504
K. M. G. Mostofa et al.
fluorescence of peak C into the photo-bleached fluorescence peak (peak M p ),
which can show the highest fluorescence intensity in some natural waters (Mostofa
et al. 2007a, 2005b; Moran et al. 2000; Komada et al. 2002; Burdige et al. 2004).
Due to photodegradation, the new photo-bleached peak M p is shifted at shorter
excitation–emission wavelengths (Fig. 3a, d, e). Such a change in the fluorescence
peak caused by photodegradation is termed as `blue-shift`. Blue-shift is commonly observed in surface lake or seawaters where photodegradation is important
due to exposure to natural sunlight (Fig. 3d, e). Photoinduced effects can decrease
the fluorescence intensity (FI) of fulvic acid-like (peak C), FWAs-like (peak
W), and tryptophan-like (peak T) compounds, which are commonly observed in
natural waters in field and experimental observations (Fig. 5; Table 4) (Hayase
and Shinozuka 1995; Mostofa et al. 2005a; 2005b, 2010, 2007a, 2011; Stedmon
et al. 2007a; Brooks et al. 2007; Garcia et al. 2005; Winter et al. 2007; Mostofa
KMG et al., unpublished data; Skoog et al. 1996; Moran et al. 2000; Osburn et al.
2009; Lepane et al. 2003; Abboudi et al. 2008; Poiger et al. 1999; Fu et al. 2010;
Borisover et al. 2009; Yamashita and Tanoue 2008; Vodacek et al. 1997; Yamashita
et al. 2007; Shank et al. 2010).
Fulvic Acid-like Components in Natural Waters
Fluorescence intensity losses of fulvic acid-like substances by photoinduced degradation are 1–84 % in rivers, 16–83 % in lakes, 19–67 % in estuaries, and 9–84 % in sea waters studied experimentally in the course of short
(hours) to long-term (days or months) irradiation (Table 4). In lake water
after 12 days irradiation, the losses of fulvic acid-like fluorescence intensity have been 36 % at the surface (2.5 m) and 48 % in deeper waters (70 m)
for DOM fractions of <0.1 μm. In the case of DOM molecular-weight fractions below 5 kDa, the corresponding losses have been 16 % in surface waters
(2.5 m) and 50 % in deeper waters (70 m). The low fluorescence intensity
decrease in the case of surface-water DOM with molecular weight below 5 kDa
may be explained by the fact that the corresponding samples have been collected during an ongoing summer stratification period (September). Therefore,
the photosensitive DOM fractions had probably already undergone photoinduced decomposition before sample collection. The higher fluorescence intensity decrease observed for deep-water DOM may be accounted for by the fact
that deep waters undergo photoinduced degradation processes to a lesser extent
because of the reduced sunlight irradiance compared to surface waters (Laurion
et al. 2000). As a consequence, deep-water samples may contain significant
amounts of photosensitive DOM components, which have not been degraded in the
natural environment and can undergo photoinduced decomposition when irradiated in the laboratory (Table 4). For similar reasons, photoinduced DOM mineralization is very difficult to be observed in surface lake water samples and is much
easier to be detected upon irradiation of groundwater (Vione et al. 2009). In estuarine water it has been observed a fluorescence intensity decrease in fulvic acid-like
K. M. G. Mostofa et al.
fluorescence of peak C into the photo-bleached fluorescence peak (peak M p ),
which can show the highest fluorescence intensity in some natural waters (Mostofa
et al. 2007a, 2005b; Moran et al. 2000; Komada et al. 2002; Burdige et al. 2004).
Due to photodegradation, the new photo-bleached peak M p is shifted at shorter
excitation–emission wavelengths (Fig. 3a, d, e). Such a change in the fluorescence
peak caused by photodegradation is termed as `blue-shift`. Blue-shift is commonly observed in surface lake or seawaters where photodegradation is important
due to exposure to natural sunlight (Fig. 3d, e). Photoinduced effects can decrease
the fluorescence intensity (FI) of fulvic acid-like (peak C), FWAs-like (peak
W), and tryptophan-like (peak T) compounds, which are commonly observed in
natural waters in field and experimental observations (Fig. 5; Table 4) (Hayase
and Shinozuka 1995; Mostofa et al. 2005a; 2005b, 2010, 2007a, 2011; Stedmon
et al. 2007a; Brooks et al. 2007; Garcia et al. 2005; Winter et al. 2007; Mostofa
KMG et al., unpublished data; Skoog et al. 1996; Moran et al. 2000; Osburn et al.
2009; Lepane et al. 2003; Abboudi et al. 2008; Poiger et al. 1999; Fu et al. 2010;
Borisover et al. 2009; Yamashita and Tanoue 2008; Vodacek et al. 1997; Yamashita
et al. 2007; Shank et al. 2010).
Fulvic Acid-like Components in Natural Waters
Fluorescence intensity losses of fulvic acid-like substances by photoinduced degradation are 1–84 % in rivers, 16–83 % in lakes, 19–67 % in estuaries, and 9–84 % in sea waters studied experimentally in the course of short
(hours) to long-term (days or months) irradiation (Table 4). In lake water
after 12 days irradiation, the losses of fulvic acid-like fluorescence intensity have been 36 % at the surface (2.5 m) and 48 % in deeper waters (70 m)
for DOM fractions of <0.1 μm. In the case of DOM molecular-weight fractions below 5 kDa, the corresponding losses have been 16 % in surface waters
(2.5 m) and 50 % in deeper waters (70 m). The low fluorescence intensity
decrease in the case of surface-water DOM with molecular weight below 5 kDa
may be explained by the fact that the corresponding samples have been collected during an ongoing summer stratification period (September). Therefore,
the photosensitive DOM fractions had probably already undergone photoinduced decomposition before sample collection. The higher fluorescence intensity decrease observed for deep-water DOM may be accounted for by the fact
that deep waters undergo photoinduced degradation processes to a lesser extent
because of the reduced sunlight irradiance compared to surface waters (Laurion
et al. 2000). As a consequence, deep-water samples may contain significant
amounts of photosensitive DOM components, which have not been degraded in the
natural environment and can undergo photoinduced decomposition when irradiated in the laboratory (Table 4). For similar reasons, photoinduced DOM mineralization is very difficult to be observed in surface lake water samples and is much
easier to be detected upon irradiation of groundwater (Vione et al. 2009). In estuarine water it has been observed a fluorescence intensity decrease in fulvic acid-like