Direct temperature-resolved mass spectrometry (DT-MS) was used to evaluate the molecular-level photodegradation of dissolved organic matter (DOM) isolated from three sites in a Chesapeake Bay subestuary (swamp- and marsh-influenced up-river, midestuarine, and bay mouth). From each site, filtered (<0.1 or <0.2 μm) water samples were irradiated in solarsimulated ultraviolet light followed by isolation of the DOM using C18-solid-phase extraction and subsequent DT-MS analysis. To provide background DOM photoreactivity data for the water samples, we also determined dissolved inorganic carbon (DIC) photoproduction and chromophoric dissolved organic matter (CDOM) photobleaching. DIC photoproduction was correlated with initial DOM light absorbance, initial dissolved organic carbon (DOC) concentration, and photobleaching. Changes in DT-MS characteristics within the extracts (in particular, the loss of an "aromatic" signal believed to be from reworked terrestrial material) were found to correlate linearly with the absorbance of the corresponding water samples. A relationship between photobleaching and DT-MS characteristics was also observed, with the upstream samples asymptotically approaching a constant "molecular-level" value as photobleaching increased. Both relationships appeared to be independent of absorbance wavelength in the ultraviolet. Following irradiation, the swamp/marsh-dominated upstream samples resembled the down-stream samples in terms of absorption spectra and MS-determined molecular-level characteristics. These shifts indicate that terrestrially-derived DOM may be more difficult to differentiate from marine DOM upon photodegradation, which has implications regarding evaluating the terrestrial impact within the marine DOM pool.
Bibliographical noteFunding Information:
The authors thank John Helms, David Ussiri, Joy Davis, Hussain Abdulla and Nianhong Chen for assistance in sample collection, preparation and analysis. Carl Johnson (WHOI) is thanked for DT-MS analyses. This work was supported by NSF grants OCE0241946 and OCE0555245 to ECM and KM, and OCE0327423 to KM.
- Chesapeake Bay
- Dissolved organic matter
- Light absorption
- Mass spectrometry
- Optical properties