Time-series evolution of planktonic biomasses and toxic organisms in a brackish ecosystem of the Mediterranean Sea

In the framework of the EU Project "Strategy" (EUROHAB), a short-term study has been 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 10/03/03, in coincidence with the first detection of Alexandrium minutum, a dinoflagellate known as a producer of PST (Paralytic Shellfish Toxins), and lasted until 04/06/03, when the bloom collapsed. Water samples and phytoplankton net hauls were taken off approximately at ten days intervals, in the Verde Pond, one of the five basins of the Marinello ecosystem, in order to evaluate the incidence of toxic and non-toxic dinoflagellates species over the whole planktonic community. The evolution of the main climatic and trophic parameters (Temperature, Salinity, Dissolved Oxygen, POC, C/N, DIN, PO4-P, NO3-N, NO2-N, NH4-N, etc.) was investigated too. The specific identity of A. minutum was also confirmed on field mixed samples, through the use of species-specific PCR-primers targeting the 5.8S rDNA-ITS regions. The hypothesis of a recent introduction of A. minutum into the Verde Pond is considered on the basis of any previous detection of this species in past years. Alexandrium blooms were mostly characterized by A. minutum with a maximum cell density of 6x105 cells l-1 on April 11, and A. tamarense was the associated species (max 2.5x104 cells l-1 on March 25). A marked dominance of dinoflagellates or small flagellates over the other taxa was registered during the A. minutum bloom, whereas there was a minimum incidence of diatoms. The load of dissolved inorganic nitrogen was maximum in the pre-bloom phase (29.45?M on March 19), then drastically decreased to 7.92. The oxygen maximum (11.98mg l-1) was registered in coincidence with the Alexandrium minutum bloom, followed by a gradual decline (7.74 mg l-1 on June 4). The amounts of POC ranged between 265,88 and 658,00?g/l showing a discontinuous temporal trend, that highly correlated with the dinoflagellate percentual incidence (r= 0,83; P<0,05).