Ecological changes of the Pátka Reservoir

The Pátka Reservoir, which has played a key role in the water supply of Lake Velence for decades, has faced severe ecological challenges in recent years. Our new study, published in the prestigious Q1-ranked journal Hydrobiologia, focuses on uncovering the ecological transformation of the reservoir over the past four decades, seeking answers to the causes of the drastic deterioration in water quality. The research, led by Dr. Magyari Enikő Katalin, Professor at the Department of Environmental and Landscape Geography at Eötvös Loránd University (ELTE), with PhD student Eszter Tombor as lead author, employed modern paleoecological and geochemical methods to shed light on the reservoir's past.

By analyzing a 54-cm long sediment core, our research group reconstructed the state of the reservoir from 1983 to the present. Based on chironomid remains preserved in the sediment, pollen, and a comprehensive geochemical analysis, we identified three distinct periods:

• 1983–1997: An unstable period following the establishment of the reservoir, disrupted by drainage in the early 1990s and subsequent refilling. The sediment was dominated by opportunistic chironomids (Chironomus species) indicating oxygen-poor environments, characterized by low biological productivity.
• 1997–2015 (Reference state): This was the "golden age" of the reservoir. Within the mesotrophic water body, stable water levels facilitated the development of rich submerged macrophyte vegetation, and bottom-water oxygenation improved. Sensitive faunal elements associated with macrophytes (e.g., Cladotanytarsus species) appeared, and ecological diversity increased. Paleoecological and geochemical data consistently show that this was the reservoir's most favorable ecological state, characterized by moderate nutrient loading, aquatic vegetation, stable sedimentation, and balanced benthic communities. This period was designated as the reference state for restoration.
• 2015–2022: In the uppermost layers, we observed rapid deterioration in water quality and a drastic increase in organic matter content (TOC, TbN, TS), during which the reservoir reached a hypoxic, hypertrophic state. This is further confirmed by the return of Chironomus dominance. Mass fish mortality and the loss of the water supply function led to the reservoir being drained in 2024.

Our findings indicate that the deterioration in water quality was not primarily caused by agricultural activity in the surrounding areas, but rather by in-lake fish management (intensive feeding and stocking) and nutrient loading from inflowing waters. The causes of these processes are strongly linked to global warming, which has resulted in increasingly extreme precipitation patterns in the region. This is further compounded by the diversion of the Császárvíz stream, which supplies the reservoir, in its upper course. As a consequence, the proportion of treated municipal wastewater in the stream channel is currently high.

The authors emphasize that the ecological rehabilitation of the Pátka Reservoir is essential for the safe water replenishment of Lake Velence. Proposed measures include carefully conducted ecological dredging, regulation of angling activities, and increasing the proportion of predatory fish, so the reservoir can once again reach its stable, healthy state observed between 1997 and 2015.

Further details can be found in our published study: https://doi.org/10.1007/s10750-026-06111-4.

Funding of this research was provided by RRF-2.3.1-21-2022-00014.