Response of a shallow water reservoir to anthropogenic stressors: implications for the water supply of Lake Velence, a major recreational lake in Hungary

Abstract Reservoir pollution and eutrophication are pressing global issues threatening water supply and aquatic ecosystems. This study reconstructs multi-decadal environmental changes in the Pátka Reservoir (Central Hungary) using a 54-cm sediment core dated by 137 Cs/ 210 Pb. Multi-proxy analyses included chironomid and pollen assemblages, chlorophyll degradation products (SPDU), major and trace elements and geochemical indicators (total organic carbon [TOC], total bound nitrogen [TbN], total sulphur [TS] and C/N ratio). Results revealed distinct ecological phases: the basal layers (1983–early 1990s) showed dominance of semi-aquatic and opportunistic taxa, likely linked to the reservoir’s drainage (1992) and refilling (1994). Between 1997 and 2015, macrophyte-associated taxa suggested mesotrophic–eutrophic conditions, while upper layers (2015–2022) were dominated by taxa indicating hypoxia and high trophic levels. Rising TOC, TbN, TS and SPDU [sedimentary pigment degradation unit] concentrations reflect intensifying eutrophic conditions, corroborated by monitoring data. Key drivers of ecological change were nutrient loading and water-level fluctuations in the 1990s. Given the reservoir’s role in supplying Lake Velence and its ecological significance, reference conditions were defined and restoration strategies proposed, including angling regulation, ecological dredging and nutrient input control. Since March 2024, the reservoir has remained drained due to poor water quality, highlighting urgent management needs.