ABSTRACT Mangrove river swamp village is one of the mangroves in Siak Regency, Riau Province. This study aims to determine the structure of the phytoplankton community in the mangrove ecosystem, which was conducted in April 2019. Samples were taken 3 times, once / week, in 3 different regions. The results found that there were 19 species of phytoplankton in mangrove waters of swamp river …