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

• Published in: Journal of geophysical research. Biogeosciences Vol. 126; no. 12
• Main Authors: Gorsky, A. L., Lottig, N. R., Stoy, P. C., Desai, A. R., Dugan, H. A.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-12-2021
• Subjects: Anoxia; Anoxic conditions; Buoys; Carbon dioxide; Covariance; Data collection; Dimictic lakes; Dystrophic lakes; eddy covariance; Emission measurements; Emissions; Fluctuations; flux; Frequency measurement; Gases; Greenhouse effect; Greenhouse gases; High frequencies; High frequency; Ice; Ice cover; ice‐off; lake; Lake ice; Lakes; Methane; Sampling; Seasons; Spring; Spring (season); Vortices; Water circulation; Water column; United States > US; Wisconsin
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2021JG006537

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.
Plain Language Summary
A large portion of greenhouse gas emissions from lakes occurs during ice‐melt when manual sampling can be too dangerous and during fall mixing when it can occur too quickly to fully capture. Significant amounts of gas can accumulate in the bottom waters without oxygen availability and quickly release as the lake mixes during the spring and fall. We measured dissolved carbon dioxide and methane concentrations in the water column below ice and throughout the open water period from January to October 2020 in a small lake in northern Wisconsin, United States. We combined manual sampling of gas concentrations in the lake with nearly continuous measurements of the flux of methane between the lake and the atmosphere from before ice‐melt to late October 2020. During the late under‐ice period, we did not observe an increase in gases over time, calling for more high frequency measurements during this dynamic period. Further, despite observing only partial lake mixing in the spring, we still measured elevated methane emissions over a 19‐day period after ice‐off. With predicted decreases in ice duration due to warming temperatures, the role of shoulder seasons to the annual budget of greenhouse gases in lakes will continue to be important.
Key Points
Eddy covariance captures methane efflux during ice‐breakup and fall turnover
Methane and carbon dioxide storage remained relatively constant in late winter
Trout Bog only partially mixed during spring turnover and fully mixed in autumn