Three distinct groups of picoplankton were identified in the Patagonian Shelf region: Prochlorococcus, Synechococcus, and picoeukaryotes. Prochlorococcus populations were detected at four stations within the Brazil Current but did not extend further south than 40°S. Cyanobacterial abundance, primarily Synechococcus, was greatest in warmer, more oligotrophic conditions corresponding to regions outside the shelf break. Picoeukaryotes dominated at most other stations. Heterotrophic bacteria abundances were two orders of magnitude higher than autotrophs, indicating rapid nutrient recycling. Cyanobacterial abundance correlated positively with temperature and negatively with nutrients, influenced by hydrographic features and seasonal blooms in the