Mangroves are major sources of terrigenous dissolved organic matter (DOM) to the ocean. However, our understanding of processes shaping the molecular composition of DOM at the land-ocean interface is still limited. We used a complementary approach combining ultrahigh-resolution mass spectrometry (FT-ICR-MS) and high-field nuclear magnetic resonance spectroscopy (NMR) for molecular formula and structural analysis of solid-phase extracted DOM recovered from a mangrove-fringed river-to-ocean transect in North Brazil. We hypothesized that marine, terrestrial and porewater-influenced DOM contains source-specific molecular fingerprints. We detected thousands of molecular formulas and various structural features. The increasing relative abundance of aliphatic structures from the river to the coastal ocean reflects the increasing influence of marine, microbial DOM, whereas the decreasing relative abundance of terrestrial aromatic structural motifs suggests estuarine dilution and/or removal of these compound groups by processes such as flocculation and photochemical degradation. Aromatic structural motifs were underestimated by a factor of three by ¹H-detecting NMR experiments compared to FT-ICR-MS analysis. Carbohydrates were more readily detected by NMR analysis. Only two-dimensional NMR was suitable to trace porewater discharge as a major source for polycyclic aromatic hydrocarbons to the coastal ocean. Our findings highlight that combined FT-ICR-MS and NMR analyses yield highly complementary results for the characterization of DOM transformations along land-ocean interfaces.

Primary Presenter: Nico Mitschke, University of Oldenburg (nico.mitschke@uni-oldenburg.de)

Authors:

Nico Mitschke, University of Oldenburg  (nico.mitschke@uni-oldenburg.de)

Thorsten Dittmar, University of Oldenburg  (thorsten.dittmar@uni-oldenburg.de)

Michael Seidel, University of Oldenburg  (m.seidel@uni-oldenburg.de)