Unusual co-occurrence of Prorocentrum lima and P. rhathymum in a mucilaginous river plume in a tropical Atlantic estuary (northeast Brazil)

A mucilaginous, water-discolouring bloom dominated by the toxic dinoflagellates Prorocentrum lima complex and Prorocentrum rhathymum was recorded for the first time in the tropical Atlantic Ocean, within the estuarine system of Camamu Bay, Bahia, Brazil. The bloom appeared as a brown greasy foam plume in the Serinhaém River near the town of Ituberá, an area of high anthropogenic influence and significant shellfish harvesting. Manual surface sampling and subsequent light and electron microscopy analyses revealed estimated concentrations of 2.04×10⁶ cells·L⁻¹ for P. rhathymum and 1.69×10⁶ cells·L⁻¹ for P. lima complex. Environmental conditions at the bloom site included a temperature of 25.3 °C, salinity of 15.6, and elevated nitrogen-to-phosphorus ratios indicating phosphorus limitation. The coexistence of both species in free-floating, toxigenic aggregations highlights the ecological plasticity of benthic dinoflagellates and underscores emerging risks to seafood safety and ecosystem health in tropical estuarine systems undergoing rapid urbanisation. These findings emphasise the need for continuous biomonitoring in vulnerable coastal areas.