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

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11.4.5 Dissolved Organic Carbon (DOC)

In contrast with DIC and CH 4 , DOC increases steadily below
65 m depth (Fig. 11.6). It has been studied in detail by
Albéric et al. ( 2000 ). As its amount is rather low
(< 400 μmolL−^1 when DIC is about 14 ,μ 000 molL−^1 at the
same depth), it will no further be discussed in the carbon
budget.

11.4.6 Particulate Organic Carbon (POC)
Fluxes

In the contrary with dissolved species, POC flux is highly
variable within the year. Sedimentation fluxes, as measured
with sediment traps, present a maximum at the mid of spring
(April-May) corresponding to the sinking of diatoms result-
ing from a bloom occurring some weeks before. The flux
(Fig. 11.7) is about 01 .. 1 mmolm−−^21 .day .day−1 in April 1994
and only 310 × −−^42 mmol..mday−^1 in September 1994
(Viollier et al. 1997 ). A rough annual mean value is
00 .. 1 mmolm−−^21 .day.
In addition,^14 C analysis for sediment trap particles
(from July–Aug. 2006 and Nov.2006–Mar.2007 periods, at
4 depths: 23, 58, 70, 88 m) and for plankton (sampled with
a 20 μm mesh net, 0–10 m depth range) gave remarkable
low PMC values: 74 .. 63 ± 0 PMC (mean for 6 trap samples)
and 67 .. 00 ± 3 PMC and 63 .. 90 ± 3 PMC (plankton from
April 2007 and Feb. 2008 respectively). This proves the

recycling of old carbon (likely issuing from the mineral

water located at 68 m depth) within the lake through

photosynthesis.

This variability is not in contradistinction with the steady

state observed for dissolved species. For elements diffusing

from the bottom of the lake resulting from POC mineraliza-

tion, the very low eddy diffusion coefficient Kz (between

0.01 and 00 .. 5 mday^21 −) attenuates variations with time

within a few meters.

0

10

20

30

40

50

60

70

80

90

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

DOC (mM)

Depth (m)

Fig. 11.6 Typical DOC profile in
Lake Pavin (from Albéric et al.
2000 )

POC flux

0

0.02

0.04

0.06

0.08

0.1

0.12

113 143 173 203 233 263 293323 353383 413443 47 3

Julian day

Flux (mol m

-2

day

-1)

Fig. 11.7 POC flux at 85 m depth from April 20, 1994 to April 20,

1995. The period of high fluxes corresponds to the spring diatoms
      bloom

D. Jézéquel et al.