Colored and Chromophoric Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 376

and Hargreaves 1997 ; Tranvik and Kokalj 1998 ; Bertilsson and Tranvik 2000 ).
Post bloom increases in DOM concentration from phytoplankton biomass cause
increased DOM absorption, which in turn decreases the UV transmission through
the water column (Whitehead and Vernet 2000 ). Mineralization of DOM by pho-
toinduced and microbial processes can decompose the chromophores in CDOM,
thereby reducing the absorption of UV and visible radiation. The consequence is
an increase of UV transparency of surface waters (Nelson et al. 1998 ; Vodacek
et al. 1997 ; Kieber et al. 1990 ; Moran et al. 2000 ; Skoog et al. 1996 ; Reche et al.
1999 ; Whitehead and Vernet 2000 ; Twardowski and Donaghay 2001 ; del Vecchio
and Blough 2002 ; Mostofa et al. 2007 ; Mostofa et al. 2007 ; Patsayeva et al.
1991 ; Kouassi and Zika 1990 ; Kouassi et al. 1990 ; Morris and Hargreaves 1997 ;
Allard et al. 1994 ; Zepp et al. 1995 ). CDOM accounts on average for 17-98 %
of the total light attenuation coefficient in lake, estuarine and coastal seawaters
(Odriozola et al. 2007 ; Lund-Hansen 2004 ; Obrador and Pretus 2008 ; Effler et al.
2010 ). CDOM shows variable and important contributions in summer (10-90 %)
along the Patagonia shelf-break front (Ferreira et al. 2009 ). Absorption by CDOM
and water together contributes to 88-94 % of UV radiation attenuation in tributar-
ies, but only 37-77 % in lakes (Smith et al. 2004 ). These studies show that light
attenuation by CDOM significantly depends on the occurrence of suspended par-
ticulate matter and phytoplankton with high contents of Chl a and water. The total
content of DOC could be a useful estimate of UV transparency in natural surface
waters.
3.1.2 Nature and Molecular Weight Fractions of CDOM
The absorption and scattering coefficients for UV and PAR are significantly
dependent on the chemical nature and variability of organic compounds
(Ishiwatari 1973 ; Lawrence 1980 ; Zepp and Schlotzhauer 1981 ; Hayase and
Tsubota 1985 ; McKnight et al. 1994 ; Dubach et al. 1964 ; Zanardi-Lamardo et
al. 2004 ; Singh et al. 2010 ). Fulvic and humic acids are typically a mixture of
organic compounds with molecular weights ranging from <100 to >300,000
Daltons in natural waters (Hayase and Tsubota 1985 ; Ghassemi and Christman
1968 ; Thurman 1985 ; Ma and Ali 2009 ). Absorption coefficients of allochtho-
nous fulvic and humic acids are significantly dependent on the molecular weight
fractions (Fig. 3 ) (Ishiwatari 1973 ; Lawrence 1980 ; Zepp and Schlotzhauer
1981 ; Hayase and Tsubota 1985 ; Dubach et al. 1964 ; Ghassemi and Christman
1968 ). In addition, autochthonous CDOM of algal origin showed efficient absorp-
tion over the entire spectrum at 250-700 nm during 33 days of dark incubation
(Fig. 4 ) (Zhang et al. 2009 ). This suggests that autochthonous CDOM has a simi-
lar absorption pattern as allochthonous fulvic acids. The absorption coefficients
(l/g/cm) of fulvic acid extracted from sediment pore waters are much higher at
320 nm than at 480 nm in all the molecular weight fractions. However, absorption
is low in low molecular weight fractions (<10 kDa) and generally increases with
increasing molecular weight up to >300 kDa (Fig. 3 ) (Hayase and Tsubota 1985 ).