Carbon dioxide emissions from an oligotrophic temperate lake: An eddy covariance approach

In recent years, several studies have suggested that most lakes are net carbon sources to the atmosphere rather than carbon sinks. Quantifying and understanding the environmental drivers of carbon dioxide (CO2) flux from lakes is important in order to have a better understanding of the current and future greenhouse-gas budget of aquatic systems and the global ecosystem as a whole. In this study, we present observations of CO2 fluxes in an oligotrophic lake in Northern Michigan during two full growing seasons. We used the eddy covariance technique to measure continuous fluxes of CO2 and calculate the advective fluxes between the lake and the surrounding forest. We found that, at our measurement location far from shore, to the effects of horizontal advection were significantly lower than EC-observed vertical turbulent fluxes and contributed minimally to estimate of the seasonal totals. We found that during the summers the lake was an overall net carbon source, though at rates at much lower magnitude than nearby terrestrial ecosystems. Using a hierarchical modelling approach, we determined that net carbon flux from the lake is primarily correlated with wind speed, indicating the key role of mixing in the upper water layer. Variables indicative of microbial activity and lake gas storage were more highly correlated with the positive fraction of carbon flux than with carbon uptake.