Factors structuring the heterotrophic flagellate and ciliate community along a brackish water primary production gradient

A seasonal study of dominant protozoa, heterotrophic flagellates and ciliates, was performed along a primary production gradient in the northern Baltic Sea. The abundances of protozoa increased with increasing primary production from north to south. Small cells dominated in the law productive north, while larger cells became more dominant in the south. The highest biovolume concentration of protozoa was observed in summer in the north and during spring in the south. The seasonal succession of protozoa followed a general pattern: choanoflagellates, large flagellates and ciliates showed peaks during spring and autumn, while small bacterivorous nanoflagellates peaked during the summer. An in situ experiment indicated that the inverse relationship between loricated choanoflagellates and other small flagellates may be explained by a seasonal change in the predator community and a seasonal change in the access to surface-attachment sites. Principal component regression analyses including all data showed that 46% of the variation of small flagellates and 20% of the variation of large flagellates could be explained by temperature and bacterial biomass. Ciliates showed a significant relationship to latitude and salinity, explaining 12-24% of the variation. In conclusion, the field data indicated that the protozoan community in general was resource controlled. © The Author 2006. Published by Oxford University Press. All rights reserved.