The Importance of Spring Mixing in Evaluating Carbon Dioxide and Methane Flux From a Small North-Temperate Lake in Wisconsin, United States

In limnological studies of temperate lakes, most studies of carbon dioxide (CO2) and methane (CH4) emissions have focused on summer measurements of gas fluxes despite the importance of shoulder seasons to annual emissions. This is especially pertinent to dimictic, small lakes that maintain anoxic conditions and turnover quickly in the spring and fall. We examined CO2 and CH4 dynamics from January to October 2020 in a small humic lake in northern Wisconsin, United States through a combination of discrete sampling and high frequency buoy and eddy covariance data collection. Eddy covariance flux towers were installed on buoys at the center of the lake while it was still frozen to continually measure CO2 and CH4 across seasons. Despite evidence for only partial turnover during the spring, there was still a notable 19-day pulse of CH4 emissions after lake ice melted with an average daytime flux rate of 8-30 nmol CH4 m(-2) s(-1). Our estimate of CH4 emissions during the spring pulse was 16 mmol CH4 m(-2) compared to 22 mmol CH4 m(-2) during the stratified period from June to August. We did not observe a linear accumulation of gases under-ice in our sampling period during the late winter, suggesting the complexity of this dynamic period and the emphasis for direct measurements throughout the ice-covered period. The results of our study help to better understand the magnitude and timing of greenhouse gas emissions in a region expected to experience warmer winters with decreased ice duration.

Automated summary

This study focused on the dynamics of CO2 and CH4 in a small lake, especially the sudden increase in CH4 emissions after ice melt in spring. It emphasizes the importance of direct gas measurements during the ice-covered period.