1.
U
01
E
CHAPTER 3 . Photooxidation of Dissolved Organic Matter
79
uptake to be about 1 x 10 14 moles yr- I • As a comparison, terrestrial organic carbon enters the ocean at about 3.4 x 1013 moles yr- I (Meybeck 1982; Sarmiento and Sundquist
1992). As there is more than sufficient photo oxidation taking place in the ocean to
oxidize all the terrestrial eDaM, photooxidation is probably the dominant removal
mechanism for this eDaM as well as a removal mechanism for some marine-derived
eDaM (Zafiriou 1994 pers. comm.).
In open ocean waters, organic matter from terrigenous sources represents only a
small fraction of the total organic matter. This fraction may be significantly different
in coastal waters and in fresh-water rivers and lakes. Analysis of lignin-derived phenols,
unique biomarkers of terrestrial plant carbon, indicated that terrestrially-derived carbon accounts for a major source of fresh-water DOC (Meyers-Shulte and Hedges 1986).
Furthermore, Herndl and co-workers (1997) have shown that in the oligotrophic northern Adriatic Sea, humic substances contribute between 10-15% to the total DOC pool,
while their contribution to the DOC pool range between 15-47% in the shallow Lake
Neusiedl, a lake surrounded by a reed belt of Phragmites australis (Fig. 3.3a). These
authors have also shown that mainly the humic fraction of the total DOC pool was
subject to photo oxidation (Fig. 3-3b), and that this fraction is efficiently altered upon
exposure to solar radiation, resulting in an increase in the 250/365 nm absorbance ratio. An increase in the 250/365 nm absorbance ratio means a transformation to less
colored compounds (see Eq. 3.1), i.e. to compounds with a lower fraction of aromatic
functional groups, which tend to be better bioavailable. In this study, the humic and
non-humic fraction of DOC was separated by XAD-extraction.
Hence, in coastal waters and in fresh-water rivers and lakes, photo oxidation of nonbioavailable humic substances may significantly increase the abundance of bioavailable
organic carbon. Furthermore, coastal waters and lakes often exhibit higher concentrations of iron due to larger riverine inputs (De Vitre et al. 1988; Davison 1993; Sigg et aI.
1991). Thus, questions arise about the roles of iron in the photo oxidation of eDaM, in
particular the following questions:
40
20
a
CJ NH
-
H
30
15
-0- DOC
. .
u
01
20
E 10
N
0
01
:110
5
0
0
VI
"- ~ 00 00 0- 0- 0
0
VI
"- ~ 00 00 0- 0- 0
0
vi 0 ;;; -0 ,...: -0 ,...: ,...: iii 0 ;;; -0 ,...: -0 ,...:
M
M
,...: ~
N
N
Date: 1995
Date: 1995
Fig. 3.3. a Contribution of the non-humic (NH) and the humic (H) fraction to the DOC pool in Lake
Neusiedl; b Photooxidation of the different DOC fractions from Lake Neusiedl as measured by O2 consumption (from Herndl et al. 1997)
b
,...;
N
U
01
E
CHAPTER 3 . Photooxidation of Dissolved Organic Matter
79
uptake to be about 1 x 10 14 moles yr- I • As a comparison, terrestrial organic carbon enters the ocean at about 3.4 x 1013 moles yr- I (Meybeck 1982; Sarmiento and Sundquist
1992). As there is more than sufficient photo oxidation taking place in the ocean to
oxidize all the terrestrial eDaM, photooxidation is probably the dominant removal
mechanism for this eDaM as well as a removal mechanism for some marine-derived
eDaM (Zafiriou 1994 pers. comm.).
In open ocean waters, organic matter from terrigenous sources represents only a
small fraction of the total organic matter. This fraction may be significantly different
in coastal waters and in fresh-water rivers and lakes. Analysis of lignin-derived phenols,
unique biomarkers of terrestrial plant carbon, indicated that terrestrially-derived carbon accounts for a major source of fresh-water DOC (Meyers-Shulte and Hedges 1986).
Furthermore, Herndl and co-workers (1997) have shown that in the oligotrophic northern Adriatic Sea, humic substances contribute between 10-15% to the total DOC pool,
while their contribution to the DOC pool range between 15-47% in the shallow Lake
Neusiedl, a lake surrounded by a reed belt of Phragmites australis (Fig. 3.3a). These
authors have also shown that mainly the humic fraction of the total DOC pool was
subject to photo oxidation (Fig. 3-3b), and that this fraction is efficiently altered upon
exposure to solar radiation, resulting in an increase in the 250/365 nm absorbance ratio. An increase in the 250/365 nm absorbance ratio means a transformation to less
colored compounds (see Eq. 3.1), i.e. to compounds with a lower fraction of aromatic
functional groups, which tend to be better bioavailable. In this study, the humic and
non-humic fraction of DOC was separated by XAD-extraction.
Hence, in coastal waters and in fresh-water rivers and lakes, photo oxidation of nonbioavailable humic substances may significantly increase the abundance of bioavailable
organic carbon. Furthermore, coastal waters and lakes often exhibit higher concentrations of iron due to larger riverine inputs (De Vitre et al. 1988; Davison 1993; Sigg et aI.
1991). Thus, questions arise about the roles of iron in the photo oxidation of eDaM, in
particular the following questions:
40
20
a
CJ NH
-
H
30
15
-0- DOC
. .
u
01
20
E 10
N
0
01
:110
5
0
0
VI
"- ~ 00 00 0- 0- 0
0
VI
"- ~ 00 00 0- 0- 0
0
vi 0 ;;; -0 ,...: -0 ,...: ,...: iii 0 ;;; -0 ,...: -0 ,...:
M
M
,...: ~
N
N
Date: 1995
Date: 1995
Fig. 3.3. a Contribution of the non-humic (NH) and the humic (H) fraction to the DOC pool in Lake
Neusiedl; b Photooxidation of the different DOC fractions from Lake Neusiedl as measured by O2 consumption (from Herndl et al. 1997)
b
,...;
N