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shows an entirely opposite behavior (maximum fluorescence at peak C and low
fluorescence at peak A) (Fig. 3 k, l).
2.6 Relationship Between FDOM and DOM
Fulvic acid-like fluorescence intensity (FI) is significantly correlated with DOC
concentration both linearly and non-linearly as a consequence of the effects of
sunlight on river water. However, non-linear correlation is more significant than
the linear one (Fig. 5 ). The extrapolation of the linear relationship suggests that
fulvic acid contributes approximately 45-63 % of the total DOM in Kago (KG)
upstream, approximately 53 % in Nishi-Mataya (NM) upstream and approx-
imately 73 % in the downstream waters of the Yasu River (YR) (Mostofa et al.
2007a ). The fulvic acid contribution in Japanese rivers estimated from DOC
and fulvic acid-like FI is similar to that reported in other studies of river water
(40-80 %) (Mostofa et al. 2009a ; Malcolm 1985 ; Peuravuori and Pihlaja 1999 ;
Mostofa 2005 ). The higher presence of allochthonous fulvic acid and humic acids
in river water is responsible for the good correlations with DOM found in natu-
ral waters (Mostofa et al. 2005a ; Fu et al. 2007 , 2010 ; Westerhoff et al. 2001 ).
A gradual decrease in fulvic acid-like FI with decreasing DOC concentration
(Fig. 5 ) can be explained if losses in FI and DOC are mostly involving the fluoro-
phores in fulvic acid. The latter consist of some repeating functional groups with
highly variable composition, having aromatic rings and highly unsaturated aliphat-
ics with extensive π -electron systems (Mostofa et al. 2009b , 2010 ; Senesi 1990a ;
Malcolm 1985 ; Corin et al. 1996 ; Wu et al. 2005 ). The fluorophores present in
Fig. 5 Changes in DOC concentration and fulvic acid (FA)-like fluorescence intensity (FI) dur-
ing photodegradation of DOM modeled by the first-order kinetics. Initial DOC concentration and
FI are presented by the open circle (the Nishi-Mataya, NM upstream); open diamond (the Kago,
KG upstream); and open triangle (Yasu River, YR downstream) for their respective samples col-
lected from Lake Biwa watershed, Japan. Data source Mostofa et al. ( 2007a )
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