The European beaver Castor fiber is well‐known as an ecosystem engineer that greatly affects landscape structure, biodiversity as well as physical and chemical properties of surface water bodies. Beaver ponds alter surface water bodies by raising water elevation, decreasing flow velocity and altering the morphology of streams or drainage ditches, which can reduce the concentrations of organic carbon (OC) and nutrients (N, P). Recent studies indicated that mercury transforms into hazardous and neurotoxic methylmercury (MeHg) in beaver impoundments by biological processes in anaerobic conditions. However, the knowledge about nutrients and MeHg levels in impounded forest waterbodies is scarce in Lithuania. We aimed to ascertain the alteration in concentrations and stocks of OC, nutrients and MeHg in water and sediments from upstream and downstream, as well as within beaver dams and ponds during the growing seasons of 2016–2018. Results showed higher concentrations of dissolved organic carbon (DOC) and nutrients (P and N) in upstream water samples compared to those of downstream from beaver dams. Meanwhile, in sediments mean stocks of OC, P and N were the highest in the middle part of the ponds and in beaver dams. Moreover, the concentrations and stocks of MeHg in sediments were higher in beaver dams than in any other parts of beaver impoundments (upstream, mid‐pond, pond periphery and downstream). We conclude that dam bottom sediments were rich in OC, N and P, and at the same time, contained toxic MeHg. Therefore, beaver dams could act as a trickle filter by improving water quality, in our case, DOC, N and P leaching, from riparian forests and soils, but may also act as hotspots of mercury methylation.
The lead (Pb) is a heavy metal, which causes severe disorders in humans and other organisms. Recently, the item of wetland buffer zone has emerged as an integrated development approach to nature conservation including buffer zone pollution by Pb. The main sources of Pb pollution in Lithuania are Pb ammunition used by hunters and natural and anthropogenic activities. Considering plant capability to accumulate Pb, they are used for bioindication. The objective of the study was to determine Pb content in the aboveground biomass of vegetation collected from the reference sites in the buffer zones of wetlands. The research was conducted in three localities of the different wetland buffer zones -drying ditch, natural pond, and stream, in the three parts of Lithuania: in the Northwestern part of Lithuania in the territory of the Žemaitija National Park, in the northern part of the Žemaitija Upland at the border of Latvia, and in the Central Lithuania, in the vicinity of the largest artificial waterbody -Kauno Marios and the Nemunas River. The content of Pb in the working solutions was determined by the atomic absorption method on an atomic absorption spectrometer at a wavelength of 283.0 nm. Significant differences were calculated using the three-way ANOVA (Tukey's HSD test). The relationships were calculated using a regression analysis, the data were significant at P ≤ 0.01 and P ≤ 0.001. The data of the research show that the content of Pb corresponds to the background content of uncontaminated sites, the values of which are reported by other scientists and is equal to 0.95-6.84 mg kg −1 . The average content of Pb in the buffer zones of various surface water source types was 3.75-3.76 mg kg −1 . The content of Pb in the plants at 20-30 m from the surface water bodies increased by 1.3-1.5 times, and then at 40 m it decreased by the same indicator.
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