Environmental Engineering Reference
In-Depth Information
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.
1
a-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.
1
d-f) (Mostofa
et al.
2005a
,
b
,
2007
; Moran et al.
2000
). It can be noted that photoinduced degra-
dation 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:
λ
MAX
(2.1)
PM
Z
=
ϕ
λ
Q
S,Z,
λ
A
CDOM,
λ
D
λ
λ
MIN
where
ϕ
λ
is the spectrum of the apparent quantum yield for photoinduced min-
eralization (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 photoin-
duced 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:
λ
MAX
(2.2)
PM
=
ϕλ
Q
A
,
λ
(
A
CDOM,
λ
/
A
TOT,
λ
)
D
λ
λ
MIN
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
).
