Dynamics of redox processes in a seasonal stratified lake with focus on anaerobic methane oxidation coupled with denitrification
11.7 in Isotopenmethoden zur Analyse von Stoffumsatz und Fließprozessen
Freshwater ecosystems constitute a significant source of methane and contribute to the non-anthropogenic methane emitted to the atmosphere. Nevertheless, methane fluxes can be considerably reduced as a result of microbial oxidation in the water column of lakes and rivers. In seasonally stratified lakes with an anoxic hypolimnion, methane oxidation may take place under anoxic conditions and it is frequently attributed to the recently discovered process known as nitrate/nitrite-dependent anaerobic methane oxidation (n-damo). However, redox conditions and the availability of suitable electron acceptors such as nitrate, nitrite and hydrogen sulfide in stratified lakes may change throughout the year. Here we report on seasonal redox dynamics in a stratified lake in southern Germany with the objective to evaluate whether n-damo is a key biogeochemical process leading to reduced methane fluxes into the atmosphere.
To reach our goals we conducted stable isotope analyses of methane, nitrate, and sulfate and interpreted the obtained results in combination with water chemistry data for the water column and sediments of an oligotrophic lake between spring and autumn of 2018 and 2019. Depth-profiles taken after lake stratification in summer showed highest concentrations of methane at the bottom of the lake, elevated nitrate concentrations in the upper part of the water column, and an anoxic zone between 16m and the lake sediments at 23m depth.
Methane produced in the sediments had an average δ13C value of -82 ±1 ‰. In the anoxic zone from 16 to 23m, δ13C values of methane increased by 13 ‰ with increasing depth in concert with decreasing CH₄ concentrations, while a decrease in nitrate concentrations was associated with increasing δ15N and δ18ONO3 values. This is clear evidence that the oxidation of methane was coupled with denitrification (e.g. n-damo).
Our results also showed that the zone in which n-damo occurred, moves upwards in the water column with the seasonal development of the water column stratification. In April, nitrate occurs throughout the water column. At the end of the summer, anoxic conditions prevail below 10m and dissolved nitrate has been completely reduced to a depth of 18 m, facilitating bacterial sulfate reduction (BSR) to hydrogen sulfide (HS⁻) between 18 and 23 m. Dissolved hydrogen sulfide enables sulfide-induced nitrate reduction to take place above the BSR zone, and limits the zone of anaerobic oxidation of methane to a few meters below the oxycline. As a result, between August and December, the microbial reduction of nitrate in the water column of a stratified lake is driven by chemolithotrophic and heterotrophic denitrification, with HS⁻ and CH₄ as principal electron donors, respectively.
Our two year study of stable isotope measurements and chemical data in a water column of a stratified lake highlights the co-occurrence of n-damo, chemolithotrophic denitrification and bacterial sulfate reduction, and shows the complete seasonal redox dynamics in a stratified lake.