Mid Sweden University

Carbon Dioxide (CO2), as one of the major anthropogenic greenhouse gases, is widely acknowledged to contribute to global warming and climate change. Historically, the major focus on the role of the aquatic environment in the carbon cycle has been on the atmosphere-ocean exchange. More recent findings suggest the importance of freshwater (lakes, rivers and streams) as a source for atmospheric CO2. The freshwater contribution is, however, poorly understood, mainly due to a paucity of data, especially from running waters.

To address this issue, Eddy Covariance (EC) measurements in a large boreal river in Northern Sweden (Indalsälven), are being made as part of a two-year long continual study of the carbon dioxide exchange between the air and water. This is one of the first known studies of its kind where EC measurements are conducted in a river setting. Continual data acquisition began in April 2018, monitoring a variety of general meteorological parameters, turbulent emissions of carbon dioxide, latent, and sensible heat, together with water-side measurements of CO2. The aim of the study is to investigate the temporal control on river carbon dioxide emissions covering timescales from hours to seasons.

This thesis focuses on the influence of the wind on the air-water CO2 exchange during the first quarter of 2019. The data indicates a positive influence of wind speed on the gas transfer velocity, k, with a threshold between a low dependency and high dependency state occurring around 5 ms-1. A further influence of wind stress, decreased by the alignment of current flow with wind direction is also demonstrated, indicating that higher wind speeds are required when flow is aligned with the wind direction to attain similar levels of k600 than for stationary water. The average value of k600 for over-water wind directions for the four month period considered was 11.7 cm.h-1, which is towards the upper end of the range of values expected for such a river from the existing literature.

These findings help to provide clarity on the processes controlling the CO2 exchange between the atmosphere and boreal rivers, which can be used to improve the accuracy of carbon flux predictions on a global scale.