Rapid freshwater discharge on the coastal ocean as a mean of long distance spreading of an unprecedented toxic cyanobacteria bloom

Cyanobacterial toxic blooms are aworldwide problem. The Río de la Plata (RdlP) basinmakes up about one fourth of South America areal surface, second only to the Amazonian. Intensive agro-industrial land use and the construction of dams have led to generalized eutrophication ofmain tributaries and increased the intensity and duration of cyanobacteria blooms. Herewe analyse the evolution of an exceptional bloomat the lowRdlP basin and Atlantic coast during the summer of 2019. A large array of biological, genetic,meteorological, oceanographic and
satellite data is combined to discuss the driving mechanisms. The bloomcovered thewhole stripe of the RdlP estuary and the Uruguayan Atlantic coasts (around 500 km) for approximately 4 months. It was caused by the Microcystis aeruginosa complex (MAC), which produces hepatotoxins (microcystin). Extreme precipitation in the upstreamregions of Uruguay and Negro rivers' basins caused highwater flows and discharges. The evolution of meteorological and oceanographic conditions as well as the similarity of organisms' traits in the affected area
suggest that the bloom originated in eutrophic reservoirs at the lower RdlP basin, Salto Grande in the Uruguay river, and Negro river reservoirs. High temperatures and weak Eastern winds prompted the rapid dispersion of the bloom over the freshwater plume along the RdlP northern and Atlantic coasts. The long-distance rapid drift allowed active MAC organisms to inoculate freshwater bodies from the Atlantic basin, impacting environments relevant for biodiversity conservation. Climate projections for the RdlP basin suggest an increase in precipitation and river water flux, which, in conjunction with agriculture intensification and dams' construction, might turn this extraordinary event into an ordinary situation.

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