High proliferation of Pseudo-nitzschia cf. arenysensis in the Adriatic Sea: ecological and morphological characterisation

Abstract

The aim of this study is to characterise the diatom Pseudo-nitzschia community during a bloom period in relation to environmental conditions. High proliferation of Pseudo-nitzschia spp. was observed in September 2017 at the shellfish breeding area in Krka River estuary (Central eastern Adriatic Sea). The peak of abundance (1.8 x 106 cells L-1) was recorded at 7 m depth, and the increased abundance persisted for four weeks. Morphological analyses of field samples based on scanning electron microscopy (SEM) revealed that Pseudo-nitzschia cf. arenysensis prevailed (94%) in the Pseudo-nitzschia assemblage. Several strains were successfully isolated from net samples in order to better define morphological features and phylogenetic characterisation. The isolated Pseudo-nitzschia strains corresponded morphologically to P. cf. arenysensis from the field samples, based on our SEM observations. Phylogenetic analysis demonstrated that the Croatian strains grouped with P. arenysensis using the ITS and LSU rDNA sequences. The Spearman rank correlation showed that salinity was an important environmental factor affecting the vertical distribution of Pseudo-nitzschia spp. in this highly variable area. Availability of increased concentration of orthophosphates and ammonium and a low Si:TIN ratio may have promoted the bloom of P. cf. arenysensis in the estuary.