AME 24:297-311 (2001)  -  doi:10.3354/ame024297

ABSTRACT: Vertical distribution, standing stocks, size-class structure, community structure, mixotrophy and cell content of ciliate assemblages were studied at 9 stations along a transect in the Mediterranean Sea in June 1999. The aim of the study was to relate the trophic conditions in the Mediterranean to the ciliate community structure and the grazing impact of ciliates. The vertical distribution was more or less uniform in the Eastern Basin but presented an extended upper layer with higher density and a maximum at 50 to 75 m in the Western Basin. The integrated abundance (11.2 to 26.9 x 10⁶ cells m^-2) and biomass (41.5 to 84.8 mg C m^-2) decreased by a factor of 2 from west to east. A total of 55 tintinnid species were identified. Aloricates <30 μm represented 62% of integrated abundance and 16% of biomass. Mixotrophs made up 17% of integrated abundance and 18% of biomass. From west to east, there was no evident change in the structure of the ciliate community with respect to (1) aloricate size-classes, (2) mixotroph size-classes, (3) contribution of mixotrophs to total abundance. The cell content of all ciliates was examined for Synechococcus and photosynthetic algae under epifluorescence inverted microscopy. Tintinnids contained similar quantities of algae and Synechococcus (1.04 ± 0.59 algae tintinnid^-1, 0.94 ± 0.87 Synechococcus tintinnid^-1) and the same was true for aloricates. Based on cell content, it was estimated that (1) the ingestion rate for tintinnids was: 0.61 photosynthetic algae h^-1 and 0.41 Synechococcus h^-1 and for aloricates: 0.14 photosynthetic algae h^-1 and 0.13 Synechococcus h^-1; (2) tintinnids ingested significantly more prey than aloricates by a factor of 5; and (3) ciliates consumed 26% of primary production in the Western, 41% in the Central and 70% in the Eastern Basin.

KEY WORDS: Planktonic ciliates · Mediterranean · Distributions · Mixotrophy· Size classes · Cell content