Organic carbon (OC) is exported from terrestrial to freshwater ecosystems where it is degraded and lost as CO₂, and seemingly, at faster rates than in soils or marine systems. Across freshwaters, variations in OC degradation have been related to compositional changes in OC. Recent evidence has highlighted that anthropogenic disturbances are likely to increase and modify the source of the lateral OC transfers along hillslopes of upland catchments and through streams and rivers. Such disturbances affect contrastingly different geographical regions and might shift C sinks into sources. Thus, there is an urgent need to constrain, understand and predict the fate of the lateral C fluxes.
Here, I will give an overview of how my past, current, and planned research of organic carbon transformations in freshwaters contribute to that need. A major emphasis of my research is to understand the spatial and temporal distribution of these transformations. Although there seems to be a gradual decrease in the half-life of organic carbon with increasing time spent in the aquatic continuum, organic carbon reactivity appears to be passive or active, depending on when and where it is examined. I will show how reactivity is linked to the chemical composition of organic matter but also how other indirect controls (e.g. inorganic nutrients) modulate these transformations. We will also see some examples on how chemical diversity might be modified in conjunction with landscape elements such as the connectivity of Arctic lakes. Because of climate and land use changes, hydrological conditions are also fluctuating and are predicted to redistribute decay rates of organic carbon. Flow intermittency duration and extent is increasing and some ecosystems might eventually undergo permanent drying. Our first assessments indicate that these areas are still biogeochemically active and might represent a loss of a long-term C sink. All in all, this knowledge points future research towards focusing on the transference between terrestrial and aquatic ecosystems, the need for an integrative catchment and regional perspective and the relevance of highly vulnerable ecosystems and C pools with the aim to constrain global lateral C fluxes.

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