The researchers studied the phytoplankton in the Yasna Polyana Reservoir in Bulgaria over ten years. They found that green algae and diatoms were the most common types of phytoplankton. The community structure remained relatively stable, with certain species like Cyclotella sp. and Ceratium hirundinella being dominant. The phytoplankton numbers ranged from 11,000 to 4,296,000 individuals per liter, and the biomass varied from 0.05 to 16.5 milligrams per liter. The reservoir was classified as having low to moderate nutrient levels, and the phytoplankton were limited by phosphorus. Differences were noted between this reservoir and other Bulgarian reservoirs in terms of phytoplankton composition.