The role of a eutrophic lowland reservoir in shaping the composition of river phytoplankton

The manuscript documents the role of a shallow eutrophic reservoir, Siemianówka Dam Reservoir, in deterioration of water quality in a dammed lowland river basin. Changes in phytoplankton composition and abundance were observed along a 152.5 km section of the Narew River. Favourable conditions for phytoplankton growth (TN > 1.5 mg dm−3, TP > 100 μg dm−3, water temperature between 18-25°C, pH 6-9) in the reservoir resulted in a more than one hundred fold increase in phytoplankton biovolume. The significant effect of limnoplankton on potamoplankton was manifested by the dominance of Cyanobacteria from mid-summer to autumn in the river reach below the dam. Water retention time, daily water outflow and molar TN:TP ratio were the factors favouring mass development of Cyanobacteria in the reservoir. However, the meandering character and low flow velocities in the outflowing river, together with the way of operating the dam gates, favoured the Cyanobacteria development in the river reach below the dam. The study demonstrated that water retention time exceeding 3 months in shallow dam reservoirs significantly increases the risk of river water quality deteriorating as a result of mass development of Cyanobacteria.