Abstract
<jats:p>Long-term eutrophication remains one of the major pressures affecting freshwater lake ecosystems in Europe. Lake Durowskie (north-western Poland) has experienced sustained nutrient loading from agricultural runoff, urban activities, and hydrological connections with upstream lakes, resulting in a hypertrophic state. Since 2009, restoration measures combined with systematic ecological monitoring have been implemented to improve the lake’s ecological condition. This study evaluates long-term phytoplankton and periphyton dynamics as indicators of ecological recovery in Lake Durowskie during 2008–2025. Samples were collected from multiple lake and inflow sites and analysed in terms of taxonomic composition, abundance, biomass, and ecological characteristics. Ecological status was assessed using biological indices, including the Shannon–Wiener diversity index (H′), Pielou evenness index (E), Jaccard similarity index, Nygaard mixed trophic index, and the diatom index (DI). The results indicate relatively high phytoplankton diversity and evenness across the lake. Chlorophyta and Bacillariophyceae contributed most to species richness, whereas total biomass was largely influenced by dinoflagellates, particularly Ceratium hirundinella and Peridiniopsis berolinense. Long-term observations revealed increasing algal species richness since 2022 and low Jaccard similarity values (17–25%), indicating significant community restructuring. Although the Nygaard index consistently classified the lake as hypertrophic, improvements in the periphyton diatom index at several sites suggest gradual ecological improvement. Overall, the results indicate a transitional recovery phase under persistent eutrophic pressure and confirm the value of phytoplankton and periphyton as indicators for long-term monitoring of hypertrophic lake ecosystems.</jats:p>