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

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10.2.3 Winter Influence, Interannual
Variability and Meromixis

The winter overturn has undeniably a great influence on the
state of the water column during the following year. It seems
that the ability of the seasonal overturn to keep the interface
mixolimnion-monimolimnion around 60 m depth is linked
both to the meteorological conditions of the winter and to the
presence of the spring. A seasonal overturn was monitored
during the 2006–2007 winter thanks to a thermistor chain
recording continuously the temperature of the lake, even
underneath the ice. Figure 10.2 shows that the seasonal over-
turn of the mixolimnion occurred in two stages: in the initial
stage, water adjacent to the density maximum at these depths
(3.9 °C) (Eklund 1965 ) dipped at the end of January to a
depth in the range of 50 m. The 10 m layer above the chemo-
cline (between 50 and 60 m) was therefore not mixed. As the
temperature of the surface falls below 4 °C during this
period, an inverse stratification then occurred between the

surface and 50 m depth in the mixolimnion (data not

provided).

In a second time, the sudden inflow of a large quantity of

low-salinity water around February 25, 2007 (see Fig. 10.2)

eventually flattened the salinity gradient, which enabled the

seasonal overturn to reach the chemocline (at 60 m). A quick

calculation of heat balance between 40 and 60 m suggests to

attribute the temperature drop at these depths to a cold external

water inflow and thus definitely to the presence of a spring.

During the 2006–2007 winter, it thus seems likely that the

sublacustrine spring helped fix the mixing depth at the time

of spring mixing as well as diffusion at the mixolimnion–

monimolimnion interface. Therefore, the difference between

the 2006 and 2007 temperature profiles (shown in Fig. 10.1)

may be explained by winter overturn differences not only

due to changing meteorological conditions, but to varying

hydrological processes, favouring either the occurrence of

water inflow from the spring or an absence of entering subla-

custrine water.

Table 10.1 Simulated tracer flux (arbitrary unit) in the presence or absence of the spring above the monimolimnion

Starting time

After 1-year simulation

No spring With a spring

Tracer mass (arbitrary unit) between 30 and 80 m 59.04 55.69 56.38

Tracer mass (arbitrary unit) between 30 and 60 m 0.67 3.35 2.66

60

26/01/07 05/02/07 15/02/07

3.9°C

3.85°C

Date

Depth (m)

25/02/07 07/03/07

50

40

30

20

10

Fig. 10.2 Observation of isotherms
dipping between January 25 and
March 10, 2007, until a depth of
58 m. Only those isotherms in the
vicinity of the maximum density
temperature are shown

C. Bonhomme et al.