Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 20

4.1 Physical and Chemical Properties of DOM
Naturally-originated organic compounds such as humic substances (fulvic and
humic acids) of terrestrial plant origin, autochthonous DOM of algal or phyto-
plankton origin, proteins, amino acids, peptides and polysaccharides exhibit,
to varying degrees, several major properties (Mostofa et al. 2009a , b ; Malcolm
1985 ; Xue et al. 1995 ; Mandal et al. 1999 ; Filella 2008 ). They are: (i) physically
heterogeneous; (ii) polyfunctional, due to the existence of a variety of functional
groups and the presence of a broad range of functional reactivity; (iii) polyelec-
trolytical, with high electric charge density due to the presence of a large number
of dissociated functional groups; (iv) structurally labile, because of their capacity
to associate intermolecularly and to change molecular conformation in response
to changes in pH, pE, ionic strength, trace metal binding, and so on; (v) polydis-
perse in size.
Water Color:
The yellow color in natural waters is due to the occurrence of humic sub-
stances (fulvic and humic acid) and of autochthonous fulvic acids (C-
and M-like) of algal or phytoplankton origin, which absorb light in the blue
and ultraviolet (Kalle 1966 ; Jerlov 1968 ). These substances were formerly
referred to collectively as yellow substances or gelbstoff (Kirk 1976 ; Kalle
1966 ). Water color is generally related to the occurrence and contents of these
substances in natural waters (Eloranta 1978 ; Jones and Arvola 1984 ). Ocean
color is an important feature of water that was recently determined using
remote sensing applications (Hopkinson et al. 2002 ; Morel et al. 2007 ; Morel
and Gentili 2009 ; Van der Woerd et al. 2011 ; Volpe et al. 2011 ; Son et al.
2011 ). It is mostly due to the effect of autochthonous fulvic acids of algal or
phytoplankton origin as well as partly to allochthonous fulvic and humic acids
(humic substances). A recent study has shown that autochthonous fulvic acids
(C-like and M-like) of lake algal origin under dark incubation can exhibit yel-
low color (Mostofa et al. 2009b ). Note that autochthonous fulvic acids (C-like
and M-like) are characterized based on their similar fluorescence properties
to allochthonous fulvic acids (C-like and M-like), which will be discussed
in detail in the FDOM chapter (see chapter “ Fluorescent Dissolved Organic
Matter in Natural Waters ”).
Attenuation of Spectral UV Irradiance
DOM is the key factor that controls the downward irradiance flux through the
water column of UV-B (280-320 nm), UV-A (320-400 nm), total UV (280-
400 nm) and photosynthetically available radiation (PAR, 400-700 nm) (Kirk
1976 ; Morris et al. 1995 ; Siegel and Michaels 1996 ; Morris and Hargreaves 1997 ;
Tranvik 1998 ; Bertilsson and Tranvik 2000 ; Laurion et al. 2000 ; Markager and
Vincent 2000 ; Huovinen et al. 2003 ; Sommaruga and Augustin 2006 ; Hayakawa