Environmental Engineering Reference
In-Depth Information
2005
). The proportion of each species depends on pH: at high pH the reaction shifts
to the right hand side of (Eq.
5.8
) and
HCO
3
−
dominates at pH between 7 and 9,
approximately 95 % of the carbon in water. At pH > 10.5,
CO
3
2
−
predominates
(Dreybrodt
1988
). The equilibrium constants for this system are altered by the salinity
of the medium: the values for seawater are an order of magnitude higher than those of
freshwater toward the right-hand-side of the reaction (Raven et al.
2002
; Millero and
Roy
1997
).
It is well known that the stable carbon isotope composition (
δ
13
C value) of
organic matter, produced either by phytoplankton or terrestrial plants during
photosynthesis, is significantly varied depending on the taxon-specific photo-
synthetic pathways (such as C
3
, C
4
, and crassulacean acid metabolism, CAM).
It also varies depending on: variety of phytoplankton; diffusion of CO
2
; incor-
poration of CO
2
by phosphoenolpyruvate carboxylase or Ribulose Bisphosphate
Carboxylase-Oxygenase (Rubisco), and respiration; sources and interconversion of
CO
2
and HCO
3
−
(depending on a variety of environmental conditions including
light intensity, temperature, DOM and POM contents, water depth, atmospheric
CO
2
concentration and so on) (O'Leary
1981
; Cooper and McRoy
1988
; Farquhar
et al.
1989
; Raven and Farquhar
1990
; Yoshioka
1997
; Raven et al.
2002
; Hu
et al.
2012
). Note that the
δ
13
C values of [CO
2
]
aq
and DIC are
−
16.5 to
−
14.5 ‰
and
−
7.4 to
−
4.5 ‰, respectively (Yoshioka
1997
). The values of
δ
13
C of organic
matter in marine macroalgae and seagrass collected from the natural environment
can vary from -2.7 ‰ to -35.3 ‰ (Raven et al.
2002
; Hu et al.
2012
; Beardall
2003
; Hemminga and Mateo
1996
; Raven
1997
; Dunton
2001
). Plants with C
4
characteristics show
δ
13
C values of
−
6 to
−
19 ‰ whilst plants with C
3
character-
istics exhibit
δ
13
C values of
−
24 to
−
34 ‰ (Smith and Epstein
1971
).
Such variation in the
δ
13
C value can be caused by (Farquhar et al.
1989
; Raven
and Farquhar
1990
): (i) the isotope fractionation factor (
α
), which is the ratio of
the reaction rates of
12
CO
2
AND
13
CO
2
with Rubisco (
α
=
1.029 for gaseous CO
2
and
α
=
1.030 for dissolved CO
2
); (ii) the relative contribution of phospho-
enolpyruvate carboxylase (PEPC) activity to the photosynthetic carbon assimila-
tion; and (iii) the supply of CO
2
to Rubisco is restricted by the boundary layer,
stomata, and intercellular gas spaces that can differ for CO
2
diffusion in the gas
phase (
α
=
1.0044), and in the aqueous phase (
α
=
1.0007).
The
δ
13
C values of POM are varied spatially and seasonally. They increase
with increasing pH of lake water, which may reflect a shift by phytoplank-
ton from using CO
2
to using
HCO
3
−
for photosynthesis (Zohary et al.
1994
;
Doi et al.
2006
). The pH is increased with increasing water temperature dur-
ing the time span of the summer stratification period, which may be con-
nected with photoinduced degradation of DOM and POM (see also chapter
H
2
O
2
(Mostofa and Sakugawa
2009
; Fujiwara
et al.
1993
) might be one of the key factors for enhancing alkalinity or pH in
waters. Therefore, uptake of
HCO
3
−
for phytoplankton photosynthesis at high pH
might be the effect of its dominant presence in waters. A significant increase in the
2O
2
•−
+
2H
+
→
H
2
O
2
+
O
2
