INTRODUCTION The Southwest Atlantic is an area of high primary productivity, where upwellings and oceanic fronts control the intensity of organic production (Gregg et al., 2005; Rivas et al., 2006). Along the Argentinian coasts, dinoflagellates and diatoms dominate the primary production (e.g., Lutz and Carreto, 1991; Segura et al., 2013; Krock et al., 2015). These microorganisms represent the bottom link in the food web and are essential for marine ecosystems. Dinoflagellates are unicellular biflagellate organisms that thrive in both freshwater and marine environments. They display a wide range of morphologies, live in a variety of habitats (Evitt, 1985; Taylor et al., 2008), and are found at all latitudes, though they are particularly abundant in coastal waters (Zonneveld et al., 2013). Most dinoflagellates are autotrophic and/or mixotrophic (50%), but some are either heterotrophic, parasitic, or ABSTRACT. The presence of upwelling systems and oceanic fronts makes the Southwest Atlantic Ocean a region of high primary productivity. These same conditions are present in San Jorge Gulf (SJG) along the southern Argentinian coast, where dinoflagellates and diatoms dominate primary production. The distribution of these microorganisms, including the cysts produced by some dinoflagellates during their life cycles, is controlled in marine environments by oceanographic parameters that include salinity, surface water temperature, ice cover duration, and productivity. The objective of this study is to document the modern distribution of dinoflagellate cyst assemblages in surface sediments so that the environmental preferences of each taxon can be inferred and used to reconstruct paleoenvironmental conditions. The dinoflagellate cyst (dinocyst) assemblages of 52 surface samples collected in 2014 aboard R/V Coriolis II in the SJG were described and compared to surface oceanographic conditions and grain size data. The results indicate dinocyst concentrations vary between 64 cysts g-1 and 45,848 cysts g-1 dry sediment, with Spiniferites ramosus and Operculodinium centrocarpum the dominant species, accompanied by Spiniferites mirabilis, Dubridinium sp., cysts of Polykrikos kofoidii, and cysts of Brigantedinium simplex, Brigantedinium auranteum, and Brigantedinium spp. We have defined two spatial domains based on the distribution of dinocysts in and near the SJG: northern/southern-central gulf and offshore domains. We found an increase in dinocyst concentrations along a north-south gradient in the SJG and minimum concentrations at offshore sites. In addition, multivariate analyses reveal the relationships among the relative abundances of dinocysts, fine grain size data (<63 μm; silts and clays), and primary productivity, as well as offshore upwelling, which appear to control most of the distribution of dinocysts.
