Fluorescence, CDOM

Fluorescent CDOM and Validation of Remote Sensing Products... 

UV absorption and fluorescence, leads to an enhancement of microbial decomposition and remineralization, whereas photodegradation of DOM with minimal CDOM has no net effect or reduces subsequent microbial decomposition and remineralization. Continental runoff is rich in CDOM, and photodegradation results in substrates that enhance microbial decomposition and the remineralization of terrestrial DOM (Miller and Moran, 1997 Smith and Benner, 2005). Very little CDOM in the open ocean appears to be of terrestrial origin (Hernes and Benner, 2006), and the photodegradation of surface water DOM results in a reduction in microbial decomposition, whereas photodegradation of deep water DOM results in an increase in microbial decomposition (Benner and Biddanda, 1998). 

Finally, a variety of zooplankton have recently been shown to produce colored (chromophoric) DOM (CDOM), a fraction of which is fluorescent (FDOM) (Steinberg et al, 2004 Urban-Rich et al, 2004, 2006). Although the nitrogenous composition of CDOM/FDOM is little known, fluorescent humic-hke material (likely fulvic and humic acids) is produced during grazing, influencing the optical characteristics of coastal waters (Urban-Rich et al, 2006). A recent culture study shows that humic-N can be a source of N for coastal phytoplankton (See et al, 2006). 

Urban-Rich, J., McCarty, J. T., and Shailer, M. (2004). Effects of food concentration and diet on CDOM accumulation and fluorescent composition during grazing experiments with the copepod, Calanusfmmarchicus. ICES J. Mar. Sci. 61, 542—551. 

Figure 4 Correlations of with (a) surfactant concentration, (b) in situ chlorophyll fluorescence, (c) dissolved organic carbon (DOC), and (d) colored dissolved organic matter (CDOM) for seawater samples collected from Monterey Bay, USA ( ) and on a transect from Narragansett USA to Bermuda ( ) (Frew, 1997) (reproduced by permission of Cambridge University Press from The Sea Surface and Global Change, 1997, pp. 121-172).

G.M. Ferrari, M.D. Dowell (1998). CDOM absorption characteristics with relation to fluorescence and salinity in coastal areas of the Southern Baltic Sea. Estuarine, Coastal Shelf Sci., 47, 91-105. 

Figure 2. Spectral diffuse attenuation of downwelling irradiance from Figure 1 compared with for pure seawater estimated by Smith and Baker [18], The phytoplankton concentration (based on chlorophyll a fluorescence) was highest in the upper 30 m. The curve labeled 1-30 m minus phytoplankton was calculated by regression of spectral Xjj against chlorophyll fluorescence for a range of depths, a method that also removes effects of scattering and absorption (including that of CDOM) that covary with phytoplankton fluorescence.

Figure 7. CDOM fluorescence of water from two lakes (Hargreaves, unpublished) emission scans for excitation at 370 nm (Shimadzu 551 fluorometer), before and after subtraction of water blank. Samples deionized water (DIW), L. Giles water (ca. 1 g m DOC), L. Lacawac water (ca. 5 g m DOC) The Raman scattering peak at 417 nm represents a shift in wavenumber by 3400 cm from the excitation wavenumber. The broad peak is contributed predominantly by the fulvic acid fraction of DOM. The peak wavelength and fluorescence index ratio for these samples (L. Giles, 452 nm peak and ratio = 1.5 L. Lacawac, 455 mn peak and ratio = 1.4) suggest a slight difference in CDOM...

The excitation-emission matrix spectra (EEMS) of CDOM fluorescence also are altered on exposure to solar radiation, generally with reductions in fluorescence that approximately parallel absorption losses [36-38]. Hypsochromic shifts (shifts to shorter wavelengths) occur in both excitation and emission maxima on irradiation [36-38]. Interactions between photochemical and microbial degradation [38] are involved. 

A. Vodacek (1992). An explanation of the spectral variation in fresh water CDOM fluorescence. Limnol. Oceanogr., 37,1808-1813. 

These photochemical intermediates and products are produced at relatively low efficiencies. About 98-99% of the photons absorbed by CDOM are released as heat, while another 1 % are re-emitted as fluorescence. These percentages (or fractions) of absorbed photons giving rise to particular photoresponses are known as quantum yields (O). The O for the production of H2O2 and O2 (the two reduced oxygen species produced with highest efficiency), are approximately one to two orders of magnitude... 

Upon excitation, CDOM fluoresces. This fluorescence is dependent on the excitation wavelength, but in general broad emission bands are observed between 250 nm and 500 nm [37]. The common fluorescent regions observed are depicted in Fig. 2. In sunlight the fluorescence intensity fades more rapidly than the absorbance. This is attributed to photoreactions of DOM that result in bleaching of the fluorescence [38]. 

SFS technique has been explored in the last years, e.g., to classify the composition of mixtures of heterogeneous CDOM present in natural freshwaters and in their different subtractions [27,28,33,34]. Similar attempts to identify certain structural and functional constituents in natural HMs have been carried out also with the aid of total luminescence spectroscopy [35]. At present, utilizations of fluorescence techniques are increasingly related to solve certain environmental questions, e.g., how does CDOM vary spatially between different aquatic environments/seasons, what is the causative factor for the leaching of natural carbons, are there any spectral signatures that could lead to the identification of CDOM sources In connection with the heterogeneity of CDOMs between different water samples there is no single spectral parameter that could explain the variation but chemometric applications are needed for interpretation of multidimensional excitation-emission matrices [33,35-39]. 

CDOM concentrations are at minimal levels in the surface of the ocean gyres due to long exposure to sunlight and low biological activity. Thus, both production of new fluorescent materials and the consequences of photobleaching are more readily observed in the open ocean away from the influence of rivers. In the Equatorial Pacific Ocean, diel variability in the composition and concentration of CDOM was observed in the surface waters (Coble, unpublished data). Samples collected at dawn showed the presence of both humic-like (peaks C and Ac) and protein-like fluorescence (peak B), which was greatly diminished or absent from at the same station at noon (Figure 3.6). The mean concentration observed in these surfaces waters was 0.3 ppb QSE, but lower values of less than 0.1 ppb QSE were observed at the Hawaii Open Time Series (HOTS) station (Coble, 1996). 

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