Comparing Spatial and Temporal Variation of Lake-Atmosphere Carbon Dioxide Fluxes Using Multiple Methods

Lakes emit globally significant amounts of carbon dioxide (CO2) to the atmosphere, but quantifying these rates for individual lakes is extremely challenging. The exchange of CO2 across the air-water interface is driven by physical, chemical, and biological processes in both the lake and the atmosphere that vary at multiple spatial and temporal scales. None of the methods we use to estimate CO2 flux fully capture this heterogeneous gas exchange. Here, we compared concurrent CO2 flux estimates from a single lake based on commonly used methods. These include floating chambers (FCs), eddy covariance (EC), and two concentration gradient-based methods labeled fixed (F-pCO(2)) and spatial (S-pCO(2)). At the end of summer, cumulative carbon fluxes were similar between EC, F-pCO(2), and S-pCO(2) methods (-4, -4, and -9.5 gC m-2), while methods diverged in directionality of fluxes during the fall turnover period (-50, 43, and 38 gC m-2). Collectively, these results highlight the discrepancies among methods and the need to acknowledge the uncertainty when using any of them to approximate this heterogeneous flux. Plain Language Summary Lakes comprise a small percentage of the landscape, but they are active and complex areas of carbon cycling. Lakes receive mixed carbon inputs from upstream sources, process this carbon internally, store it in sediments and biomass, and export it downstream. In addition, some fraction of the carbon in lakes exchanges into and out of the atmosphere, linking lakes with the global atmosphere. The exchange of carbon dioxide across lake surfaces has globally significant implications, but quantifying these rates has yet to be fully resolved. Here, we compared four methods of estimating diffusive carbon dioxide exchange between the atmosphere and the lake surface. Flux rates generally agreed during the summer, but estimates diverged in the fall, a critical time period with elevated carbon cycling rates. These discrepancies among methods may arise because of the high degree of spatial and temporal variability in gas exchange and our limited ability to portray and scale these processes accurately. In the future, we need to improve both the resolution of observations and how we process those observations to better measure carbon gas exchange between lakes and the atmosphere.