JPR Advance Access originally published online on September 19, 2005
Journal of Plankton Research 2005 27(10):973-986; doi:10.1093/plankt/fbi069
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Relationships between microzooplankton and mesozooplankton: competition versus predation on natural assemblages of the Gulf of Trieste (northern Adriatic Sea)
1 Department of Biology, University of Trieste, Via Giorgieri 10, I-34127 Trieste, Italy and 2 Laboratory of Marine Biology (LBM), via Auguste Piccard 54, I-34010 Trieste, Italy
* Corresponding Author: labbioma{at}units.it
Received May 12, 2005; accepted in principle July 26, 2005; accepted for publication September 8, 2005; published online September 19, 2005
Communicating editor: K.J. Flynn
We performed, on a seasonal basis, 16 dilution experiments and, simultaneously, copepod or cladoceran grazing experiments on natural assemblages from Gulf of Trieste (northern Adriatic Sea). The autotrophic fraction was almost entirely composed of diatoms in late winter. As the seasons progressed, relative abundance of nanoplankton and cyanobacteria increased. Microzooplankton was always present in the diet of mesozooplankton, even if in percentages usually not exceeding 6% of diet intake on carbon basis. Microzooplankton took advantage of ephemeral increases of autotrophic biomass when prey were in the optimal size range but did not consume diatoms when these were large. When autotrophic resources were scarce, micrograzers used heterotrophic biomass which, in turn, fuelled the upper trophic levels through predation by mesozooplankton on microzooplankton. Microzooplankton grazing was the most important loss term of primary production in the Gulf of Trieste (on average, microzooplankton consumed 
100% of primary production, while mesozooplankton only 76%), which can be considered a mesotrophic coastal system.
This paper is one of six on the subject of the role of zooplankton predator–prey interactions in structuring plankton communities.