The anaerobic oxidation of methane (AOM) is an important sink of methane that plays a significant role in global warming. However, evidence for the AOM in freshwater habitats is rare, especially in dam and weir (small-scale dam) reservoirs. Here, the AOM process was examined in freshwater sediments of a small-scale dam reservoir located in Rzeszów, SE Poland. The AOM rate was determined in the main experiment with the addition of the 13 CH 4 isotope marker (He+ 13 CH 4 ). Sediments were collected three times: in spring (in May, 15 • C), in summer (in July, 20 • C) and in autumn (in September, 10 • C). Further analysis considers the impact on AOM rate of potential electron acceptors present in pore-water (NO 2 − , NO 3 − , SO 4 2− , and Fe 3+ ions). The work suggests that an AOM process does take place in the studied reservoir sediments, with this evidenced by the presence in the headspace of an increased 13 CO 2 concentration deemed to derive from 13 CH 4 oxidation. Rates of AOM noted were of 0.36-1.42 nmol·g −1 ·h −1 , with the most intensive oxidation in each sediment layer occurring at 20 • C. While none of the potential electron acceptors considered individually were found to have had a statistically significant influence on the AOM rate, their significance to the dynamics of the AOM process was not precluded.

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The results presented here concern the anaerobic oxidation of methane (CH 4) coupled with denitrification (i.e. a process abbreviated to DAMO) in the freshwater sediments of dam reservoirs located in Rzeszów, Maziarnia and Nielisz, SE Poland. The DAMO rate was determined experimentally by adding a 13 CH 4 isotope marker and NO 3 as an electron acceptor. The sediments were collected once, in autumn (September), with incubation of the 0-5, 5-10 and 10-15 cm layers then carried out at 10 o C, as the temperature corresponding to the in situ conditions at the given time of the year. The DAMO rates were set against the results for the anaerobic oxidation of methane (AOM), which were obtained by incubation of reservoir sediments with the 13 CH 4 isotope marker alone. The DAMO rates noted were of 0.03-0.69 nmol•g-1 •h-1 for Rzeszów Reservoir; 0.04-0.47 nmol•g-1 •h-1 for Maziarnia Reservoir and 0.19-1.04 nmol•g-1 •h-1 for Nielisz Reservoir. Overall, it was typical for the DAMO rates to be about twice as high as the rates of AOM with no electron acceptor added. The addition of NO 3 did not accelerate the methane oxidation significantly in any of the sediment layers from Maziarnia Reservoir, while the effects in Rzeszów Reservoir sediments were confined to the 10-15 cm layer. While the DAMO rates were progressively higher in the deeper layers of sediment from Maziarnia Reservoir, the trend was the reverse (downward) with depth at the Rzeszów and Nielisz sites. The results indicate that the process abbreviated as DAMO takes place in dam reservoirs and is related, not only to the presence of NO 3-, but also to the sediment parameters.

This paper presents the results of research conducted in 2018–2019 on the anaerobic oxidation of methane (AOM) in reservoir sediments. Located in SE Poland, Maziarnia, Nielisz and Rzeszów Reservoirs were selected for the purposes of the research. Rates of AOM were determined via 50-day incubation of sediment from the 0–5, 5–10 and 10–15 cm layers, to which a 13CH4 isotope tracer was added. The sediments had been collected from a single station at each reservoir in places that had earlier reported high levels of emission of CH4 to the atmosphere. Results demonstrate ongoing AOM processes in the kinds of freshwater ecosystem represented by reservoirs, further implying the existence of an important sink for CH4. More specifically, however, AOM rates were found to differ among both the reservoirs, and the layers of sediment, studied. Preliminary analysis of selected environmental factors capable of affecting AOM failed to suggest the availability of electron acceptors (NO3−, SO42−, Fe3+) as key controlling factors. Important factors also proved to be sediment pH, the quality of organic matter (especially the content of organic electron acceptors), the salinity of pore water, and—primarily—the presence of the microorganisms actually responsible for AOM. The results here are important, given the low level of knowledge of AOM process in reservoirs. They therefore help supply key information on the functioning of these ecosystems and the role in global climate change they play.