Environmental Engineering Reference
In-Depth Information
of allochthonous humic substances (fulvic and humic acids), carbohydrates, amino
acids and so on (Mostofa et al.
2009
; Malcolm
1985
; Ittekkot et al.
1985
; Guéguen
et al.
2006
). The terrestrial run-off through rivers may bring the plant material to
many surface waters, such transport depending on precipitation and on the type and
density of terrestrial plants. On the other hand, algal biomass and microbes which
develop in the photic zone may release autochthonous fulvic acids, amino acids,
carbohydrates, proteins, fatty acids, peptides, organic acids and other compounds
(Zhang et al.
2009
; McCarthy et al.
1996
; Biddanda and Benner
1997
; Wakeham
et al.
1997
; Rosenstock et al.
2005
; Hama and Handa
1992
). Sometimes such a pro-
cess is photolytically enhanced. Major seasonal differences in the spectral slope val-
ues show that phytoplankton degradation is one of the important sources of CDOM
in summer, whereas in other seasons CDOM mainly reaches lake water from river
input (Zhang et al.
2009
; Zhang and Qin
2007
). At the same time, sediment-trap stud-
ies demonstrate that only 1-35 % of the organic carbon (viz. algae) synthesized in the
photic zone reaches the sediment surface in marine and lacustrine waters (Bernasconi
et al.
1997
; Hernes et al.
2001
; Lehmann et al.
2002
). Such algal material releases the
same autochthonous organic substances in pore waters as it does in the upper parts
of the water column (Burdige et al.
2004
; Li W et al., unpublished data). Some
DOM components from the pore-water sediment surface can mix up with surface
waters during the vertical mixing (overturn) period in lakes or oceans. Allochthonous
and autochthous DOM has very variable absorption properties (Zhang et al.
2009
;
Vodacek et al.
1997
; Zepp and Schlotzhauer
1981
; Hayase and Tsubota
1985
;
McKnight et al.
1994
; del Vecchio and Blough
2004
; Vodacek et al.
1995
; Belzile
et al.
2002
; Vincent et al.
1998
; Pienitz and Vincent
2000
). Therefore, DOM composi-
tion also depends on the occurrence and nature of sediments in natural waters.
4.3 Photoinduced Degradation of CDOM in Natural Waters
Photoinduced processes can decompose the chromophores in CDOM and thus decrease
the CDOM absorption (Vähätalo and Wetzel
2004
; Zhang et al.
2009
; Shank et al.
2010
; Moran et al.
2000
; Winter et al.
2007
; del Vecchio and Blough
2002
; Norman
et al.
2011
; Zagarese et al.
2001
). A theoretical model for the photoinduced degradation
of CDOM and its effects on absorption properties are discussed in this section.
4.3.1 Theoretical Model for Photoinduced Degradation of CDOM
The CDOM chromophores absorb photons and sometimes undergo degradation,
while DOM undergoes partial mineralization to hydrogen peroxide (H
2
O
2
), CO
2
,
DIC (sum of dissolved CO
2
, H
2
CO
3
, HCO
3
, and CO
3
2
−
), COS, CO, ammo-
nium, gaseous hydrocarbons, organic peroxides (ROOH), low molecular weight
(LMW) DOM, and so on in upper surface waters (Fig.
1
) (Vähätalo and Wetzel
2004
; Ma and Green
2004
; Moran and Zepp
1997
; Mostofa and Sakugawa
2009
;
−
