385
Colored and Chromophoric Dissolved Organic Matter in Natural Waters
under snow-free sea ice (Uusikivi et al. 2010). Note that transmittance, T(λ), is
the ratio of the downwelling irradiance at the lower surface of the ice, E d (z ice , λ),
to the incident irradiance, E d (0, λ) (Perovich 1993). The transmittance depends
on the spectral reflection coefficient, α(λ)—that is, the fraction of E d (0, λ) that
is reflected—and on the attenuation of irradiance by snow and ice, according to
Beer’s Law.
4 Factors Affecting Absorption of Radiation by CDOM
CDOM absorption differs considerably in a variety of natural waters and depends
on several factors, which can be distinguished as follows: (i) Contents and molecular nature of CDOM; (ii) Occurrence and type of sediments; (iii) Photoinduced
degradation; (iv) Microbial degradation; and (v) Salinity.
4.1.1 Contents and Molecular Nature of DOM
The CDOM absorption depends on total DOM contents and on its molecular
nature (Fig. 7) (Vodacek et al. 1997; Ishiwatari 1973; Lawrence 1980; Zepp and
Schlotzhauer 1981; Hayase and Tsubota 1985; McKnight et al. 1994; Dubach
et al. 1964; del Vecchio and Blough 2004; Belzile et al. 2002; Vincent et al. 1998;
Pienitz and Vincent 2000). DOC contents are very much correlated with CDOM
absorption (a CDOM ) in natural waters, with the exception of surface waters during the summer stratification period (Rochelle-Newall and Fisher 2002; del
Vecchio and Blough 2004; Vodacek et al. 1995; Ferrari et al. 1996; Ferrari 2000;
Klinkhammer et al. 2000; Chen et al. 2002; Kowalczuk et al. 2010). The results
typically suggest that the CDOM fraction often increases linearly with the DOC
content, whilst the non-CDOM fraction of DOC remains relatively constant at
approximately 50–100 μM C (Fig. 7). It has been shown that humic-like CDOM
components with excitation maxima at longer wavelengths have significantly
higher non-absorbing DOC compared to humic-like CDOM components with
excitation maxima at shorter wavelengths (Kowalczuk et al. 2010). The relationship between the DOC concentration and the intensity of one of the protein-like
components can result in significantly reduced non-absorbing DOC fraction
(Kowalczuk et al. 2010). This study suggests that the relative proportion of humiclike CDOM components (characterized by excitation maximum at longer wavelengths) and the main protein-like component have the highest impact on the
absorption at 350 nm (Kowalczuk et al. 2010). Moreover, two phenomena are
responsible for the observed differences in CDOM absorption in surface waters.
First, CDOM properties (chromophores in CDOM) are significantly altered by
exposure to natural sunlight in surface waters, which reduces the CDOM absorption. Second, new CDOM is produced from algal or phytoplankton biomass under
Colored and Chromophoric Dissolved Organic Matter in Natural Waters
under snow-free sea ice (Uusikivi et al. 2010). Note that transmittance, T(λ), is
the ratio of the downwelling irradiance at the lower surface of the ice, E d (z ice , λ),
to the incident irradiance, E d (0, λ) (Perovich 1993). The transmittance depends
on the spectral reflection coefficient, α(λ)—that is, the fraction of E d (0, λ) that
is reflected—and on the attenuation of irradiance by snow and ice, according to
Beer’s Law.
4 Factors Affecting Absorption of Radiation by CDOM
CDOM absorption differs considerably in a variety of natural waters and depends
on several factors, which can be distinguished as follows: (i) Contents and molecular nature of CDOM; (ii) Occurrence and type of sediments; (iii) Photoinduced
degradation; (iv) Microbial degradation; and (v) Salinity.
4.1.1 Contents and Molecular Nature of DOM
The CDOM absorption depends on total DOM contents and on its molecular
nature (Fig. 7) (Vodacek et al. 1997; Ishiwatari 1973; Lawrence 1980; Zepp and
Schlotzhauer 1981; Hayase and Tsubota 1985; McKnight et al. 1994; Dubach
et al. 1964; del Vecchio and Blough 2004; Belzile et al. 2002; Vincent et al. 1998;
Pienitz and Vincent 2000). DOC contents are very much correlated with CDOM
absorption (a CDOM ) in natural waters, with the exception of surface waters during the summer stratification period (Rochelle-Newall and Fisher 2002; del
Vecchio and Blough 2004; Vodacek et al. 1995; Ferrari et al. 1996; Ferrari 2000;
Klinkhammer et al. 2000; Chen et al. 2002; Kowalczuk et al. 2010). The results
typically suggest that the CDOM fraction often increases linearly with the DOC
content, whilst the non-CDOM fraction of DOC remains relatively constant at
approximately 50–100 μM C (Fig. 7). It has been shown that humic-like CDOM
components with excitation maxima at longer wavelengths have significantly
higher non-absorbing DOC compared to humic-like CDOM components with
excitation maxima at shorter wavelengths (Kowalczuk et al. 2010). The relationship between the DOC concentration and the intensity of one of the protein-like
components can result in significantly reduced non-absorbing DOC fraction
(Kowalczuk et al. 2010). This study suggests that the relative proportion of humiclike CDOM components (characterized by excitation maximum at longer wavelengths) and the main protein-like component have the highest impact on the
absorption at 350 nm (Kowalczuk et al. 2010). Moreover, two phenomena are
responsible for the observed differences in CDOM absorption in surface waters.
First, CDOM properties (chromophores in CDOM) are significantly altered by
exposure to natural sunlight in surface waters, which reduces the CDOM absorption. Second, new CDOM is produced from algal or phytoplankton biomass under