The overlay of cooler nutrient enriched Beagle-Magellan water with warmer nutrient depleted shelf water and a strong stratification of the water column in the San Jorge Gulf region, Argentina, coincided with relatively high dinoflagellate abundances in April 2012, up to 34,000 cells L −1 . This dinoflagellate proliferation was dominated by Ceratium spp., but environmental conditions also favored to a lesser amount the occurrence of toxigenic dinoflagellates, such as Alexandrium tamarense and Protoceratium reticulatum, whose toxins were hardly detected in any other areas along the expedition transect of the R/V Puerto Deseado between 38 and 56°S (Ushuaia-Mar del Plata) in March/April 2012. Generally vegetative cells of A. tamarense and P. reticulatum co-occurred with their respective phycotoxins in the water column and their cysts in the upper sediment layers. Two strains of A. tamarense were isolated from the bloom sample and morphologically characterized. Their PSP toxin profiles consisted of C1/2, gonyautoxins 1/4 and to a lesser amount of neosaxitoxin and confirmed earlier data from this region. The ratios between autotrophic picoplankton and heterotrophic bacteria were higher in shelf waters in the north than in Beagle-Magellan waters in the south of San Jorge Gulf.

Keywords:Amnesic shellfish poisoning South-western Atlantic Pseudo-nitzschia diversity P. australis P. brasiliana Domoic acid A B S T R A C TThe marine diatom genus Pseudo-nitzschia, the major known producer of the neurotoxin domoic acid (DA) responsible for the amnesic shellfish poisoning (ASP) syndrome in humans and marine mammals, is globally distributed. The genus presents high species richness in the Argentine Sea and DA has been frequently detected in the last few years in plankton and shellfish samples, but the species identity of the producers remains unclear. In the present work, the distribution and abundance of Pseudo-nitzschia species and DA were determined from samples collected on two oceanographic cruises carried out through the Argentine Sea ($39-47 S) during summer and spring 2013. Phytoplankton composition was analysed by light and electron microscopy while DA was determined by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The genus Pseudo-nitzschia was recorded in 71 and 86% of samples collected in summer and spring, respectively, whereas DA was detected in only 42 and 21% of samples, respectively. Microscopic analyses revealed at least five potentially toxic species (P. australis, P. brasiliana, P. fraudulenta, P. pungens, P. turgidula), plus putatively non-toxigenic P. dolorosa, P. lineola, P. turgiduloides and unidentified specimens of the P. pseudodelicatissima complex. The species P. australis showed the highest correlation with DA occurrence (r = 0.55; p < 0.05), suggesting its importance as a major DA producer in the Argentine Sea. In the northern area and during summer, DA was associated with the presence of P. brasiliana, a species recorded for the first time in the Argentine Sea. By contrast, high concentrations of P. fraudulenta, P. pungens and P. turgidula did not correspond with DA occurrence. This study represents the first successful attempt to link toxigenicity with Pseudo-nitzschia diversity and cell abundance in field plankton populations in the south-western Atlantic.

In the Argentine Sea, blooms of toxigenic dinoflagellates of the Alexandrium tamarense species complex have led to fish and bird mortalities and human deaths as a consequence of paralytic shellfish poisoning (PSP). Yet little is known about the occurrence of other toxigenic species of the genus Alexandrium, or of their toxin composition beyond coastal waters. The distribution of Alexandrium species and related toxins in the Argentine Sea was determined by sampling surface waters on an oceanographic expedition during austral spring from ~39°S to 48°S. Light microscope and SEM analysis for species identification and enumeration was supplemented by confirmatory PCR analysis from field samples. The most frequent Alexandrium taxon identified by microscopy corresponded to the classical description of A. tamarense. Only weak signals of Group I from the A. tamarense species complex were detected by PCR of bulk field samples, but phylogenetic reconstruction of rDNA sequences from single cells from one station assigned them to ribotype Group I (Alexandrium catenella). PCR probes for Alexandrium minutum and Alexandrium ostenfeldii yielded a positive signal, although A. minutum morphology did not completely match the classical description. Analysis of PSP toxin composition of plankton samples revealed toxin profiles dominated by gonyautoxins (GTX1/4). The main toxic cyclic imine detected was 13-desMe-spirolide C and this supported the association with A. ostenfeldii in the field. This study represents the first integrated molecular, morphological and toxinological analysis of field populations of the genus Alexandrium in the Argentine Sea.