ABSTRACT The study was held in January to February 2019. The purpose of this study was to determine the abundance of toxic benthic dinoflagellates on seaweed Sargassum sp. each station and to see the relationship of abundance of toxic benthic dinoflagellates with the quality of the waters in the seaweed ecosystem of Sargassum sp. in the waters of Nirwana Beach, West Sumatra Province. The m…