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JPR Advance Access published online on September 19, 2005
Journal of Plankton Research, doi:10.1093/plankt/fbi069
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org
Received May 12, 2005
Accepted September 8, 2005

Article

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

S. Fonda UMANI 1*, V. Tirelli 2, A. Beran 2, and B. Guardiani 2

1 Department of Biology, University of Trieste, via Giorgieri 10, I-34127 Trieste (Italy); Laboratory of Marine Biology (LBM), via Auguste Piccard 54, I-34010 Trieste (Italy)
2 Laboratory of Marine Biology (LBM), via Auguste Piccard 54, I-34010 Trieste (Italy)

* To whom correspondence should be addressed.
S. Fonda UMANI, E-mail: labbioma{at}units.it


  Abstract

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 micro-grazers 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 ca. 100 % of primary production, while mesozooplankton only 76 %), which can be considered a mesotrophic coastal system.

Communicating Editor: KJ Flynn
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