312
18.6 Phagotrophic Ciliates as a Trophic
Link Between Nanoplankton
and Metazoa
The in situ growth rates of the dominant planktonic protists
were estimated in unfi ltered (i.e. raw, with ciliates and zoo-
plankton) and fractionated (<50 μm: without metazooplank-
ton, and < 10 μm: without large-size ciliates and
metazooplankton) water samples incubated in diffusion cham-
bers. We assumed that differences in the growth rates between
treatments refl ected the mortality caused by metazooplankton
grazing. Experiments were conducted on fi ve dates during
spring to estimate both fl agellate and ciliate mortality due to
naturally occurring densities of potential consumers. We found
that the growth rates of heterotrophic nanofl agellates in
the < 10 μm fraction were signifi cantly higher than in the < 50
μm fraction whereas no signifi cant difference was recorded
for autotrophic nanofl agellates. We concluded that heterotro-
phic nanofl agellates were subjected to high predation by large-
sized ciliates ( Urotricha pelagica , Strombidium viride ,
tintinnids, Colpoda sp.), unlike autotrophic nanofl agellates.
We recorded a strong negative correlation between the growthTable 18.2 Major ciliates 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 )
Ciliates
Oligotrichida
Strombidium sp. 84.0 91,700
Strobilidium gyrans 65.9 65,000
Strombidium viride 40.1 9500
Pelagohalteria viridis 29.0 6400 110–1320 29–225
Strobilidium spp. 24.2 3500 6–130 5.4–60
Halteria sp. 20.0 1800 12–440 10–72
Prostomatida
Urotricha sp.1 59.4 82,400
Urotricha spp. 19.7 2100
Pseudobalanion planctonicum 13.7 850
Prorodon sp. 60.0 13,000
Scuticociliatida
Uronema sp. 22.8 1600 5.8–320 4.8–42
Cyclidium spp. 13.1 830 14–64 4.8–54
Haptorida
Cyclotrichium sp. 197.0 1,200,000
Trachellophyllum sp. 42.0 3200
Mesodinium pulex 35.0 10,000
Askenasia volvox 32.0 8700
Mesodinium sp.1 24.7 1400
Peritrichida
Vorticella sp.1 24.5 4900 30–1610 25–211
Vorticella sp.2 48.0 38,100 460–5910 430–790
Others
Colpoda sp. 34.6 10,800
Undetermined 150.0 470,000
Modifi ed from Carrias et al. ( 1996 )
Table 18.3 Relative abundance (Ab) and grazing impact (Gr) of attached fl agellates and attached ciliates as a percentage of total abundance and
grazing impact of total bacterivorous nonpigmented fl agellates and total bacterivorous ciliates (Lake Pavin 1993)
Attached fl agellates Attached ciliates
Depths Ab Gr Ab Gr
5 m 14.0 22.9 0.9 8.1
15 m 11.5 23.2 20.9 56.8
40 m 15.1 26.9 22.7 41.4
Modifi ed from Carrias et al. ( 1996 )
J. Carrias et al.