309Mixotrophic oligotrichs ( Strombidium viride , Pelagohalteria
viridis, Strobilidium caudatum, Strombidium sp1 ) largely
dominated the ciliate biomass at 1 m and 15 m depth during
summer. These ciliates contain pigmented endosymbionts
and are dependent on light for their growth. In spring and
autumn, large-sized ciliates ( Colpoda sp., Lembadion mag-
num , Cyclotrichium sp., Paradileptus elephantinus ) and the
prostomatid Urotricha sp. formed most of the ciliate biomass
at 1 m and 15 m depth. In the deep hypolimnion, the bacte-
rivorous scuticociliate Uronema nigricans dominated the
biomass, but only in May and June (Fig. 18.3 ).
18.4 Bacterivory by Phagotrophic Protists
We assessed bacterivory by phagotrophic protists using
fl uorescent microspheres as tracers. We evaluated cell-spe-
cifi c grazing rates of the different taxa and estimated the
importance of different groups of protists to total protistan
bacterivory. These measures were done on seven occasions
from June to November 1993 and at three depths represen-
tative of the epilimnion (5 m), metalimnion (15 m) and
hypolimnion (40 m). Protistan bacterivory was dominated
by phagotrophic nonpigmented fl agellates (Table 18.1 ),
accounting for 84 % of the total protistan bacterivory.
About 62 % of nonpigmented fl agellates taxa were found
to ingest particles. The highest grazing impact was
18.9 × 10^6 bacteria L −1 h −1 during a peak of heterotrophic
fl agellates in June. At this time, bacterial productivity
might be totally consumed by heterotrophic fl agellates.
Grazing impact was higher in the metalimnion than in the
epi- and hypolimnion. A clear seasonal pattern was
observed with highest values of grazing impact during
June and in autumn, at the three depths. Monas -like cells
were the dominant grazers, followed by small attached
fl agellates (bicoecids and choanofl agellates), fi xed mainlychoanoflagellatesbicoecids Small undetermined HNFKathablepharis ovalisMonas-like cells0%20%40%60%80%100%20%40%60%80%100%0%0m
5m
Apr 24May 2May 7May 15May 22May 29June 5
June 12June 19July 11July 19Aug 1Aug 12Sep 11Sep 29Oct 17Nov 27Fig. 18.2 Relative contribution of
different HNF taxa to total HNF
abundance. From Carrias et al.
( 1998a ). This material is reproduced
with permission of John Wiley &
Sons, Inc
18 The Importance of Phagotrophic Protists in Lake Pavin