Pseudo-nitzschia blooms in the Zrmanja River estuary (eastern Adriatic Sea)

The spatial and temporal distribution of the diatom genus Pseudo-nitzschia was investigated in the small, highly stratified Zrmanja River estuary (eastern Adriatic Sea) from 1998 to 2004. During this period, Pseudo-nitzschia spp. mostly dominated the phytoplankton assemblages (abundance close to 1.3 x 106 cells L-1, appeared in 51% of the samples). The maximum abundance of Pseudo-nitzschia spp. was during mid-winter (February 1999), when it contributed up to 85% of the total diatom abundance. Pseudo-nitzschia bloom was significantly correlated with phosphorous concentration and temperature, whereas there was no significant correlation with total inorganic nitrogen, silicate and salinity. The very broad range of environmental factors over which Pseudo-nitzschia spp. were observed suggests there was more than one Pseudo-nitzschia species in the diatom assemblage. Morphological analysis has shown that the dominating Pseudo-nitzschia species corresponded to Pseudo-nitzschia calliantha. Two other species were encountered; one identified as Pseudo-nitzschia heimii, while other has yet to be identified. This study was the first to find P. heimii in the Adriatic Sea and the first to record the presence of a potentially toxic diatom P. calliantha in an Adriatic estuary.