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

308

estimated during a short-term study (Bettarel et al. 2003 ).
Finally, we used small microcosms to access growth rates of
protists and their potential trophic relationships with meta-
zoa (Carrias et al. 2001 ).

18.2 Distribution and Composition
of Heterotrophic Nanofl agellates
(HNF)

The seasonal distribution of free and attached HNF was
assessed in the epilimnion of Lake Pavin from April to
November 1991. Total abundance varied from 0.1 × 10^3 to
2.5 × 10^3 cells ml −1 and averaged 0.8 × 10^3 cells ml −1 (Fig.
18.1 ). Free-living HNF largely dominated the community,
accounting for 80 % of the cells. Monas -like cells and small-
unidentifi ed cells were the dominant taxa during spring (Fig.
18.2 ). Bicoecids and choanofl agellates were attached to large
diatoms ( Aulacoseira italica , Asterionella formosa ) and to
the colonial cyanobacteria Anabaena fl os-aquae , and
accounted for 9 and 11 % of HNF densities, respectively. The
dynamics of the free-living HNF were different from that of
epiphytic HNF. Free-living HNF reached peak abundance in
spring at the onset of thermal stratifi cation and the sedimen-
tation of the spring diatom bloom. The development of epi-
phytic HNF was recorded at the end of spring during the
decline of free-living HNF and in autumn (Fig. 18.1 ). During
these periods the free-living forms were probably subjected
to high predation pressure from metazooplankton while large
size diatoms and colonial cyanobacteria provided attached
forms a refuge against metazooplankton predation. Because
attached fl agellates had higher ingestion rates than free-
living bacterivorous fl agellates (see below), their bacterivory
may be of great signifi cance in Lake Pavin.

18.3 Spatio-Temporal Distribution

and Community Structure of Ciliates

The distribution of ciliated protozoa was investigated during

the period of April to December 1992 in the epi- (1 m depth),

meta- (15 m depth), and hypolimnion (40 m depth) of Lake

Pavin. The densities of ciliates ranged from 200 to 24,000

ciliates L −1 (Mean = 4000 ciliates L −1 ) and are typical of

unproductive lakes. Values were comparable at depths of 1

and 15 m, with highest densities during the period of water

overturn. Values remain low during the thermal stratifi cation

suggesting that there was intense predation pressure by the

macrozooplankton ( Cyclops abyssorum prealpinus ,

Acanthodiaptomus denticornis , Daphnia longispina ,

Ceriodaphnia quadrangula ) during this period. A total of 28

genera or species of ciliates were identifi ed during this study.

Oligotrichs ( Strobilidium spp., Strombidium viride ,

Pelagohalteria viridis ) largely dominated the community at

1 and 15 m depth (Fig. 18.3 ) accounting for 40 % of total

abundance. Small-sized algivorous prostomatids ( Balanion

planctonicum and Urotricha spp.) were relatively more

abundant at 1 m (33 % of total abundance) than at 15 m

(22 %). These ciliates are probably the main predators of

nanoplanktonic algae. Bacterivorous scuticociliates (mainly

Cyclidium spp.) accounted for 18 % of total abundance at

1 m and 19 % at 15 m. In addition to these three dominant

groups, various taxa ( Vorticella spp., Colpoda sp., Askenasia

volvox , Chilodonella sp., Lembadion magnum ) developed

for short periods and occurred especially at 15 m depth. In

the deep hypolimnion (40 m depth), the highest densities

occurred during the major spring development of the scuti-

cociliate Uronema nigricans. Scuticociliates largely domi-

nated the hypolimnetic ciliate community, accounting for

64 % of the total abundance. In term of biomass, the ciliate

community represented from 0.2 to 16.4 μgC L −1.

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0.6 0 m5 m

0 m

5 m

0.4

0.2

Fixed HNF (100.0

3 ml

-1)

Free HNF (10

3 ml

-1)

Spring Summer Autumn

Fig. 18.1 Seasonal distribution of free and attached
(fi xed) HNF at 0 and 5 m depth in the epilimnion of
Lake Pavin (Modifi ed from Carrias et al. ( 1998a ) )

J. Carrias et al.