Toxic Trichodesmium bloom occurrence in the southwestern South Atlantic Ocean

•Six Trichodesmium’ blooms were sampled in the South Atlantic Ocean in December 2014.•We have assessed the trichomes abundance, environmental variables and neurotoxin concentration.•The toxicity tests have showed that large biomass of Trichodesmium is harmful for shrimp larvae.•Blooms of Trichodesmium may also be harmful for local plankton community.•This current study provides the first record of neurotoxic ocean blooms in the western of South Atlantic.

Harmful Trichodesmium blooms have been reported on the continental slope of the southwestern South Atlantic Ocean; we sampled six such blooms. The highest saxitoxin concentration was observed where the number of colonies was proportionally greater relative to the total density of trichomes. Trichodesmium blooms are harmful to shrimp larvae and may lead to plankton community mortality. This study is the first record of neurotoxic blooms in the open waters of the South Atlantic.