LAUSR

Abstract: A non-toxic red tide with resulting anoxia was the cause of a major harmful algal bloom in St Helena Bay, South Africa, in February and March 2015. The red tide was observed along approximately 200 km of the Namaqua coastline extending well north of the Olifants River and southward into the bay. A maximum cell concentration of 14.32 × 106 cells l−1 was recorded in the southern reaches of the bay, and oxygen concentrations declined to as low as 0.06 ml l−1 in the shallow waters off Dwarskersbos. The hypoxia induced ecological fallout was significant with massive marine mortalities, including an estimated 415 tons of rock lobster (Jasus lalandii). The causative organism was identified morphologically, ultrastructurally and phylogenetically (using partial sequences of the 18S small subunit rDNA and the 28S large subunit rDNA) as Prorocentrum triestinum (Dinophyceae). Light and electron microscopy (both scanning and transmission) also facilitated identification of P. triestinum and addressed possible confusion with similar taxa. The global incidence of P. triestinum blooms is low, with hot spots limited to coastal areas in eastern China and the Mediterranean Sea, where blooms are sometimes associated with anthropogenic nutrient pollution. The harmful bloom documented here is the first exceptional bloom event with anoxic-related marine mortalities induced by P. triestinum in a major eastern boundary upwelling system such as the Benguela.