| Summary: | Protists are a taxonomic group of organisms world wild distributed with high abundance and biodiversity; their countless forms, sizes, and trophic activities constitute a continuum of species ranging from bacterial-sized cells for the smallest known species of chlorophytes to meters in length for the largest colonies of radiolaria. The enormous size range of protists, their many nutritional modes, and their rapid metabolic rates result in their pivotal ecological roles as primary producers and consumers at and near the base of marine food webs. Protists, in particular the heterotophic ones, are, with few metazoans larval stage, the major components of microzooplankton, on which this study focuses. Microzooplankton assemblage is described as a group of planktonic organisms in the size range of 10/20-200 µm; they are consumers of bacteria, cyanobacteria, other protists, viruses, and some metazoans. The quantitative importance of microzooplankton as consumers of primary production in the ocean has been recognized in the last decades; at the same time copepod predation on heterotrophic dinoflagellates and ciliates constitutes a trophic link more important than the phytoplankton–copepod link in many situations. Phagotrophic protists are the primary trophic link between minute cyanobacterial and bacterial production and higher organisms, a concept formalized more than 35 years ago in the microbial food web by Pomeroy (1974). In this study I want to provide more information on microzooplankton assemblages, describing the community composition and the role of this important component in different environments. I point out some of the factors influencing the distribution of these organisms with the aims to increase the current knowledge on microzooplankton and to contribute to the understanding of the phenomena that regulate the efficiency of the trophic web of the marine ecosystem. In the first chapter of the thesis entitled “Microzooplankton composition in the winter sea ice of the Weddell Sea” sympagic ...
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