The chemical characteristics of bulk (sterile-filtered) and high molecular-weight dissolved organic matter (HMW DOM) were analyzed for freshwater (St. Louis River, Minnesota to Lake Superior) and saline (Elizabeth River, Virginia to Chesapeake Bay) river-to-receiving basin transects. Dissolved organic carbon concentrations and UV-Visible spectroscopy of bulk DOM demonstrated a reduction in organic carbon, colored DOM and aromatic compounds downstream in both transects. The proportion of DOM recoverable via ultrafiltration as HMW material also decreased downstream in both transects, although there was an offset in recoveries between the transects that may be explained by the effects of ionic strength and/or differences in ultrafiltration technique. The analysis of HMW DOM by Fourier transform infrared spectroscopy illustrated similar trends between transects, with a general shift from aromatic/carboxylic compounds nearshore to aliphatic/carbohydrate materials offshore. The parallel changes observed along saline and freshwater transects imply that similar processes play significant roles in the down-gradient alteration of DOM and that ionic strength or pH changes cause second-order effects.
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Acknowledgments We thank Hussain Abdulla and Joy Davis for isolating and processing sample from the Chesapeake Bay and Brent Dalzell for training and assistance in processing sample from the Lake Superior transect. Thanks also to the captains and crew of the R.V.’s Faye Slover and Blue Heron for their smooth voyages at sea. Research was supported by NSF grant OCE-0453777 (ECM) and the University of Minnesota Duluth (via start-up funds to ECM).
- Chesapeake Bay
- DOM characterization
- HMW DOM recovery
- Lake Superior