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In the framework of the EU Project STRATEGY, a short-term study was carried out in the Marinello ecosystem, a small brackish area located on the Tyrrhenian coast of Sicily (Italy). The investigation was aimed at understanding the dynamics of phytoplankton toxic blooms in relation to other planktonic species and environmental conditions. The study started on 10 March 2003, in coincidence with the first detection of Alexandrium minutum, a dinoflagellate known as a producer of Paralyzing Shellfish Toxins (PST) and lasted until 4 June 2003, when the bloom collapsed. The specific identity of A. minutum was confirmed on field mixed samples, through the use of species-specific PCR-primers targeting the 5.8S rDNA-ITS regions. Water samples and phytoplankton net hauls were taken approximately at 10 days intervals in the Verde Pond, one of the five basins of the Marinello ecosystem, in order to evaluate the incidence of toxic and nontoxic dinoflagellate species over the whole planktonic community. The evolution of the main environmental and trophic parameters (temperature, salinity, dissolved oxygen, POC, C/N, DIN, PO4-P) was simultaneously investigated. Alexandrium blooms were mostly characterized by A. minutum (max. 6 × 105 cells l−1 on April 11) and Alexandrium tamarense as an associated species (max. 2.5 × 104 cells l−1 on March 25). During the bloom, dinoflagellates or small flagellates dominated over the other taxa, with a minimum incidence of diatoms. The load of dissolved inorganic nitrogen was maximum in the pre-bloom phase (29 µM on March 19), after which it decreased sharply. An oxygen supersaturation event was registered in coincidence with the A. minutum bloom. The amounts of POC ranged between 266 and 658 µg l−1 showing a discontinuous temporal trend. A recent introduction of A. minutum into the Verde Pond is suggested on the basis of the absence of this species in past years. [ABSTRACT FROM AUTHOR]