ECOLOGY AND DIVERSITY OF MARINE MICROZOOPLANKTON

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 microzooplankton were studied during late winter in the northern Weddell Sea, for diversity, abundance and carbon biomass. In order to asses the role of microzooplanktonic component as food supply for the upper levels of the trophic web in this particular environment, the ice-cores were collected on an ice floe along three dive transects, and sea water was taken from under the ice through the central dive hole from which all transects were connected. The areal and vertical microzooplankton distributions in the ice and water were compared. They showed high abundance (max 1300 cells L-1) and biomass (max 28 µg C L-1) in the ice-cores, and were lower in the water, below sea ice (maxima, 19 cells L-1; 0.15 µg C L-1, respectively). The highest amounts were found in the lower 10 cm section of ice cores. The microzooplankton community within sea ice comprised mainly aloricate ciliates, foraminifers and micrometazoans. In winter, microzooplankton represent an important fraction of the sympagic community in the Antarctic sea ice. They can potentially control microalgal production and can contribute to particulate organic carbon concentrations when released into the water column due to ice melting in spring. Continued reduction of the sea ice might undermine these roles of microzooplankton, leading to reduction or completely loss in diversity, abundance and biomass of these sympagic protists. The second Chapter of this work focuses on the microbial community along the Ligurian coast in correspondence of two marine canyons. Community structures along the water column were studied using microscopic techniques, and their relationships with the environmental factors recorded along the canyons were investigated. The study considered pico-, nano- and micro-planktonic fractions, their abundances and