Joanna Siwek podpisana: Joanna P. Siwek. Bibliogr.

In recent decades, the population of the Eurasian beaver (Castor fiber) has undergone a rapid recovery from near extinction to abundance across large areas of Europe. The ability of this species to build dams of varying durability and extent has a significant impact on ecological processes, making beaver reintroduction an important environmental factor in recolonised areas. Although the effects of beaver dams on stream water chemistry have been studied extensively, general conclusions are often limited by site-specific conditions. Therefore, the aim of this study was to assess the impact of the flooding extent and age of beaver pond sequences on the physico-chemical properties of the water in and below beaver ponds compared to upstream channel sections across the seasons. The study was conducted on nine beaver-inhabited streams distributed across the Western Carpathians (Poland and Slovakia). The beaver ponds were divided into two types based on flooding extent: overflowing the river banks, or confined to the river channel (in-channel). Ponds were classified into three age categories: young (≤3 years old), moderate (4-9), and old (≥10). Water samples were collected above, within, and below the beaver pond sequences under baseflow conditions during four seasons in 2022-2023. The results showed that beaver dams have a strong impact on some physico-chemical stream water parameters during warm periods, when high temperatures accelerate biogeochemical processes. In particular, beaver ponds were associated with an increase in water temperature and decreases in dissolved oxygen, pH, and concentrations. The age of the beaver ponds had a more pronounced effect on water chemistry than the pond type. There was a greater decrease in dissolved oxygen and pH (throughout the study period) and concentrations (in spring and summer) in older ponds compared to younger ones. These changes might be regulated by the decomposition and aerobic/anaerobic oxidation of organic matter, which is more abundant in older ponds. Overall, the results suggest that seasonal conditions and pond age are important factors influencing the biogeochemical effects of beaver dams in mountain streams. However, most of the effects observed within the ponds were, to some extent, attenuated in downstream river sections.