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

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rates of small algivorous Urotricha spp. and autotrophic nano-
fl agellates in all of the fractions studied, supporting the
hypothesis that small Urotricha is a very important nanoalgi-
vore in lacustrine pelagic environments (Weisse et al. 1990 ;
Sommaruga and Psenner 1993 ). The growth rates of large-
sized ciliates were higher in the < 50 μm fraction than in the
unfi ltered water. By assuming that this difference refl ected the
mortality caused by metazooplankton, we concluded that
Cyclops abyssorum prealpinus , the dominant metazooplank-
ton species, had a preponderant impact on large-sized ciliates
during spring. Therefore, our results suggest that large-size
ciliates may be able to effi ciently transfer energy between het-
erotrophic nanofl agellates and metazooplankton.

18.7 Conclusions

The roles of phagotrophic protists in pelagic ecosystems are
summarized in Fig. 18.4. The precise position of different
groups of phagotrophic protists in the planktonic food web
will provide us a much better understanding of microbial
ecology and ecosystem functioning. Heterotrophic nanofl a-
gellates are the main consumers of bacteria. Free heterotro-
phic nanofl agellates are particular important during spring
while attached heterotrophic nanofl agellates appeared in
summer and autumn. Because of their attachment to large-
size colonial algae, attached fl agellates and ciliates are prob-
ably diffi cult to ingest by ciliates and metazooplankton. In

contrast, free heterotrophic nanofl agellates and autotrophic

nanofl agellates are grazed by large-size ciliates and small-

size algivorous ciliates, respectively. Large-size ciliates are

in turn consumed by copepods ( Cyclops abyssorum prealpi-

nus) in spring and probably by cladocerans ( Daphnia longi-

spina and Ceriodaphnia quadrangula ) during summer and

autumn. Although we have no data, we assume that small-

size ciliates are potential prey for metazooplankton (Sanders

et al. 1996 ). Thus, phagotrophic protists play a pivotal role in

the planktonic food webs by transforming small-size prey

(bacteria, small fl agellates) into larger preys (fl agellates and

large-size ciliates), which can then be consumed by

metazooplankton.

Acknowledgements We are grateful to Robert W. Sanders for com-

ments and valuable suggestions to improve the manuscript.

References

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of ciliated protozoa in relation to the bacterial and phytoplanktonic

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Bennett S-J, Sanders RW, Porter KG (1990) Heterotrophic, autotrophic,

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Viruses

Infection

Lysis

Phytoplankton

Bacteria

Free HNF

Large-size ciliates

Metazooplankton

ANF

Small-size ciliates

Attached HNF and ciliates

Macroaggregates

Sedimentation

Excretion

DOM

?

?

?

?

Fig. 18.4 Relationships among lake Pavin planktonic communities including viruses and protists. DOM dissolved organic matter, HNF heterotro-
phic nanofl agellates, ANF autotrophic nanofl agellates

18 The Importance of Phagotrophic Protists in Lake Pavin

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