An experimentalist's approach to global limnology: filling the gaps in process understanding of large-scale organic matter transformations (invited)

In the classical plenary lecture ‘Global limnology’ (J.A. Downing, 2007, SIL in Montréal), a new holistic direction for freshwater science was pointed out, focused at upscaling of limnological processes to planet Earth. Since then, the literature on the global importance of freshwater networks, especially as routes for carbon degassing, burial and transport, has grown explosively. This has been reflected in increased use of large-scale models, intensified collection of field survey data and in a revival of holistic concepts such as the River continuum. However, I here argue that the field has come to a point where mechanistic understanding of basic ecosystem processes again is limiting the development. Recent experimental work on bacterial and photochemical degradation processes is used to exemplify how small-scale reductionist studies can fundamentally challenge the picture painted by current large-scale models, especially with respect to the reactivity of dissolved organic matter along the land-sea continuum. Finally, I discuss how the apparent antagonism between holistic and reductionist studies can be overcome by efforts to better represent mechanistic process understanding in models.