Variability of summer CH4 and CO2 flux rates in and between three large Swedish lakes : A spatio-temporal field study

Sammanfattning: Understanding of natural greenhouse gas (GHG) cycles is crucial for making GHG budgets, which work as basis in climate change and global warming policy programs. Lakes as a source for GHG activity have only recently been included in global GHG budgets, and most studies of lake GHG flux rates are conducted on lakes <10 km2, which only comprise roughly half of the global lake area—making data of large lake flux rates scarce. CO2 and CH4 are the primary contributors of GHGs, and lakes house production processes and receive these gasses via lateral transport. This study utilized a floating chamber method with CO2 sensors to study CH4 and CO2 flux rates from three large Swedish lakes. To do this, chambers were anchored at shallow depth, as well as passively drifted on open water. Sampling was conducted during two periods in the summer 2019, late June–early July and August. For CH4, spatial difference was found between deep and shallow transects within lakes, no temporal difference was found between study periods. Difference between lakes within the deep and shallow chamber groups was found. One possible instance of deep-water ebullition was recorded, and a correlation between CH4 flux rate and water temperature was observed. For CO2, no difference between deep and shallow chambers or measurement periods was found. One instance within the deeper chamber group was found to be different between two of the lakes, despite all three lakes being of different size, depth and trophic state. The study results indicate CO2 water concentrations near saturation with atmosphere during the sampling periods. No correlation between CO2 flux rate and water temperature was observed. Unexpected small-scale variability patterns in CO2 flux was observed while chambers were passively drifting. While some observed patterns for the two gases could be explained by previous findings, some of our observations could not be explained on the basis of previous literature, highlighting the need for further study of GHG flux rates from large lakes.