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

A seasonal study of dominant protozoa, heterotrophic flagellatesand ciliates, was performed along a primary production gradientin the northern Baltic Sea. The abundances of protozoa increasedwith increasing primary production from north to south. Smallcells dominated in the low productive north, while larger cellsbecame more dominant in the south. The highest biovolume concentrationof protozoa was observed in summer in the north and during springin the south. The seasonal succession of protozoa followed ageneral pattern: choanoflagellates, large flagellates and ciliatesshowed peaks during spring and autumn, while small bacterivorousnanoflagellates peaked during the summer. An in situ experimentindicated that the inverse relationship between loricated choanoflagellatesand other small flagellates may be explained by a seasonal changein the predator community and a seasonal change in the accessto surface-attachment sites. Principal component regressionanalyses including all data showed that 46% of the variationof small flagellates and 20% of the variation of large flagellatescould be explained by temperature and bacterial biomass. Ciliatesshowed a significant relationship to latitude and salinity,explaining 12–24% of the variation. In conclusion, thefield data indicated that the protozoan community in generalwas resource controlled.