Longitudinal dynamics and transformation of riverine dissolved organic matter from source to sea

Transformation and degradation of dissolved organic matter (DOM) are of considerable magnitude in large rivers but studies investigating a river system from source to sea are scarce. DOM composition changes from headwaters to tide-impacted river stretches due to natural processes, but is also influenced by anthropogenic impacts on river morphology and water quality. We tested the hypotheses that (1) aromatic, oxygen-rich, and large molecules of terrestrial DOM in upstream regions are transformed to more saturated, nitrogen-rich, and smaller molecules towards the tidal and coastal parts, and (2) chlorophyll a concentration and salinity are important explanatory variables of DOM transformation. We tracked the longitudinal dynamics of DOM composition and relevant drivers along the Elbe River in Central Europe from the Czech headwater region via the lowland freshwater section and the tidal region to the coastal waters of the North Sea applying a Lagrangian sampling approach. Chlorophyll a concentration and oxygen saturation increased longitudinally in the river but showed a distinct minimum in the estuary upstream of the salinity gradient whereas dissolved nutrients were depleted by algae in the freshwater part and were released at algal die-off in the estuary. DOM was dominated by aromatic, oxygen-rich components of terrestrial origin in the upstream region. However, the imprint of this terrestrial signal constantly decreased with increasing river stretch while the proportion of organic nitrogen increased towards the estuary and coastal regions. Analyses of DOM transformations along the river-estuary-ocean gradient revealed that decarboxylation was the most frequent transformation and that phytoplankton and salinity were major explanatory variables of DOM quality. Overall, our unique data set demonstrated a distinct sequence of DOM transformation along the land-ocean gradient highlighting the large activity of riverine and estuarine systems in terms of organic carbon dynamics.