212
dissolved Fe concentrations measured in the
monimolimnion.
In order to explain the variation of turbidity profiles
observed in the monimolimnion (Fig. 12.3b), we modified our
former steady-state model into a pseudo steady-state model. In
this second model, the particle sedimentation flux in the water
column was increased during 1 month (from April 11 to May
11) due to a diatom bloom (Fig. 12.8). As expected, the modi-
fication of the particle sedimentation flux only affects the par-
ticulate matter distribution in the water column, but does not
impact the dissolved Fe profile. The disturbance tends to
smooth the distribution patterns of particulate matter along the
depth profile, with smaller peaks of SPM at the redox bound-
ary and at the lake bottom than during steady state periods. An
important result of the model is that the system returns very
rapidly to steady state since only 2 months after diatom
blooms, the disturbance is not seen anymore (Fig. 12.8).12.4 Discussion
12.4.1 Global Iron and Phosphorus
Biogeochemistry in Lac PavinThe coupling between chemical, isotopic, mineralogical and
modeling studies allows us to propose an integrated view of
the Fe and P biogeochemical cycles in Lac Pavin, and their
connection to Mn, S and C cycles (Fig. 12.9). In Lac Pavin
water column, dissolved Fe(II) and SRP show a strong con-
centration gradient from high values at the lake bottom to
low values at the redox transition zone (Fig. 12.1b). This02468Turbidity (NTU)012340102030405060708090012345Fe (μM), Fe/P (molar)Depth (m)Turbidity (NTU)MX33 - June 2011
MX34 - Sept. 2011
MX41 - June 2013(A) (B)Fig. 12.3 (a) Turbidity
profiles in Lac Pavin water
column, compared to Fe
concentrations (black
squares) and Fe/P ratio (white
circles) of suspended
particulate matter (SPM).
Turbidity measurement and
SPM collection were both
performed in October 2010.
(b) Three turbidity profiles
illustrating some of the
variability of particulate
matter distribution
Fe (μM)P (μM)Fe = 3.07 PO 4 3-
R = 0.920123400 .5 1 1.5Fig. 12.4 Fe and P concentrations of suspended particulate matter in
Lac Pavin water column. The concentrations are reported in μmol.L−1,
relative to the volume of water filtered. The black squares represent the
particles around and below the peak of turbidity (i.e. depth > 48 m). The
straight line is the best fit for these samples. The grey squares represent
the particles sampled in the mixolimnion, at a depth shallower than
40 m in the water column. The data plotted correspond to a compilation
of various samples collected in October 2010 (MX30), November 2011
(MX35) and June 2013 (MX41)
V. Busigny et al.