Assessing the threats of small lowland reservoirs under climate change on the example of Supraśl Water System (NE Poland)

The Supraśl River is a stable water ecosystem in North-Eastern Poland. This is indicated by the flow rate over many years, as well as the water quality monitored for years. Despite this, there are periods of oxygen deficiency in the water, causing massive fish kills. Our research shows that climate change resulting in an increase in the frequency of hydro-meteorological extremes may contribute to negative processes taking place in aquatic ecosystems. On the example of Supraśl Water System (SWS), we present an analysis of the phenomenon of suspension caused by the underground inflow, supplying the surface waters with a significant load of organic matter. The accelerated decomposition of peat formations in the Supraśl River catchment after heavy rains makes it easier for organic matter to penetrate into the aquatic ecosystem. Intensive flushing of valley and river bed sediments during summer floods has a particularly negative impact on the ecosystem of inland waters. If there are organic sediments on the bottom, their re-suspension is very dangerous. Then there is the immediate demand for oxygen, which can lead to deoxygenation of the water. Although this is a natural process, alarm and monitoring procedures should be established and countermeasures sought to respond in time and prevent mass fish kills.