This project has developed during GLEON 17 in the catchment/lake connection working group.
There are about 117 million lakes on Earth that have long been regarded as isolated waters, and even been termed mesocosms. More recently, they have been considered an integral part of the Earth surface, being hydrologically connected and responsible for transporting and transforming carbon along the land to ocean aquatic continuum. While it is widely accepted that lakes can act as active carbon transformers it is not known yet where and when lakes function as such and where and when they function as passive open channels with losses of untransformed terrestrial carbon to downstream waters, the atmosphere and the sediments. At present, global carbon estimates that include processes at the land to ocean aquatic continuum are based on two different assumptions, one where lakes are considered as passive open channels and another one where lakes are considered as efficient carbon transformers. Depending on which assumption is made large differences in estimates of the terrestrial carbon sink occur. Here we show with carbon mass balances and sensitivity analyses how estimates of the terrestrial carbon sink vary depending on the main function of lakes.