Dissolved organic matter (DOM) is central to the functioning of global ecosystems, but it is one of the most complex mixtures on Earth, making it difficult to predict its future composition and role in the environment. In this talk, I will discuss how the molecular composition of DOM is predictably transformed from plant to microbially-derived compounds and influences microbial respiration and the persistence of carbon in northern waters. I will then explore how global change from catchment- to continental-scales is predicted to shift the future production and consumption of DOM in ways that influence the health of freshwaters.