AME 32:287-297 (2003)  -  doi:10.3354/ame032287

Planktonic ciliates in the Baltic Sea in summer: distribution, species association and estimated grazing impact

Outi Setälä1,2,*, Kai Kivi3

1Finnish Environment Institute, PO Box 140, 00251 Helsinki, Finland
2Finnish Institute of Marine Research, PO Box 33, 00931 Helsinki, Finland
3Division of Hydrobiology, PO Box 65, 00014 University of Helsinki, Finland

ABSTRACT: Samples for studying the ciliate communities of the open Baltic Sea were taken on 2 transects from the Kattegat to the entrance of the Gulf of Finland during 1988 and 1990. The abundance of HCIL (heterotrophic ciliates) was highest close to the surface, the maximum values ranging from ca. 7000 (entrance of the Gulf of Finland in 1988) to ca. 20000 cells l-1 (Arkona Basin in 1990). The dominating HCIL groups were small strobilidiids, strombidiids, or prostomatiids. The photoautotrophic ciliate Mesodinium rubrum was most abundant in the surface water of Arkona Basin in 1990 (ca. 26000 cells l-1) but was also found at a high concentration in deeper layers during both cruises in daytime (down to 80 m in 1988). Large ciliates dwelling in the oxic/anoxic boundary layer at ca. 100 m depth were found in both years, the highest numbers being ca. 1000 cells l-1 (1988). HCIL community grazing was estimated by using a biovolume-dependent, mostly experimentally derived exponential function. We estimated that in 1988 the ciliate community cleared close to 50% of the water volume per day, whereas in 1990 the highest values reached up to 125% cleared d-1. In both years, small (≤30 μm ciliates dominated the communities and were also responsible for most of the estimated grazing. Three distinct ciliate associations were revealed from the data of both years by correlation analysis. A deep-water association characterized by large ciliates was found at the oxic/anoxic water interface at the 2 deepest stations, Bornholm and Gotland Basins, in both years, whereas the other associations found were found closer to the surface. Some of these groups may represent true feeding guilds bound together by utilization of same resources, while others could be united by abiotic factors or internal dynamics (e.g. predator-prey relationships) of the association.

KEY WORDS: Heterotrophic ciliates · Distribution · Clearance · Species associations

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