The influence of lowland pre-dams on riverine water quality - the cascade on the Cybina River in Poland case study

A cascade of four preliminary small lowland reservoir was created in the course of the Cybina River in Poznan (Western Poland) in the end of the XXth century in order to improve the water quality before it reaches to the Maltanski Reservoir, where international rowing and canoeing competitions are held each year. The deposition of suspended solids together with planktonic cells as well as the uptake of dissolved nutrients by phytoplankton proliferating in the shallow reservoirs were supposed to aim at water transparency improvement and nutrient concentrations decrease. Unfortunately, thirty years after the cascade was formed, reservoirs became a highly eutrophic, with severe blooms during summer. The chlorophyll-a content reached over 300 mg m^-3 in 2023, severely affecting the primary production in the reservoir situated below the cascade. At the same time, nutrient loads increased in the river downstream the cascade, as the ponds periodically supplied the river with additional 1,5 kgP per day and over 13 kgN per day. Climate changes, influencing the shortage of water in the Cybina River during warm months, also affect the water quality by increasing the water retention time and thus extending the blooms period. These results indicated an urgent need to undertake additional actions to improve the water purification in the system in order to maintain good water quality in the Maltanski Reservoir. Nature-based solutions shall be considered e.g. increasing the role of macrophytes in nutrient assimilation in the shallow reservoirs.