Colored and Chromophoric Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 397

(del Vecchio and Blough 2004 ; Twardowski and Donaghay 2001 ; del Castillo et al.
1999 ; Uher et al. 2001 ; Blough et al. 1993 ; Sholkovitz 1976 ). In addition, a nonlin-
ear dependence may result from the conservative mixing of multiple water masses
containing variable CDOM (del Vecchio and Blough 2004 ; Blough and del Vecchio
2002 ; Hujerslev et al. 1996 ; Chen et al. 2007 ; Blough et al. 1993 ). Photoinduced
degradation can greatly decrease the CDOM absorbance in intermediate- to high-
salinity surface waters under stratified conditions during the summer period (Vodacek
et al. 1997 ; del Vecchio and Blough 2004 ; Chen et al. 2007 ; Osburn and Morris 2003 ;
Osburn et al. 2009 ). Dissolved lignin phenols are significantly affected by salinity
and two key phenomena are generally detected: First, a nonconservative decrease in
dissolved high molecular weight (HMW) lignin phenols at salinity <25 psu is likely
due to flocculation and microbial degradation. In contrast, LMW dissolved lignin
phenols mix conservatively (Hernes and Benner 2003 ). Second, at salinity >25 psu
photooxidation is a dominant factor influencing lignin composition and concentration
(Hernes and Benner 2003 ).
CDOM photoreactivity can increase with salinity across an estuarine gradient.
Shortwave CDOM absorption loss (e.g. at 280 nm) does not change with salin-
ity, but longwave CDOM absorption loss (e.g. at 440 nm) is often decreased by
10-40 % with increasing salinity (Osburn and Morris 2003 ; Osburn et al. 2009 ).
In another study, a decrease in CDOM photobleaching at 280 nm is detected when
humic CDOM is added to an artificial salinity gradient used to mimic coastal mix-
ing (Minor et al. 2006 ). The decrease of the absorption properties of CDOM with
salinity can be accounted for by several factors: (i) Mixing of CDOM-rich river-
ine water with CDOM-poor coastal water (del Vecchio and Blough 2004 ; Gonsior
et al. 2008 ; Blough et al. 1993 ); (ii) Photodegradation of chromophores present
in riverine CDOM after they reach the coastal regions during the summer strati-
fication period (Vodacek et al. 1997 ; Moran et al. 2000 ; del Vecchio and Blough
2004 ; Blough and del Vecchio 2002 ; Whitehead and Vernet 2000 ; del Vecchio and
Blough 2002 ; Osburn et al. 2009 ); (iii) Microbial degradation, in particular of the
autochthonous fraction that is the major part of CDOM in marine waters (Table 1 )
(Moran et al. 2000 ; Winter et al. 2007 ; Moran and Hodson 1994 ; Brown 1977 ;
Opsahl and Benner 1998 ); (iv) Flocculation and precipitation of riverine CDOM
because of increased salinity (Blough et al. 1993 ; Sholkovitz 1976 ; Sieburth and
Jensen 1968 ; Fox 1991 ); and possibly (v) Enhanced CDOM photodegradation in
saline waters.
The mechanism behind the latter process apparently involves two factors:
first, irradiated CDOM can induce photoinduced production of hydrogen perox-
ide (H 2 O 2 ) that is a HO

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source via photolysis or the photo-Fenton reaction, and
the photoinduced generation of H 2 O 2 is enhanced by salinity. Trace metal ions
(M) in salinity or sea waters can complex with DOM (M-DOM) forming a strong
π -electron bonding system between metal ions and the functional groups in DOM
(see chapter “ Complexation of Dissolved Organic Matter with Trace Metal Ions
in Natural Waters ” for in details explanation). This π -electron in M-DOM com-
plex is rapidly excited photolytically, which is responsible for high production of
aqueous electrons (e aq -) and subsequently the high production of superoxide ion