Environmental Engineering Reference
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The decrease in CDOM absorption is relatively lower in the waters of Bay and
oceans (Blough and del Vecchio 2002 ; del Vecchio and Blough 2002 .
Generally speaking, CDOM absorption losses in lakes, estuaries, Bay and
oceans are significantly lower than for upstream CDOM. The main reason might
be the autochthonous sources of most of the CDOM in these waters, which make
the corresponding CDOM less susceptible to photoinduced degradation. Indeed,
the fraction of autochthonous CDOM is maximum (25-98 %) in lakes and oceans,
whilst allochthonous humic substances (mostly fulvic acids) are 2-75 % (see
also chapter Dissolved Organic Matter in Natural Waters ”) (Mostofa et al. 2009 ;
McCarthy et al. 1996 ; Biddanda and Benner 1997 ; Moran et al. 1991 ; Moran and
Hodson 1994 ; Benner and Kaiser 2003 ). In addition, the amount of allochthonous
CDOM (mostly fulvic and humic acids) is considerably decreased in the transport
from rivers to lakes and oceans because of photoinduced decomposition by natural
sunlight (Vähätalo and Wetzel 2004 ; Vodacek et al. 1997 ; Mopper et al. 1991 ;
Wetzel et al. 1995 ; Moran et al. 2000 ; Skoog et al. 1996 ; Mostofa et al. 2007 ;
Bertilsson and Tranvik 2000 ; Amon and Benner 1996 ; Twardowski and Donaghay
2002 ; Waiser and Robarts 2004 ; Wu et al. 2005 ; Brooks et al. 2007 ). The experi-
mental results demonstrate the photoreactive nature of CDOM, with half-lives from
2.1 to 5.1 days due to photobleaching in the upper layer and duplication times from
4.9 to 15.7 days due to photohumification. Such results highlight the highly dynamic
nature of CDOM in the Southern Ocean (Ortega-Retuerta et al. 2010 ). In addition,
the high susceptibility to photobleaching of CDOM in Antarctic ice waters might be
the effect of the presence of fresh CDOM in bulk ice samples, due to elevated in situ
production (Norman et al. 2011 ). The fresh CDOM in Antarctic ice waters is char-
acterized by low S and high a 375 . In contrast, aged material present in brine and sea-
water samples is characterised by high S values and low a 375 (Norman et al. 2011 ).
Therefore, photoinduced degradation is one of the key factors that can regulate the
CDOM absorption depending on its composition for a variety of natural waters.
4.3.3 Changes in Spectral Slope Due to Photoinduced Degradation
Photoinduced degradation can alter the spectral slope of CDOM ( S ) either
in natural surface waters or in experimental observations under solar irradia-
tion (Fig. 8 ) (Vodacek et al. 1997 ; Helms et al. 2008 ; Zhang et al. 2009 ; Shank
et al. 2010 ; Moran et al. 2000 ; Zhang and Qin 2007 ; del Vecchio and Blough
2002 ; Mostofa KMG et al., unpublished data; Xie et al. 2004 ; Twardowski and
Donaghay 2002 ; Tzortziou et al. 2007 ; Whitehead et al. 2000 ). Two key phenom-
ena are generally observed: the first one is an increase of S because of CDOM
photobleaching by solar radiation (Fig. 8 ) (Helms et al. 2008 ; Zhang et al. 2009 ;
Shank et al. 2010 ; Moran et al. 2000 ; del Vecchio and Blough 2002 ; Xie et al.
2004 ; Twardowski and Donaghay 2002 ; Whitehead et al. 2000 ). It is suggested
that photobleaching can be caused by the transformation of high-molecular weight
CDOM complexes that absorb at longer wavelengths into smaller complexes that
absorb at shorter wavelengths. The opposite effect can also be observed: in some
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