Nature, origin and average age of estuarine ultrafiltered dissolved organic matter as determined by molecular and carbon isotope characterization

The Ems-Dollart estuary (on the border of the Netherlands and Germany) was chosen for a pilot study to characterize ultrafiltered dissolved organic matter (UDOM) in estuarine systems. UDOM samples were taken from four locations with salinities varying from 0.43 to 20‰. The UDOM in these samples was concentrated using cross-flow ultrafiltration and represents the size fraction of DOM between 1000 Dalton and 0.2 μm. The samples were analyzed by analytical pyrolysis, stable carbon isotope and radiocarbon analysis, and solid state 13C NMR. All UDOM samples throughout the estuary showed relatively similar pyrolyzates and 14C activities (c. 87%). Solid-state 13C NMR spectra were also similar, except for the sample with the highest salinity. UDOM δ13C values ranged from −27.78 to −25.40‰ with increasing salinity. This suggests mixing of river DOM with seawater DOM of comparable ages. The absence of pyrolysis products of unaltered polysaccharides, lignins, proteins and lipids indicates that the Ems-Dollart estuary UDOM consists of a large fraction of refractory organic matter and a very small fraction of fresh organic matter.