Phytoplankton blooms localized by Sentinel-2 images and hydrodynamic modelling – Sulejów Reservoir on the Pilica river, Poland

Reservoirs created by damming rivers significantly modify abiotic and biotic environmental components. A major consequence is the accumulation of sediments and nutrients, degrading water quality through eutrophication. This study focused on the Sulejów Reservoir in Central Poland, examining nutrient balance, phytoplankton blooms using Sentinel-2 satellite data, and wind-driven surface currents with the CCHE2D hydrodynamic model. Eight Sentinel-2 images from the 2020 vegetation season and the Normalized Difference Chlorophyll Index (NDCI) were used to assess phytoplankton distribution. Results indicate the reservoir mainly acts as a nutrient sink, but under low-flow and intense bloom conditions, it can become a nutrient source. Coupling remote sensing with hydrodynamic modeling effectively interpreted flow patterns and nutrient dynamics. For the first time, the influence of eddy structures and wind on phytoplankton distribution in the lacustrine zone was demonstrated.