Phytoplankton Dynamics in the Middle Adriatic Estuary, with a Focus on the Potentially Toxic Genus Pseudo-nitzschia

The Krka River estuary is a karstic, permanently stratified estuary due to the strong freshwater inflow. It is a special environment in which to study the phytoplankton community, especially because this area is an important aquaculture site. Among other potentially toxic phytoplankton species, the diatom genusPseudo-nitzschiaoccurs frequently and is a potential source of domoic acid (DA), causing shellfish toxicity and human intoxication. The main objective was to examine the dynamics of the phytoplankton community and, in particular, the genusPseudo-nitzschiain the upper part of the Krka estuary, through monthly sampling over two years. The phytoplankton community was analysed using light microscopy and scanning electron microscopy to determine the diversity ofPseudo-nitzschiaspecies and characterise the environmental parameters associated with a high abundance ofPseudo-nitzschiaspecies. SevenPseudo-nitzschiaspecies were identified in the investigation, with higher frequencies and abundances in the less variable layer, at a 7 m depth. Blooms ofPseudo-nitzschiawere noted in the late summer/early autumn, dominated byP. delicatissima/arenysensis. Winter assemblages were characterised byP. pseudodelicatissima/cuspidata, P. calliantha, and P. subfraudulenta, and were associated with domoic acid occurrence in shellfish.