1985 ) used a box model to explain
both the hydrologic balance and tritium measurements by
assuming that a unique spring existed within the monimo-
limnion. Assayag et al. ( 2008 ) tested various scenarios by
applying the one-dimensional vertical AQUASIM model
(Reichert 1994 ), which incorporates, in particular, the
hypothesis of a sublacustrine spring within the mixolimnion.
This hypothesis was corroborated by the observation of a
cold temperature anomaly on the vertical profiles measured
with a multiparameter (conductivity-temperature-depth)
probe in September 1996, which was not apparent on the
September 1994 profiles (Aeschbach-Hertig et al. 2002 ).
This approximately −0.06 °C anomaly was noticed at depths
between 45 and 50 m. The authors also identified a slight
drop in dissolved oxygen at these same depths, with no
recorded conductivity anomaly.
Recently, the sublacustrine spring hypothesis was con-
firmed (Bonhomme et al. 2011a), on the basis of continuous
physical monitoring of Lake Pavin carried out during years
2006 and 2007. The sublacustrine spring was identified
between 53 and 56 m depth. The temperature of the entering
water was shown to be about 1.3 °C lower than the tempera-
ture of the lower part of the mixolimnion during the mea-
surement campaign. The oxygen and saline contents of the
spring were low in comparison with the mixolimnion.
Moreover, spring puffs going up in the water column by con-
vection were observed, suggesting that the sublacustrine
spring discharged to the lake intermittently.
In the vicinity of Lake Pavin, the hole “Creux du Soucy”,
located at about 2 km from the lake, is a good candidate to
explain such cool water entries in the lake. The surface water
inside the hole has an altitude of 1235 m (35 m higher than
the surface elevation of the lake). The hole extends into a
tube that drips somewhere else at a lower altitude. The water
temperature and conductivity were measured in July 2007
and were respectively 2.1 °C and 23 mS/cm (conductivity at
25 °C is 40 mS/cm), which is consistent with the observa-
tions cited above. Old legends report that this hole may be
directly connected to Lake Pavin by a kind of siphon
(Bakalowicz 1971 ) which may explain the intermittent enter-
ing of water into the lake. This assumption is confirmed by
recent diving explorations of the hole, which revealed that
the hole is connected to a 50 m long quasi-vertical tunnel
which may directly discharge into Lake Pavin.10.2.2 Role of the Spring in the Maintaining
of MeromixisAs the spring is very close to the mixolimnion- monimolimnion
interface, the spring probably influences on the diffusion of
solutes at this interface and therefore on meromixis. To
understand the impact of the sublacustrine spring on mass
transfer at the mixolimnion–monimolimnion interface, a
modelling tool was developped to investigate the role of the
sublacustrine spring role in maintaining meromixis
(Bonhomme et al. 2011a, b). The model is based on the dif-
fusion equation for solutes. The influence of the spring is
evaluated only during the ice-free period of the lake and
compared to the diffusion of solutes that would have hap-
pened if the spring was not present.
As described before, if no spring discharges into the lake,
heat and dissolved substances naturally diffuse in the
monimolimnion- to-mixolimnion direction. The decrease in
heat and salinity gradients through diffusion at the mixo-
monimolimnion interface causes a reduction of the stability
at the interface.
On the contrary, a water inflow able to strengthen a low
conductivity value near the interface raises the interface sta-
bility by sharpening the gradients, while stimulating mixing
with the layers of the mixolimnion shallower than 50 m
depth. This additional water therefore helps insulate the mix-
olimnion from the monimolimnion by reducing vertical dif-
fusion near the interface.
The flux evaluation was performed with a fictitious inde-
pendent passive tracer presenting a gradient analogous to con-
ductivity between the mixo and the monimolimnion over a
1 -year period. This fictitious passive tracer has a concentration
equal to 1 at 80 m depth and to 0 at 30 m depth. It is only trans-
ported, it does not react with any other tracer in the environ-
ment and its concentration does not influence the water density.
The results are summarized in Table 10.1. The presence of a
spring would thus help reduce fluxes between the monimo- and
mixolimnion by some 20 %, with this effect tending to become
more pronounced over time. This flux decrease is significant at
the annual scale and serves to limit diffusion above the moni-
molimnion. On the other hand, the absence of a spring causes a
gradual stability decline by diffusion.10 Lake Pavin Mixing: New Insights from High Resolution Continuous Measurements