Laboratory Analysis Description
Total organic carbon (TOC) concentrations were analyzed by a high-temperature combustion method with a Shimadzu TOC-5000A TOC analyzer. In advance the analysis, samples were acidified with 3M HCl, and purged with N2 gas to remove inorganic C. Ancillary physical and chemical parameters were measured using standar methods as part of on-going estuarine water quality monitoring program http://www.serc.fiu.edu/wqmnetwork. Detailed methods will be found elsewhere. For escitation-emission matrix (EEM) measurements, fluorescences spectra were measured with a Jobin-Yvon-Horiba (France) Aqualog-2 fluorometer equipped with a 150-W continuous output xenon arc lamp under condition of 5.7-nm excitation and 2-nm emission slit widths and a 0.25 second response time. Forty-four emission scans were acquired at excitation wavelengths (lamda ex) between 240 and 455 nm at 5 nm intervals. Them emission wavelengths were scanned from lamda ex + 10 nm to lamda ex + 250 nm at 2 nm intervals (Coble et al., 1993 and Coble, 1996). All fluorescence spectra were acquired in ratio mode, whereby the sample (emission signal, S) and reference (excitation lamp output, R) signals were collected and the ratio (S/R) was calculated. The ratio mode eliminates the influence of possible fluctuation and wavelength dependency of excitation lamp output. Several post-acquisition steps were involved in the correction of the fluorescence spectra. First, an inner filter corrections was applied to the fluorescence data according to McKnight et al. (2001). After inner filter corrections the sample EEM underwent spectral subtraction of the Milli-Q water to remove most of the effects due to Raman scattering. Instrument bias related to wavelength dependent efficiencies of the specific instrument's optical components (gratings, mirrors, etc.) were then corrected by applying multiplication factors, supplied by the manufacturer, for both excitation and emission wavelengths for the range of observations. Finally, the fluorescence intensity values were converted to quinine sulfate unit (QSU;1QSU=1 ngL-1 of quinine sulfate monohydroxide) to facilitate inter-laboratory comparisons (Coble et al., 1993). From the 370 nm scan a fluorescence index (FI) was calculated (McKnight et al., 2001). The humification index (HIX) was quantified as the area under the emission curve between 435-480 nm divided by the area under the emission curve between 300-345 nm, for excitation at 254 nm (Zsolnay et al. 1999). The biological index (BIX), an indicator of the relative contribution of new autochthonous production to the DOM pool, was calculated as the emission at 380 nm divided by the emission at 430 nm, for excitation at 310 nm (Huguet et al. 2009). The slope ratio (SR), a measure of the average molecular weight, was calculated as the best-fir slope of the natural-log of abosorbance from 275 to 295 nm divided by the best-fit slope of the natural-log of absorbance from 350 to 400 nm (Helms et al. 2008). Milli-Q water was used as a reference for all fluorescence analysis. UV-visible measurements of the water samples were carried out with 1cm quartz UV-visible cells at room temperature (20 degrees C), using a Varian CARY 50 Bio UV-visible spectrophotometer. Milli-Q water was used as the reference.
Quality Control:
Fluorescence measurements are corrected for internal absorbance quenching. Fluorescence spectra are corrected for internal instrument configuration using excitation and emission correction factors. For DOC, Humic carbon and carbohydrate data, we create calibration curves with standards and then graph the data.
References:
Coble, P. G. (1996). Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Marine chemistry, 51(4), 325-346.
Coble, P. G., Schultz, C. A., & Mopper, K. (1993). Fluorescence contouring analysis of DOC intercalibration experiment samples: a comparison of techniques. Marine chemistry, 41(1-3), 173-178.
Helms, J. R., Stubbins, A., Ritchie, J. D., Minor, E. C., Kieber, D. J., & Mopper, K. (2008). Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter. Limnology and oceanography, 53(3), 955-969.
Huguet, A., Vacher, L., Relexans, S., Saubusse, S., Froidefond, J. M., & Parlanti, E. (2009). Properties of fluorescent dissolved organic matter in the Gironde Estuary. Organic Geochemistry, 40(6), 706-719.
References:
Coble, P. G. (1996). Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Marine chemistry, 51(4), 325-346.
Coble, P. G., Schultz, C. A., & Mopper, K. (1993). Fluorescence contouring analysis of DOC intercalibration experiment samples: a comparison of techniques. Marine chemistry, 41(1-3), 173-178.
Helms, J. R., Stubbins, A., Ritchie, J. D., Minor, E. C., Kieber, D. J., & Mopper, K. (2008). Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter. Limnology and oceanography, 53(3), 955-969.
Huguet, A., Vacher, L., Relexans, S., Saubusse, S., Froidefond, J. M., & Parlanti, E. (2009). Properties of fluorescent dissolved organic matter in the Gironde Estuary. Organic Geochemistry, 40(6), 706-719.
