Zooplankton of the open Baltic: Extended Atlas - IOW

Gdańsk Basin (Witek, 1998). Generally, ciliates are more abundant in the
coastal than in the open waters (Khlebovich, 1987; Samuelsson et al., 2006).
The highest abundances of Baltic planktonic ciliates were observed in
shallow inlets of the Southern Baltic (0.17–88 x10 3 ind./l) (Garstecki et al.,
2000). In the central Bornholm Basin, planktonic ciliates reached biomass of
about 0.13–0.3 mg C/l (Beusekom et al., 2007). In the Neva Bay, abundance
of ciliates on average was 3x10 3 ind./l, with maximum values in spring (8x10 3
ind./l); biomass ranged from 0.01 mg of wet mass per liter in autumn to 0.74
– in spring (Khlebovich, 1987). In the northern Baltic Proper, biomass of
planktonic ciliates did not exceed 0.025 mg C/l (Johansson et al., 2004). In
surface waters of the Gulf of Finland, abundance of ciliates constitute 7-20
x10 3 ind./l (Setala & Kivi, 2003). In the western Baltic, maximum biomass
(0.056 mg C/l) was registered in spring; cell numbers ranged from 2x10 3
ind./l to 28x10 3 ind./l (Smetacek, 1981).
Vertical distribution of Baltic planktonic ciliates is also rather
heterogeneous. For example, in the south-western Gdańsk Basin, ciliate
abundance in the euphotic zone was less than 28x10 3 ind./l and biomass 0.023
mg C/l. At the same time, the ciliate biomass in the deep-water layer was
similar to the ciliate biomass in the euphotic zone (Witek, 1998). In contrast
to Gdańsk Basin, in the shallow waters of the Darss-Zingst ecosystem, less
than 50% of ciliate individuals lived in the near-bottom layer, but more than
50% of the biomass (59.7 and 75.5%) was concentrated in this layer. This
difference is caused by the composition of ciliates in the bottom layer
consisting especially of large benthic and particle-associated forms
(Klinkenberg & Shumann, 1994). Rather high numbers of ciliates were
encountered even in some anoxic depths of the central Baltic Sea (Detmer et
al., 1993). Abundance of ciliates below 120 m depth was found to increase
from 160 to 480 ind./l; however, it amounted to only 10% of the surface
values of ciliates’ abundance.
It should be noted that vertical distribution of ciliates undergo changes
not only because of hydrological reasons (e.g., water mixing). Many species
of ciliates are capable of vertical migrations. For example, active vertical
migrations of autotrophic ciliate Myrionecta rubra from deep layers to the
euphotic zone were registered during the vernal bloom in the northern Baltic
(Olli et al., 1998). It has been suggested that this ciliate can act as peculiar
nutrient pump, which makes nutrients available to nonmigrating species.
Factors controlling the ciliate assemblages
Ciliates showed a significant relationship to latitude and salinity, which
explains 12–24% of their abundance variation in the northern Baltic Sea
(Samuelsson et al., 2006).
Besides the reasons given above, trophic factor (quantity/quality of
food and grazers) also exerts much influence on the ciliate abundance.
39