514
K. M. G. Mostofa et al.
substances of 19–67 % during 72 h to 70 days irradiation (Table 4) (Moran et al.
2000; Osburn et al. 2009). In unfiltered seawater samples from the Baltic Sea irradiated for 4–5 days, the corresponding fluorescence intensity decrease has been of
44–52 % at the surface (0–50 m) and 56–65 % in the deeper layers (100–240 m).
In some cases the decrease has been more marked, i.e. 61–70 % at the surface
(0–50 m) and 73-75 % in the deeper layer (100–190 m). Interestingly the addition of chloroform significantly enhanced photodegradation, yielding a fulvic
acid-like fluorescence decrease of 59–81 % in surface samples (0–50 m) and of
83–84 % in deep-water ones (Table 4) (Skoog et al. 1996). The mechanism behind
the increased FDOM photodegradation upon addition of chloroform may be the
production of phosgene in the presence of O 2 (CHCl 3 + O 2 + hυ → COCl 2 + H
Cl). Phosgene is highly reactive toward the degradation of the fluorophores, such
as the amino groups (RNH 2 + COCl 2 → RN = CO + 2HCl) or carboxylic acids
(RCO 2 H + COCl 2 → RC(O)Cl + HCl + CO 2 ) (Mostofa et al. 2011; Shriner et al.
1943). Such processes would contribute to the decrease of DOM fluorescence in
natural waters (Mostofa et al. 2009a, 2011).
Photoinduced degradation of Mediterranean Sea samples (8 h sunlight exposure) showed a decrease in the fluorescence of fulvic acid-like or humic-like
fluorophores (peak C), in the range of 9–22 % for lagoon water and approaching
34 % for coastal water (Table 4) (Abboudi et al. 2008). Similarly, photoinduced
degradation of waters collected from Mackenzie River and Beaufort Sea (Estuary,
Shelf and Gulf) demonstrates that the degradation of fulvic acid-like fluorophores
(peak C) is usually higher during summer irradiation than in spring, autumn and
winter (Table 4). The photodegradation of fulvic acid-like fluorophore (peak C)
is relatively higher in Beaufort Sea samples (47–60 % in Estuary during summer;
67–75 % in Shelf during summer; 66 % in Gulf during spring; 72 h irradiation)
than in Satilla Estuary (61–67 %, 70 days), Baltic Sea (44–52 % in surface waters,
4–5 days), and Gotland Deep seawater (32 %, 13 days) (Table 4) (Stedmon et al.
2007a; Skoog et al. 1996; Moran et al. 2000; Osburn et al. 2009; Lepane et al.
2003). The high photodegradation of fulvic acid-like substances in Beaufort Sea
samples has been explained by the occurrence of two phenomena. Firstly, in many
cases a significant fraction of the fulvic acid-like substances are of autochthonous origin, which makes them highly susceptible to photodegradation (Mostofa
KMG et al., unpublished data; Johannessen et al. 2007). Secondly, in the case of
the Beaufort Sea the fulvic acid-like substances have allochthonous origin as they
mainly derive from riverine input. Photoinduced degradation of these compounds
is poorly effective due to low water temperature in the Beaufort Sea (−0.54 to
21.81 °C in Estuary, −1.36 to 9.23 °C in Shelf, and −1.68 to 0.12 °C in Gulf
samples) (Osburn et al. 2009). Therefore, unaffected allochthonous fulvic acid-like
substances are highly susceptible to degradation upon laboratory irradiation. The
case of the Beaufort Sea may be a particular one, however, because it has been
reported that DOM (or FDOM) components are produced from microbial assimilation of phytoplankton biomass or organic matter in natural waters (Mostofa
et al. 2009a; Parlanti et al. 2000; Stedmon et al. 2007a, 2007b; Fu et al. 2010;
Rochelle-Newall and Fisher 2002; Yamashita and Tanoue 2004; Rochelle-Newall
K. M. G. Mostofa et al.
substances of 19–67 % during 72 h to 70 days irradiation (Table 4) (Moran et al.
