Variation in ultrafiltered and LMW organic matter fluorescence properties under simulated estuarine mixing transects: 2. Mixing with photoexposure

Abstract

[1] 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 endmember. 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.

Stimulation of Phytoplankton Production by Anthropogenic Dissolved Organic Nitrogen in a Coastal Plain Estuary

“…The component was not split-half validated and did not match with any previously identified components in the OpenFluor database; however, this component does appear to represent a separate FDOM class that is inherent to the chicken litter samples and is not accurately captured by the bioassay PARAFAC model. The identified component did match, visually, to the fluorescent N-peak which has been characterized as biologically labile FDOM , but is not included in the OpenFluor database. Regardless, we have isolated a fluorescence signal specific to poultry litter sources of DOM in the NRE’s watershed.…”

“…PARAFAC component unique to the chicken litter leachate (June 2014, October 2014) treatment samples (left) and the Ex and Em spectra (right). The component was not split-half validated and was identified as the nonhumic, biologically labile N-peak. , Excitation maximum at 270 nm; emission maximum at 372 nm.…”

“…This stimulation of growth attributed to a fluorescent component identified with PARAFAC shared spectral similarity to the N-peak region of fluorescence that has been previously identified as non-humic and biologically labile. 44,45 Chicken litter showed the greatest stimulation; the turkey litter DOM treatment appeared to have an inhibitory effect on primary production, not related to the DOM pool, while the effluent and river DOM treatments had no impact on phytoplankton production compared to their respective inorganic nutrient addition treatments.…”

“…The component was not split-half validated and was identified as the nonhumic, biologically labile N-peak. 44,45 Excitation maximum at 270 nm; emission maximum at 372 nm. The turkey litter treatment also contained a high initial intensity of this residual fluorescence but was not completely removed during the bioassay incubation as compared to the chicken litter treatments (Figure 4).…”