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

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11.4.7 Auxiliary Data

Unlike DIC and CH 4 , CO 2 fluxes cannot be proposed sepa-
rately in the budget for the aqueous phase, as CO 2 can be
converted into HCO 3 − and CO 32 −. Nevertheless CO 2 fluxes
are estimated for the air-water interface, as CO 2 is the only
inorganic carbon specie capable to be exchanged.

(a) In the mixolimnion, electroneutrality equation is:

[][][][]

[][][][]

HCOCONaK

Ca Mg Cl SO

33

2

22

4

2

2

22 2

−−++

++−−

+=++

+−−

if only major species (>10μmol L/ ) are considered. This
equation corresponds to the definition of alkalinity (left
member) and alkaline reserve (right member).

[]HCOC 33 −−+= 2 []OA^2 lk

If DIC and Alk are known, [CO 2 ] can be calculated by:

[]()[][

()]( )

CO KCOAlk DIC

KDIC AlkKDICAlk

2

2

2

2

14

42 2

×− +×−+

−− −

′

′′== 0

where K’ is the apparent equilibrium constant of the
reaction:

2 HCOC 32 −−=+OCOH 32 + 2 O

(b) In the monimolimnion, major species are different and
the electroneutrality equation is:

[][][][][]

[][][][

HCONaKCa Mg

Cl Fe NH H

3

22

2

4

22

2

−++++

−++

=+++

−+ +− 224 PO−]

In this compartment where pH⊕ 63. :

DICH=+[] 23 CO []HCOC 33 −−+≈[]OH^2 [] 23 CO +[]HCO 3 −

[]HCOA 3 − ≈ lk

DICA>> lkandC[]OH^22 =≈[]CO^3 DICA− lk^

It is then necessary to study also the cycle of elements
involved in Alk. These elements or compounds can be
divided in two groups:

• Elements that are present both in the mixolimnion and the
  monimolimnion. They are not reactive in the lake system

and inputs/outputs occurred only by brooks or springs.

They will be called “conservative elements”.

• Elements or species that are present in the monimolim-
  nion only. They can be either produced or consumed by
  chemical or biochemical reactions within the lake or the
  sediment: they will be called “reactive species”.

11.4.7.1 Conservative Elements

This group includes Na+, K+, Ca^2 +, Mg^2 + and Cl−. All these

species present the same pattern, for example for sodium

(Fig. 11.8):

• Constant in the mixolimnion at about 220 μmolL−^1


• Large increase within the mesolimnion


• Almost constant in the bottom layer

The large increase across the chemocline is related to the

low value of the eddy diffusion coefficient in this layer. The

constancy in the bottom layer can be associated with an input

of mineral water at ca. 67–70 m depth and a weak diffusion

from the sediment at the bottom.

Alkali trace elements have a similar behaviour. However,

the Li/Rb ratio exhibits a maximum at about 70 m depth

(Fig. 11.9). It is well known (Fouillac 1983 ) that mineral

waters in Massif Central have an unusually high Li content;

the Li/Rb maximum at ca. 70 m depth confirms the input of

a mineral water at this depth. Recently Gal et al. ( 2015 ) pre-

sented new data namely δD (H 2 O) and δ^7 Li that support this

hypothesis.

11.4.7.2 Reactive Species

The three main species of this group are Fe^2 +, NH 4 + and

HP 24 O− (HPO 42 − is present at concentrations lower than

30 μmolL−^1 and can be neglected). The cycle of these three

species is limited to the monimolimnion and is associated

with the mineralization of organic matter: N and almost P

compounds result in the mineralization of organic matter,

and Fe^2 + is formed by reduction of ferric oxides by organic

matter (dissimilatory reduction of iron).

Their profiles are similar:

• Essentially absent in the mixolimnion


• Large increase in the mesolimnion


• Significant increase in the bottom layer

Even if the production of these species occurs partially in

the water column, the major part occurs in the sediment (or

at the sediment surface) and the pattern in the monimolim-

nion reflects diffusion from the sediment towards the bottom

waters.

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

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