2000; Osburn et al. 2009). In unfiltered seawater samples from the Baltic Sea irradiated for 4–5 days, the corresponding fluorescence intensity decrease has been of
44–52 % at the surface (0–50 m) and 56–65 % in the deeper layers (100–240 m).
In some cases the decrease has been more marked, i.e. 61–70 % at the surface
(0–50 m) and 73-75 % in the deeper layer (100–190 m). Interestingly the addition of chloroform significantly enhanced photodegradation, yielding a fulvic
acid-like fluorescence decrease of 59–81 % in surface samples (0–50 m) and of
83–84 % in deep-water ones (Table 4) (Skoog et al. 1996). The mechanism behind
the increased FDOM photodegradation upon addition of chloroform may be the
production of phosgene in the presence of O 2 (CHCl 3 + O 2 + hυ → COCl 2 + H
Cl). Phosgene is highly reactive toward the degradation of the fluorophores, such
as the amino groups (RNH 2 + COCl 2 → RN = CO + 2HCl) or carboxylic acids
(RCO 2 H + COCl 2 → RC(O)Cl + HCl + CO 2 ) (Mostofa et al. 2011; Shriner et al.
1943). Such processes would contribute to the decrease of DOM fluorescence in
natural waters (Mostofa et al. 2009a, 2011).
Photoinduced degradation of Mediterranean Sea samples (8 h sunlight exposure) showed a decrease in the fluorescence of fulvic acid-like or humic-like
fluorophores (peak C), in the range of 9–22 % for lagoon water and approaching
34 % for coastal water (Table 4) (Abboudi et al. 2008). Similarly, photoinduced
degradation of waters collected from Mackenzie River and Beaufort Sea (Estuary,
Shelf and Gulf) demonstrates that the degradation of fulvic acid-like fluorophores
(peak C) is usually higher during summer irradiation than in spring, autumn and
winter (Table 4). The photodegradation of fulvic acid-like fluorophore (peak C)
is relatively higher in Beaufort Sea samples (47–60 % in Estuary during summer;
67–75 % in Shelf during summer; 66 % in Gulf during spring; 72 h irradiation)
than in Satilla Estuary (61–67 %, 70 days), Baltic Sea (44–52 % in surface waters,
4–5 days), and Gotland Deep seawater (32 %, 13 days) (Table 4) (Stedmon et al.
2007a; Skoog et al. 1996; Moran et al. 2000; Osburn et al. 2009; Lepane et al.
2003). The high photodegradation of fulvic acid-like substances in Beaufort Sea
samples has been explained by the occurrence of two phenomena. Firstly, in many
cases a significant fraction of the fulvic acid-like substances are of autochthonous origin, which makes them highly susceptible to photodegradation (Mostofa
KMG et al., unpublished data; Johannessen et al. 2007). Secondly, in the case of
the Beaufort Sea the fulvic acid-like substances have allochthonous origin as they
mainly derive from riverine input. Photoinduced degradation of these compounds
is poorly effective due to low water temperature in the Beaufort Sea (−0.54 to
21.81 °C in Estuary, −1.36 to 9.23 °C in Shelf, and −1.68 to 0.12 °C in Gulf
samples) (Osburn et al. 2009). Therefore, unaffected allochthonous fulvic acid-like
substances are highly susceptible to degradation upon laboratory irradiation. The
case of the Beaufort Sea may be a particular one, however, because it has been
reported that DOM (or FDOM) components are produced from microbial assimilation of phytoplankton biomass or organic matter in natural waters (Mostofa
et al. 2009a; Parlanti et al. 2000; Stedmon et al. 2007a, 2007b; Fu et al. 2010;
Rochelle-Newall and Fisher 2002; Yamashita and Tanoue 2004; Rochelle-Newall