476
K. M. G. Mostofa et al.
from rivers and lakes; 300–340/423–464 and 235–260/425–464 nm for various
upstream and rivers, but not in the water Raman Unit (RU: nm −1 ) calibration
(330–380/420–467 and 240–260/4420–467 nm, respectively); 295–310/443–
464 and 250–260/443–464 nm in lakes; 305/439 and 260/439 nm in estuaries,
which shifts to 360–385/478–504 and 250–270/478–504 nm in RU calibration; 320–325/422–454 and <250/422–454 nm in bay and marine waters (but
one finds 340–410/440–520 and 250/440–520 nm upon RU calibration in bay
waters from the Barataria Basin); 300–305/449 and 250/449 nm in irradiated
river waters; 315/429 nm in water extracted from sugar maple leaves; 315/447
and ~250/447 nm in plant biomass, manure and soil (Tables 1, 2) (Stedmon et
al. 2003; Stedmon and Markager 2005b; Ohno and Bro 2006; Mostofa et al.
2010; Mostofa KMG et al., unpublished data; Nakajima 2006; Hunt et al. 2008;
Kowalczuk et al. 2009; Baghoth et al. 2010; Chen et al. 2010; Dubnick et al.
2010; Fellman et al. 2010; Guo et al. 2010; Singh et al. 2010; Yamashita et al.
2010, 2011; Balcarczyk et al. 2009; Santín et al. 2009). Note that longer excitation–emission maxima have been observed at the C-region for SRFA dissolved in
sea water compared to Milli-Q water. This is presumably linked to the formation
of complexes of trace elements in seawater with the functional groups (or flurophores) bound in SRFA. Complex formation can significantly enhance electron
promotion at peak C from the ground to the excited state by longer wavelength
energy. The lower excitation energy can shift the Ex/Em maxima to longer wavelengths. This will be explained in detail in the section that deals with the effect of
salinity.
The second component is denoted as allochthonous fulvic acid (A-like) and is
typically composed of a strong fluorescence shoulder (or peak) at Ex/Em = 225–
250/413–448 nm (peak A-region) and of a minor peak at Ex/Em = 280–295/414–
442 nm (peak C-region) (Fig. 3b). The allochthonous fulvic acid (A-like) is
identified at 230/441 nm (peak A-region) in SRFA dissolved in Milli-Q waters;
225–230/414–442 nm (peak C-region) and 285–295/414–442 nm (minor peak
at peak C-region) in upstream waters (Figs. 2b, 3b); 225/432–442 nm in lakes;
240–250/416–448 nm in estuaries, and <260/(not mentioned) nm in marine waters
(Table 2) (Stedmon et al. 2003; Stedmon and Markager 2005b; Mostofa KMG
et al., unpublished data; Yamashita et al. 2010; Balcarczyk et al. 2009).
The third component of allochthonous fulvic acid exhibits fluorescence excitation–emission maxima at Ex/Em = 285–310/387–429 nm (peak C-region) and Ex/
Em = 230–260/387–429 nm (peak A-region). The fluorescence intensity at peak
A-region is >1.50 times higher than at C-region (Fig. 3c; Table 2). The Ex/Em
maxima of this component at peak C-region occur at relatively shorter wavelengths
compared to those of the allochthonous fulvic acid (C-like). Therefore, the third
component can be denoted as allochthonous fulvic acid (M-like). Allochthonous
fulvic acid (M-like) has been detected at Ex/Em = 285–310/387–429 nm at peak
C-region and 230–240/387–429 nm at peak A-region in the waters of the Yellow
River and Heilongjiang (Amur) River watershed (China); 305/396 and 240/396 nm
in Occoquan Watershed (USA); 305/428 and <240–260/414–428 nm in marine
waters; 320/410 and 250/410 nm in drinking water treatment plants, and 312/417
K. M. G. Mostofa et al.
from rivers and lakes; 300–340/423–464 and 235–260/425–464 nm for various
upstream and rivers, but not in the water Raman Unit (RU: nm −1 ) calibration
(330–380/420–467 and 240–260/4420–467 nm, respectively); 295–310/443–
464 and 250–260/443–464 nm in lakes; 305/439 and 260/439 nm in estuaries,
which shifts to 360–385/478–504 and 250–270/478–504 nm in RU calibration; 320–325/422–454 and <250/422–454 nm in bay and marine waters (but
one finds 340–410/440–520 and 250/440–520 nm upon RU calibration in bay
waters from the Barataria Basin); 300–305/449 and 250/449 nm in irradiated
river waters; 315/429 nm in water extracted from sugar maple leaves; 315/447
and ~250/447 nm in plant biomass, manure and soil (Tables 1, 2) (Stedmon et
al. 2003; Stedmon and Markager 2005b; Ohno and Bro 2006; Mostofa et al.
2010; Mostofa KMG et al., unpublished data; Nakajima 2006; Hunt et al. 2008;
Kowalczuk et al. 2009; Baghoth et al. 2010; Chen et al. 2010; Dubnick et al.
2010; Fellman et al. 2010; Guo et al. 2010; Singh et al. 2010; Yamashita et al.
2010, 2011; Balcarczyk et al. 2009; Santín et al. 2009). Note that longer excitation–emission maxima have been observed at the C-region for SRFA dissolved in
sea water compared to Milli-Q water. This is presumably linked to the formation
of complexes of trace elements in seawater with the functional groups (or flurophores) bound in SRFA. Complex formation can significantly enhance electron
promotion at peak C from the ground to the excited state by longer wavelength
energy. The lower excitation energy can shift the Ex/Em maxima to longer wavelengths. This will be explained in detail in the section that deals with the effect of
salinity.
The second component is denoted as allochthonous fulvic acid (A-like) and is
typically composed of a strong fluorescence shoulder (or peak) at Ex/Em = 225–
250/413–448 nm (peak A-region) and of a minor peak at Ex/Em = 280–295/414–
442 nm (peak C-region) (Fig. 3b). The allochthonous fulvic acid (A-like) is
identified at 230/441 nm (peak A-region) in SRFA dissolved in Milli-Q waters;
225–230/414–442 nm (peak C-region) and 285–295/414–442 nm (minor peak
at peak C-region) in upstream waters (Figs. 2b, 3b); 225/432–442 nm in lakes;
240–250/416–448 nm in estuaries, and <260/(not mentioned) nm in marine waters
(Table 2) (Stedmon et al. 2003; Stedmon and Markager 2005b; Mostofa KMG
et al., unpublished data; Yamashita et al. 2010; Balcarczyk et al. 2009).
The third component of allochthonous fulvic acid exhibits fluorescence excitation–emission maxima at Ex/Em = 285–310/387–429 nm (peak C-region) and Ex/
Em = 230–260/387–429 nm (peak A-region). The fluorescence intensity at peak
A-region is >1.50 times higher than at C-region (Fig. 3c; Table 2). The Ex/Em
maxima of this component at peak C-region occur at relatively shorter wavelengths
compared to those of the allochthonous fulvic acid (C-like). Therefore, the third
component can be denoted as allochthonous fulvic acid (M-like). Allochthonous
fulvic acid (M-like) has been detected at Ex/Em = 285–310/387–429 nm at peak
C-region and 230–240/387–429 nm at peak A-region in the waters of the Yellow
River and Heilongjiang (Amur) River watershed (China); 305/396 and 240/396 nm
in Occoquan Watershed (USA); 305/428 and <240–260/414–428 nm in marine
waters; 320/410 and 250/410 nm in drinking water treatment plants, and 312/417