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 or cladoceran grazing experiments onnatural assemblages from Gulf of Trieste (northern AdriaticSea). The autotrophic fraction was almost entirely composedof diatoms in late winter. As the seasons progressed, relativeabundance of nanoplankton and cyanobacteria increased. Microzooplanktonwas always present in the diet of mesozooplankton, even if inpercentages usually not exceeding 6% of diet intake on carbonbasis. Microzooplankton took advantage of ephemeral increasesof autotrophic biomass when prey were in the optimal size rangebut did not consume diatoms when these were large. When autotrophicresources were scarce, micrograzers used heterotrophic biomasswhich, in turn, fuelled the upper trophic levels through predationby mesozooplankton on microzooplankton. Microzooplankton grazingwas the most important loss term of primary production in theGulf of Trieste (on average, microzooplankton consumed 100%of primary production, while mesozooplankton only 76%), whichcan be considered a mesotrophic coastal system. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.