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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. 3 a, d, e). Such a change in the fluorescence
peak caused by photodegradation is termed as `blue-shift`. Blue-shift is com-
monly observed in surface lake or seawaters where photodegradation is important
due to exposure to natural sunlight (Fig. 3 d, 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KMG et al., unpublished data; Skoog et al. 1996 ; Moran et al. 2000 ; Osburn et al.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 photoin-
duced degradation are 1-84 % in rivers, 16-83 % in lakes, 19-67 % in estu-
aries, 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 inten-
sity have been 36 % at the surface (2.5 m) and 48 % in deeper waters (70 m)
for DOM fractions of <0.1 μ m. In the case of DOM molecular-weight frac-
tions 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 col-
lected during an ongoing summer stratification period (September). Therefore,
the photosensitive DOM fractions had probably already undergone photoin-
duced decomposition before sample collection. The higher fluorescence inten-
sity 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 irradi-
ated in the laboratory (Table 4 ). For similar reasons, photoinduced DOM miner-
alization 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 estua-
rine water it has been observed a fluorescence intensity decrease in fulvic acid-like
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