Ultrafiltered and low molecular weight dissolved organic matter (UDOM and LMW-DOM, respectively) fluorescence was studied under simulated estuarine mixing using samples collected from Delaware, Chesapeake, and San Francisco Bays (USA) transects. UDOM was concentrated by tangential flow ultrafiltration (TFF) from the marine (>33 PSU), mid-estuarine (similar to 16 PSU), and freshwater (<1 PSU) members. TFF permeates (<1 kDa) from the three members were used to create artificial salinity transects ranging from similar to 0 to similar to 36, with 4 PSU increments. UDOM from the end-or mid-members was added in equal amounts to each salinity-mix. Three-dimensional fluorescence excitation-emission matrix (EEMs) spectra were generated for each end-member permeate and UDOM through the full estuarine mixing transect. Fluorescence components such as proteinaceous, terrigenous, and marine derived humic peaks, and certain fluorescent ratios were noticeably altered by simulated estuarine mixing, suggesting that LMW DOM and UDOM undergo physicochemical alteration as they move to or from the freshwater, mid-estuarine, or coastal ocean members. LMW fluorescence components fit a decreasing linear mixing model from mid salinities to the ocean end-member, but were more highly fluorescent than mixing alone would predict in lower salinities (<8). Significant shifts were also seen in UDOM peak emission wavelengths with blue-shifting toward the ocean end-member. Humic-type components in UDOM generally showed lower fluorescent intensities at low salinities, higher at mid-salinities, and lower again toward the ocean end-member. T (believed to be proteinaceous) and N (labile organic matter) peaks behaved similarly to each other, but not to B peak fluorescence, which showed virtually no variation in permeate or UDOM mixes with salinity. PCA and PARAFAC models showed similar results suggesting trends could be modeled for DOM end-and mid-member sources. Changes in fluorescence properties due to estuarine mixing may be important when using CDOM as a proxy for DOM cycling in coastal systems.
