Longitudinal shifts in dissolved organic matter chemogeography and chemodiversity within headwater streams: a river continuum reprise

We tested a long-standing hypothesis within river ecology, predicted by the River Continuum Concept, that dissolved organic matter (DOM) diversity decreases with stream order. We measured DOM molecular composition across three stream orders in the headwaters of well characterized forested catchments with ultrahigh-resolution mass spectrometry to assess DOM chemogeography and chemodiversity over the spatial scales of climatic regions and fluvial networks. Stream waters with similar dissolved organic carbon (DOC) concentrations in different climatic regions had distinctive DOM compositions, but shared 69.5 % of the 3286 individual chemical formulae detected. DOM compositions common to all watersheds were characterized by abundant lignin-like and tannin-like molecules as well as carboxyl-rich alicyclic-like molecules (CRAM); 50 % of all formulae were found in all streams. Of the roughly 700 unique chemical formulae within a given fluvial network, most were outside the CRAM region within a van Krevelen diagram and 78 to 95 % were restricted to 1st-order streams where diffuse ground water sources surface, coalesce into a channel, and flow downstream. The 1st-order streams within a fluvial network also exhibited the highest formula diversity as well as the greatest numbers of formulae across a broad range of compound classes.