Blooms of Azadinium cf. spinosum Elbrächter et Tillmann (Dinophyceae) in northern shelf waters of Argentina, Southwestern Atlantic

Azadinium spinosum has been identified as a species producer of azaspiracids (AZAs), marine toxins reported to cause human poisoning etiologically similar to diarrheic shellfish poisoning. AZAs are accumulated by molluscs via food web which, in turn, may cause human poisoning after consumption of contaminated shellfish. In this study, the presence of a species of the genus Azadinium, A. cf. spinosum, is reported for the first time in the Southwest Atlantic ocean, and two blooms in northern shelf waters of Argentina are analysed. Both blooms occurred during spring in succesive years and developed along extensive areas (up to 60 km transection) of the middle shelf and near the shelf break in 1990, and also near the shelf break during 1991. The extended shelf break front of the Argentine Sea developed during spring and summer is a highly productive area rich in exploited marine resources. In November 1990, a first bloom (up to 9.03 × 106 cells L−1) produced heavy discolorations, and morphological and taxonomical studies showed that the causing organism was a new dinoflagellate genus. During September 1991 plankton composition and abundance along a transect from the coast to the Malvinas Current were analysed. Results showed that this dinoflagellate was the most important species in middle shelf waters, with high biomass and abundance values (up to 3 × 106 cells L−1) surpassing those of various chain-forming diatom Thalassiosira species, typical of the spring bloom. The description of this new taxon, according to the thecal analysis performed at that time by light and scanning electron microscopy, corresponds with little morphological differences to the thecal tabulation, cell morphology and dimensions of the recently described Azadinium spinosum. Taking into account these minor differences and due to the lack of both DNA sequence data and AZAs production, we decided to name it A. cf. spinosum. The phagotrophic dinoflagellate Gyrodinium fusus has shown to be an important grazer of A. cf. spinosum during the first bloom, suggesting that its grazing activity could have acted as a modeling factor in population levels of its prey. The presence of A. cf. spinosum in this area opens several questions as that related to toxin production given the regional importance of bivalve commercial fisheries.

► Azadinium cf. spinosum is reported for the first time in shelf waters of Argentina.
► We analyse two blooms (>106 cells L−1), which are new events on a global scale for a species of this genus.
► Blooms were registered in spring near the shelf break front of the Malvinas Current.
► A. cf. spinosum is a successful competitor with diatoms typical of the spring bloom.
► Feeding capacity of phagotrophic dinoflagellates showed to be high at bloom situation.