Relationships between microzooplankton and mesozooplankton: competition versus predation on natural assemblages of the Gulf of Trieste (northern Adriatic Sea)

We performed, on a seasonal basis, 16 dilution experiments and, simultaneously, copepod orcladoceran grazing experiments on natural assemblages from Gulf of Trieste (northern AdriaticSea). The autotrophic fraction was almost entirely composed of diatoms in late winter. As the seasonsprogressed, relative abundance of nanoplankton and cyanobacteria increased. Microzooplankton wasalways present in the diet of mesozooplankton, even if in percentages usually not exceeding 6% of dietintake on carbon basis. Microzooplankton took advantage of ephemeral increases of autotrophicbiomass when prey were in the optimal size range but did not consume diatoms when these werelarge. When autotrophic resources were scarce, micrograzers used heterotrophic biomass which, in turn,fuelled the upper trophic levels through predation by mesozooplankton on microzooplankton. Microzooplanktongrazing was the most important loss term of primary production in the Gulf of Trieste (onaverage, microzooplankton consumed 100% of primary production, while mesozooplankton only76%), which can be considered a mesotrophic coastal system.