Environmental Engineering Reference
In-Depth Information
The air-sea CO
2
exchange occurs mostly in temperate regions (Borges and
Frankignoulle
2002a
,
b
; Borges
2005
; Andersson et al.
2003
; Andersson and
Mackenzie
2004
; Zhai et al.
2005
). However, waters in upwelling regions act
both as sinks (California and Oman coasts) and as sources (Galician and Oregon
coasts) of atmospheric CO
2
(Borges and Frankignoulle
2002a
,
b
; Goyet et al.
1998
; Friederich et al.
2002
; Hales et al.
2005
). The global coastal zone is still
a net source of CO
2
to the atmosphere, due to the combination of calcification
and of net heterotrophy that is a feature of estuarine ecosystems (Frankignoulle
et al.
1998
; Borges
2005
; Cai and Wang
1998
; Raymond et al.
2000
; Sarma et al.
2001
; Mukhopadhyay et al.
2002
; Bouillon et al.
2003
; Abril et al.
2003
,
2004
;
Mackenzie et al.
2004
; Fagan and Mackenzie
2007
). Indeed, when estuaries are
included in the CO
2
exchange budget, the global shallow-water coastal ocean is a
net source of CO
2
to the atmosphere (Borges
2005
).
The production of CO
2
and its input to the atmosphere is considerably higher
during the summer and fall (or dry) seasons than in winter and spring (or wet)
seasons. In the latter case the waterbed actually acts as a net sink for atmospheric
CO
2
. The reason behind this phenomenon is that the photoinduced and micro-
bial degradation of DOM and POM are greatly enhanced in surface waters during
the summer period due to high solar radiation and longer summer day-time. CO
2
emission by boreal streams is quite high during summer and very low in spring,
which might be a consequence of photoinduced processing of DOM and POM
(Koprivnjak et al.
2010
). Obviously, the solar intensity is significantly reduced
during the winter season that also has shorter day-time. In addition, estuaries
often have high contents of DOM that undergoes strong photoinduced degrada-
tion and makes these systems to be significant sources of CO
2
to the atmosphere.
The concentration of dissolved organic carbon (DOC) explains the significant vari-
ation of lake
p
CO
2
(Sobek et al.
2005
), which might be an effect of photoinduced
and microbial release of CO
2
from DOM and POM in water as mentioned before.
Supersaturation of CO
2
in freshwater ecosystems (streams, rivers and lakes) is
possibly caused by the same photoinduced and microbial processes that degrade
DOM and POM. Indeed, freshwater ecosystems generally contain high amounts of
DOM and POM that are potentially important microbial or photoinduced sources
of CO
2
or DIC.
The situation is much different at northern latitudes: it is estimated that the
direct photo-oxidation of organic carbon to CO
2
accounted for less than 10 % of
dark respiration in the epilimnion of six boreal lakes (Granéli et al.
1996
). CO
2
emission is also mainly derived from in-lake respiration in the lake environ-
ments (del Giorgio et al.
1999
; Jansson et al.
2000
). Anyway, global warming will
increase the atmospheric temperature that can enhance both the photoinduced
and the microbial degradation of DOM and POM, during all seasons and at all
latitudes. The consequence would obviously be a further increase of atmospheric
CO
2
. Warming is also expected to reduce terrestrial and ocean uptake of atmos-
pheric CO
2
, increasing the fraction of anthropogenic emissions that remain in the
atmosphere. This would result into an additional increase of atmospheric CO
2
(IPCC
2007a
).
