Photodegradation of estuarine dissolved organic matter: a multi-method assessment of DOM transformation

A multi-method approach was applied to study changes in dissolved organic matter (DOM) at three estuarine sites with varying salinity, as well as changes resulting from experimental photodegradation. Following measurement of ultraviolet and visible absorption spectra of bulk samples, DOM was isolated using C₁₈ solid phase extraction. The extract was characterized using high performance size exclusion chromatography (HP-SEC) and molecular level characterization was conducted via direct temperature-resolved mass spectrometry (DT-MS) and electrospray ionization mass spectrometry (ESI-MS). The molecular weight distribution of DOM as determined from HP-SEC and ESI-MS varied between techniques, but generally decreased down estuary and with photodegradation for both approaches. Relative differences in molecular weight were significantly correlated with the ratio of absorption coefficients at 254/365 nm. Additionally, photobleaching was significantly correlated with mass spectral characteristics from both DT-MS and ESI-MS. Principal component analysis of DT-MS spectra showed that photoexposure removed different mass spectral characteristics depending on sampling site; however, upon photodegradation, the mass spectral characteristics of both marine DOM and terrestrially dominated DOM approached a common spectrum. We interpret this spectrum, characterized by fragments from aromatic and carbohydrate-like precursors, as photochemically refractory DOM. Our results show that multiple approaches that characterize different aspects of DOM can provide complementary information about its sources and transformation. More specifically, photobleaching results in decreased light absorbance, decreased molecular weight and shifts in the relative abundance of classes of compounds (and broad shifts in m/z values); moreover, these transformations result in photodegraded samples from a low-salinity site which are compositionally similar to samples collected from a mid-salinity site further downstream.