311ciliates was 4 × 10^6 bacteria L −1 h −1. Epilimnetic bacterivo-
rous ciliates were dominated largely by small heterotrophic
( Strobilidium spp. and Halteria sp.) and mixotrophic
( Pelagohalteria viridis ) oligotrichs. Due to the vertical zona-
tion of mixotrophic and small algivorous forms the relative
contribution of bacterivorous ciliates to the ciliate commu-
nity increased with depth. In the meta- and hypolimnion
attached vorticellids represented up to 66 % of the total
ciliate bacterivory. Grazing impact of scuticociliates was
negligible due to their relative low rates of bacterivory.
An unusual result of this study concerns the importance
of attached protozoa as bacterivores. Small heterotrophic
fl agellates (bicoecids and choanofl agellates) and vorticel-
lids were found almost exclusively on Asterionella formosa ,
and their maximal densities occurred during the sediment-
ing period of this diatom. These attached protozoa had
higher ingestion rates than free bacterivorous protozoa
(Tables 18.1 and 18.2 ) suggesting that attachment to sur-
faces is the most favorable situation for suspension-feeding
protozoa (Fenchel 1986 ). Attached protozoa were not
numerically dominant but were of great signifi cance in the
fate of bacteria (Table 18.3 ). Therefore, even in low num-
bers, epiphytic protozoa may have a major grazing impact
on free bacteria. Due to their attachment to large colonial
algae, and the tests of some taxa (bicoecids), epibiotic pro-
tozoa are probably diffi cult for zooplankton to ingest.
Consequently, attached protozoa would constitute a sink for
carbon rather than a signifi cant link to larger consumers.18.5 Flagellates Grazing Potential
and Viral Lysis in the Fate of BacteriaBacterial heterotrophic production, viral lysis, and potential fl ag-
ellate grazing impacts on bacteria were estimated during a short-
term study in the epilimnion and metalimnion of Lake Pavin. The
main objective of this study was to estimate the relative bacterial
mortality rates inferred from estimated viral infected cells and
assumed fl agellate grazing rates. On average, viral lysis repre-
sented 6.4 % (range = 3.5–10.3 %) of bacterial production in the
epilimnion and 15.6 % (range = 6.0–33.7 %) in the metalimnion.
Bacterial mortality due to fl agellates grazing represented 38.7 %
(range = 0.5–115.4 %) of bacterial production in the epilimnion,
and 66.7 % (range = 0.7–97.5 %) in the metalimnion. Therefore,
fl agellates consumed a larger proportion of bacterial production
than was lost to viral lysis. The summed bacterial mortality due to
viral lysis and fl agellate grazing ranged from 4.8 to 125.7 % in the
epilimnion, and from 10.3 to 103.5 % in the metalimnion, sug-
gesting that these two factors may at times control most of the
bacterial production, beside other potential bacterivores such as
ciliates and cladocerans.Table 18.1 Major fl agellates taxa of the plankton of Lake Pavin, mean length, biovolume, and the range of ingestion and clearance rates
Mean length (μm) Biovolume (μm^3 ) Ingestion (bacteria ind −1 h −1 ) Clearance (nl ind −1 h −1 )
Flagellates
Pigmented
Cryptomonas sp. 25.5 2800
Mallomonas reginae 20.4 1100
Dinobryon cylindricum 14.4 300 2.4–35.3 0.7–26.9
Ochromonas sp. 12.2 950 6.4–87 2.4–13.1
Rhodomonas minuta 10.4 110
Chrysosphaerella conradii 8.3 300
Uroglena sp. 8.0 150 1.3–3.9 0.5–1.2
Mallomonas sp. 7.4 210
Mallomonas akrokomos 7.3 22
Chrysidalis peritaphrena 3.9 24
Chlamydomonas sp. 3.0 14
Undetermined 2.5 8
Nonpigmented
Katablepharis ovalis 7.5 63
Bodonids 6.0 37
Choanofl agellates 5.0 38 1.7–33.6 1.1–13.0
Bicoecids 4.8 34 6.0–92.4 3.9–24.6
Monas -cells like 3.3 19 1.6–27 0.7–11.5
Undetermined 1 3.5 16 2.0–11.5 2.1–3.0
Undetermined spp. 3.0 14
Modifi ed from Carrias et al. ( 1996 )
18 The Importance of Phagotrophic Protists in Lake Pavin