Seasonal and annual variability of methane emissions to the atmosphere from the surface of a eutrophic lake located in the temperate zone (Lake Kortowskie, Poland)

Abstract. Despite studies on methane emissions from lakes, there remains considerable uncertainty in accurately estimating global emissions from this source. This uncertainty is related to the diversity of lake types, their conditions, geographical locations, as well as the various research methods employed, typically short measurement series, temporal variability of emissions, and the various forms of emissions: diffusion, ebullition, transport in macrophytes, and storage emission.

In this study, an attempt was made to supplement information on methane emissions through real-time in situ measurements using a measurement chamber connected to a mobile CRDS spectrometer. Measurements were conducted on Lake Kortowskie, which is representative of highly eutrophic lakes in the northeastern region of Poland. The methane emission measurement cycle was carried out over a full four-year period (2019–2022) at weekly intervals, alongside simultaneous observations of water indicators and meteorological measurements.

The average methane emission from the surface of Lake Kortowskie over the entire observation period was 11.79 mg m^-2 d^-1, with a median of 6.91 mg m^-2 d^-1, and a maximum of 134.4 mg m^-2 d^-1 on a single measurement date. During the four-year observation period, slight differences in annual averages were noted, along with significant variability in seasonal emissions. In the years 2019, 2020, 2021, and 2022, the average CH₄ emissions were 13.7, 10.1, 11.8, and 11.6 mg m^-2 d^-1, respectively. Seasonally, average emissions were recorded at 3.2, 12.1, 20.6, and 14.9 mg m^-2 d^-1 for winter, spring, summer, and autumn, respectively.

The studies indicated that the main environmental factors associated with methane emissions from the lake were primarily water temperature and air temperature. However, water waves height, wind speed and gusts, precipitation totals, Secchi depth, and oxygen concentration in the water also played significant roles. Regression analyses for Lake Kortowskie suggest that only changes in the main climate components, following the current trend of changes, could lead to an increase in methane emissions from the lake by over 30 % by the year 2100.