Open AccessVariation in ultrafiltered and LMW organic matter fluorescence properties under simulated estuarine mixing transects: 2. Mixing with photoexposure
Author(s) -
Boyd Thomas J.,
Barham Bethany P.,
Hall Gregory J.,
Schumann Brandon S.,
Paerl Ryan W.,
Osburn Christopher L.
Publication year - 2010
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2009jg000994
Subject(s) - estuary , dissolved organic carbon , transect , salinity , photobleaching , colored dissolved organic matter , mixing (physics) , organic matter , seawater , environmental chemistry , bay , fluorescence , chemistry , oceanography , environmental science , analytical chemistry (journal) , geology , phytoplankton , nutrient , physics , organic chemistry , quantum mechanics
Ultrafiltered and low molecular weight dissolved organic matter (UDOM and LMW‐DOM, respectively) fluorescence was studied under simulated estuarine mixing along with moderate photoexposure using Delaware, Chesapeake, and San Francisco Bays (USA) natural organic matter. UDOM was produced by tangential flow ultrafiltration (TFF) from the marine (>33 PSU), mid‐estuarine (∼16 PSU), and freshwater (<1 PSU) members. TFF permeates (<1 kDa) were used to create artificial salinity transects nominally ranging from ∼0 to ∼36, with 4 PSU increments. UDOM or permeate (as control) from freshwater and mid‐estuary was added to each salinity mix in the artificial transect to determine the impact of mixing behavior on optical properties. Three‐dimensional fluorescence excitation‐emission matrix (EEMs) spectra were generated for each end‐member permeate (LMW fraction) and UDOM through the full artificial mixing transect. Fluorescent properties representing standard‐identified peaks, fluorescence ratios and excitation‐emission characteristics were assayed as previously reported. However, in this study, each sample was additionally photobleached for three days (nominally) to determine the coupled effect of estuarine mixing and photobleaching on LMW and UDOM fluorescence. Permeates, except Delaware Bay samples, were more bleached at lower salinities (<16). This effect was especially noticeable for mid‐estuarine LMW organic material which was highly bleached at low salinities. Humic‐type UDOM was generally bleached less at low salinities, maximally at mid‐salinities, and less as it mixed toward the ocean end‐member. As with mixing alone experiments, the B peak showed virtually no variability in the LMW and UDOMs fraction and was not significantly bleached. The N and T peak behaved similarly to one another and were significantly bleached. PCA and PARAFAC models confirmed trends for individual peaks. A four‐dimensional PARAFAC model with pre‐ and post‐bleached as the fourth dimension showed increases in the T peak fluorescence after photobleaching (with some overlap of the B and N peak). Results from this study indicate that coupled mixing and photobleaching can alter CDOM fluorescence in ways which might increase the difficulty in using CDOM as a proxy for DOM in regional carbon cycling biogeochemical models.