Minor impacts of rain on methane flux from hemiboreal, boreal, and subarctic lakes.

Methane (CH) emissions (F) from northern freshwater lakes are not only significant but also highly variable in time and one driver variable suggested to be important is precipitation. Rain can have various, potentially large effects on F across multiple time frames, and verifying the impact of rain on lake F is key to understand both contemporary flux regulation, and to predict future F related to possible changes in frequency and intensity of rainfall from climate change. The main objective of this study was to assess the short-term impact of typically occurring rain events with different intensity on F from various lake types located in hemiboreal, boreal, and subarctic Sweden. In spite of high time resolution automated flux measurements across different depth zones and covering numerous commonly types of rain events in northern areas, in general, no strong impact on F during and within 24 h after the rainfall could be observed. Only in deeper lake areas and during longer rain events F was weakly related to rain (R = 0.29, p < 0.05), where a minor F decrease during the rain was identified, suggesting that direct rainwater input, during greater rainfall, may decrease F by dilution of surface water CH. Overall, this study indicates that typical rain events in the studied regions have minor direct short-term effects on F from northern lakes and do not enhance F from shallow and deeper parts of lakes during and up to 24-h after the rainfall. Instead, other factors such as wind speed, water temperature and pressure changes were more strongly correlated with lake F.