A simplified approach to detect a significant carbon dioxide reduction by phytoplankton in lakes and rivers on a regional and global scale

Carbon dioxide (CO 2 ) uptake by phytoplankton can significantly reduce the partial pressure of CO 2 ( p CO 2 ) in lakes and rivers, and thereby CO 2 emissions. Presently, it is not known in which inland waters on Earth a significant p CO 2 reduction by phytoplankton is likely. Since detailed, comparable carbon budgets are currently not available for most inland waters, we modified a proxy to assess the p CO 2 reduction by phytoplankton, originally developed for boreal lakes, for application on a global scale. Using data from 61 rivers and 125 lakes distributed over five continents, we show that a significant p CO 2 reduction by phytoplankton is widespread across the temperate and sub-/tropical region, but absent in the cold regions on Earth. More specifically, we found that a significant p CO 2 reduction by phytoplankton might occur in 24% of the lakes in the temperate region, and 39% of the lakes in the sub-/tropical region. We also showed that such a reduction might occur in 21% of the rivers in the temperate region, and 5% of the rivers in the sub-/tropical region. Our results indicate that CO 2 uptake by phytoplankton is a relevant flux in regional and global carbon budgets. This highlights the need for more accurate approaches to quantify CO 2 uptake by primary producers in inland waters, particularly in the temperate and sub-/tropical region.