280
K. M. G. Mostofa et al.
Cooper et al. 1989; Senesi 1990). This can subsequently lead to the decomposition
of those functional groups in DOM, thereby causing either losses of absorbance in
the UV and visible wavelength regions(Fig. 1a–c) (Vahatalo et al. 2000; Vähätalo
and Wetzel 2004; del Vecchio and Blough 2002; Blough and del Vecchio 2002) or
losses in fluorescence intensity of FDOM in natural waters (Fig. 1d–f) (Mostofa
et al. 2005a, b, 2007; Moran et al. 2000). It can be noted that photoinduced degradation is generally occurring in the mixing zone and decreases with an increase in
water depth in natural waters (Vahatalo et al. 2000; Graneli et al. 1996; Mostofa
et al. 2005; Bertilsson and Tranvik 2000). Photoinduced degradation can reduce
the mean molecular size of the high molecular weight DOM (Moran and Zepp
1997; Yoshioka et al. 2007; Amador et al. 1989; Amon and Benner 1994), which
subsequently produces low molecular weight (LMW) intermediate substances
(Moran and Zepp 1997; Dahlén et al. 1996; Bertilsson and Tranvik 1998; Mopper
et al. 1991). This process ultimately ends up in mineralization with formation of
e.g. COS, CO, CO 2 , DIC, ammonium, gaseous hydrocarbons and so on in natural
waters (Moran and Zepp 1997; Ma and Green 2004; Gao and Zepp 1998; Graneli
et al. 1996, 1998; Clark et al. 2004; Xie et al. 2004; Borges et al. 2008; Kujawinski
et al. 2009; Tranvik et al. 2009; Omar et al. 2010; Ballaré et al. 2011; Zepp et al.
2011; Mopper et al. 1991; Miller and Zepp 1995; Bertilsson and Tranvik 2000;
Chen et al. 1978; Fujiwara et al. 1995; Bushaw et al. 1996; Miller and Moran 1997;
Stiller and Nissenbaum 1999; White et al. 2010; Cai 2011).
The rate of photoinduced mineralization of DOM at the depth z (pm z , mol C
m −3 d −1 ), modified by Vähätalo et al. (2000) from Schwarzenbach 1993) and
Miller (1998), can be expressed as follows:
where ϕ λ is the spectrum of the apparent quantum yield for photoinduced mineralization (mol produced DIC/mol absorbed photons), Q s,z,λ is the scalar photon
flux density spectrum at a depth z (also referred to as actinic flux, mol photons
m −2 d −1 ), and a CDOM,λ is the absorption spectrum of CDOM (m −1 ). CDOM or
FDOM is the part of DOM that can absorb solar radiation. The parameters λ max
and λ min are the minimum and maximum wavelengths contributing to photoinduced mineralization.
In the whole water column the rate of photoinduced mineralization, modified
by Vähätalo et al. (2000) from Miller (1998), can be expressed as follows:
where Q a,λ represents the photons absorbed by the water column (mol photons m −2
d −1 ) and the a CDOM,λ /a tot,λ ratio expresses how much CDOM contributes to the
total absorption. In infinitely deep waters, Q a,λ roughly equals the downward vector
photon flux density just below the surface Q d,v,0−λ , (Sikorski and Zika 1993a, b).
(2.1)
pm z =
λ max
λ min
ϕ λ Q s,z,λ a CDOM,λ dλ
(2.2)
pm =
λ max
λ min
ϕλQa ,λ (a CDOM,λ /a tot,λ ) dλ
K. M. G. Mostofa et al.
Cooper et al. 1989; Senesi 1990). This can subsequently lead to the decomposition
of those functional groups in DOM, thereby causing either losses of absorbance in
the UV and visible wavelength regions(Fig. 1a–c) (Vahatalo et al. 2000; Vähätalo
and Wetzel 2004; del Vecchio and Blough 2002; Blough and del Vecchio 2002) or
losses in fluorescence intensity of FDOM in natural waters (Fig. 1d–f) (Mostofa
et al. 2005a, b, 2007; Moran et al. 2000). It can be noted that photoinduced degradation is generally occurring in the mixing zone and decreases with an increase in
water depth in natural waters (Vahatalo et al. 2000; Graneli et al. 1996; Mostofa
et al. 2005; Bertilsson and Tranvik 2000). Photoinduced degradation can reduce
the mean molecular size of the high molecular weight DOM (Moran and Zepp
1997; Yoshioka et al. 2007; Amador et al. 1989; Amon and Benner 1994), which
subsequently produces low molecular weight (LMW) intermediate substances
(Moran and Zepp 1997; Dahlén et al. 1996; Bertilsson and Tranvik 1998; Mopper
et al. 1991). This process ultimately ends up in mineralization with formation of
e.g. COS, CO, CO 2 , DIC, ammonium, gaseous hydrocarbons and so on in natural
waters (Moran and Zepp 1997; Ma and Green 2004; Gao and Zepp 1998; Graneli
et al. 1996, 1998; Clark et al. 2004; Xie et al. 2004; Borges et al. 2008; Kujawinski
et al. 2009; Tranvik et al. 2009; Omar et al. 2010; Ballaré et al. 2011; Zepp et al.
2011; Mopper et al. 1991; Miller and Zepp 1995; Bertilsson and Tranvik 2000;
Chen et al. 1978; Fujiwara et al. 1995; Bushaw et al. 1996; Miller and Moran 1997;
Stiller and Nissenbaum 1999; White et al. 2010; Cai 2011).
The rate of photoinduced mineralization of DOM at the depth z (pm z , mol C
m −3 d −1 ), modified by Vähätalo et al. (2000) from Schwarzenbach 1993) and
Miller (1998), can be expressed as follows:
where ϕ λ is the spectrum of the apparent quantum yield for photoinduced mineralization (mol produced DIC/mol absorbed photons), Q s,z,λ is the scalar photon
flux density spectrum at a depth z (also referred to as actinic flux, mol photons
m −2 d −1 ), and a CDOM,λ is the absorption spectrum of CDOM (m −1 ). CDOM or
FDOM is the part of DOM that can absorb solar radiation. The parameters λ max
and λ min are the minimum and maximum wavelengths contributing to photoinduced mineralization.
In the whole water column the rate of photoinduced mineralization, modified
by Vähätalo et al. (2000) from Miller (1998), can be expressed as follows:
where Q a,λ represents the photons absorbed by the water column (mol photons m −2
d −1 ) and the a CDOM,λ /a tot,λ ratio expresses how much CDOM contributes to the
total absorption. In infinitely deep waters, Q a,λ roughly equals the downward vector
photon flux density just below the surface Q d,v,0−λ , (Sikorski and Zika 1993a, b).
(2.1)
pm z =
λ max
λ min
ϕ λ Q s,z,λ a CDOM,λ dλ
(2.2)
pm =
λ max
λ min
ϕλQa ,λ (a CDOM,λ /a tot,λ ) dλ