Lake Nakaumi, a brackish-water lake in western Japan, can be divided into three areas based on dinoflagellate assemblage characteristics. Four dinoflagellate groups are present throughout the lake in all seasons. These ubiquitous species are adapted to radical changes in brackish-water conditions. Ubiquitous species are able to take advantage of the highly variable environment because their numbers can increase quickly when ambient conditions are appropriate for that dinoflagellate taxon. This lifestyle results in a clear competitive advantage over that of cyst forming species.