This study aimed to assess the composition, seasonality, and abundance of the phytoplankton community of a Boron (B) mine effluent storage reservoir, Çamköy Reservoir, Balıkesir, Turkey. For this purpose, phytoplankton and certain physicochemical parameters were sampled seasonally between April 2015 and January 2016. B concentrations ranged from 554 mg L -1 to 689 mg L -1 . A total of 39 taxa were identified during the study. The percent composition for each phytoplankton group was as follows: Bacillariophyta 67%, Chlorophyta 10%, Cyanobacteria 8%, Euglenophyta 8%, Mioza 5%, and Charophyta 2%. In summer 2015 no phytoplankton was detected in the samples and the excessive B concentrations (above 600 mg L -1 ) in the reservoir seem to be the reason for the lack of phytoplankton in the summer samples. The most common taxa were Navicula digitoradiata (Bacillariophyta), Surirella ovata (Bacillariophyta), and Nitzschia amphibia (Bacillariophyta). The reservoir had a low number of phytoplankton taxa and abundance compared with the natural lakes probably due to the excessive B levels. The phytoplankton community of the reservoir was composed of taxa that preferred alkaline waters.