Methane dynamics in three different Siberian water bodies under winter and summer conditions

摘要

Arctic regions and their water bodies are affected by a rapidly warming climate. Arctic lakes and small ponds are known to act as an important source of atmospheric methane. However, not much is known about other types of water bodies in permafrost regions, which include major rivers and coastal bays as a transition type between freshwater and marine environments. We monitored dissolved methane concentrations in three different water bodies (Lena River, Tiksi Bay, and Lake Golzovoye, Siberia, Russia) over a period of 2 years. Sampling was carried out under ice cover (April) and in open water (July–August). The methane oxidation?(MOX) rate and the fractional turnover rate ( k ′ ) in water and melted ice samples from the late winter of?2017 was determined with the radiotracer method. In the Lena River winter methane concentrations were a quarter of the summer concentrations (8?nmol?L ?1 vs.?31?nmol?L ?1 ), and mean winter MOX rate was low (0.023?nmol?L ?1 ?d ?1 ). In contrast, Tiksi Bay winter methane concentrations were 10?times higher than in summer (103?nmol?L ?1 vs.?13?nmol?L ?1 ). Winter MOX rates showed a median of 0.305?nmol?L ?1 ?d ?1 . In Lake Golzovoye, median methane concentrations in winter were 40?times higher than in summer (1957?nmol?L ?1 vs.?49?nmol?L ?1 ). However, MOX was much higher in the lake (2.95?nmol?L ?1 ?d ?1 ) than in either the river or bay. The temperature had a strong influence on the MOX ( Q 10 = 2.72 ± 0.69 ). In summer water temperatures ranged from 7–14? ° C and in winter from ?0.7 ?to 1.3? ° C. In the ice cores a median methane concentration of 9?nM was observed, with no gradient between the ice surface and the bottom layer at the ice–water interface. MOX in the (melted) ice cores was mostly below the detection limit. Comparing methane concentrations in the ice with the underlaying water column revealed methane concentration in the water column 100–1000 times higher. The winter situation seemed to favor a methane accumulation under ice, especially in the lake with a stagnant water body. While on the other hand, in the Lena River with its flowing water, no methane accumulation under ice was observed. In a changing, warming Arctic, a shorter ice cover period is predicted. With respect to our study this would imply a shortened time for methane to accumulate below the ice and a shorter time for the less efficient winter MOX. Especially for lakes, an extended time of ice-free conditions could reduce the methane flux from the Arctic water bodies.