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et al. 1999 ). Photoinduced degradation thus leads to hypotheses about several
characteristic chemical and optical features of FDOM, which can be listed as
follows (Mostofa et al. 2011 ): (i) In upstream and downstream rivers the fluo-
rescence is predominantly caused by fulvic and humic acids. In contrast, in the
surface layer of lakes and oceans the fluorescence of various FDOM components
is rapidly depleted by exposure to natural sunlight (Hayase and Shinozuka 1995 ;
Mostofa et al. 2005b ; Fu et al. 2010 ; Yamashita and Tanoue 2008 ). As a conse-
quence, the FDOM sampled from these environments is relatively less suscepti-
ble to undergo further photoinduced degradation in the laboratory, as was for
instance the case for Lake Biwa (Table 4 ). (ii) High losses of fulvic acid-like fluo-
rescence have been observed upon irradiation of water samples from the deeper
layers of lakes and seas. They are more pronounced compared to surface water.
A reasonable explanation for this phenomena can include two facts. First, the
higher occurrence of fulvic or humic acids in the deeper layers may result directly
from terrestrial sources through riverine input without degradation in surface
waters (Table 4 ) (Mopper et al. 1991 ). Second, the releases of autochthonous ful-
vic acid (C-like) can occur microbially from algal biomass or phytoplankton in
deeper waters (Mostofa et al. 2009a , b ; Zhang et al. 2009a ; Yamashita and Tanoue
2004 , 2008 ). Autochthonous material is highly susceptible to undergo photoin-
duced decomposition. It has recently been shown that algal-derived CDOM is a
more efficient photoinduced substrate than allochthonous fulvic acid (Mostofa
KMG et al., unpublished data; Johannessen et al. 2007 ; Hulatt et al. 2009 ). (iii)
By comparison of the initial and final photo-bleached components of fulvic acid
(C-like) using PARAFAC analysis, it is estimated that the decrease in fluorescence
was highest (28-30 %) in the longer wavelength regions (Ex/Em = 335-350/430-
450 nm) than at peak M (17 % at 310/450 nm) and peak A (20 % at 250/440 nm)
in downstream river (Mostofa et al. 2010 ). This suggests that the fluorophore at
the longer Ex/Em wavelength in fulvic acid is susceptible to undergo rapid pho-
toinduced degradation in aqueous media. Thus, photodegradation would be use-
ful in the removal of major anthropogenic fluorescent organic contaminants,
particularly the fluorophores at the longer Ex/Em wavelengths in rivers (Mostofa
et al. 2010 ). (iv) Photo-induced losses of fulvic acid-like fluorescence intensity
are gradually reduced in the transition from river to lake, estuary and sea water
(Yamashita and Tanoue 2003a ; Mostofa et al. 2007a , 2005b ; Vodacek et al. 1997 ;
Cory et al. 2007 ). The cause might be linked to the prior losses of fluorescence
intensity in stagnant lake or seawaters by photodegradation. In contrast, pho-
todegradation in rivers is less effective due to continuous transport of water.
Photodegradation changes the excitation-emission spectra by introducing a shift
to shorter wavelengths. This might constitute evidence of the alteration of existing
fluorophores or of the appearance of new fluorescent organic substances (Mostofa
et al. 2009a ). Examples of fluorescent substances arising from FDOM photodeg-
radation could be salicylic acid (Ex/Em = 314/410 nm), 3-hydroxybenzoic acid
(Ex/Em = 314/423 nm), and 3-hydroxycinnamic acid (Ex/Em = 310/407 nm).
These molecules are characterized by fluorescence at relatively short wavelengths
(Mostofa et al. 2009a ).
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