Environmental Engineering Reference
In-Depth Information
A positive correlation of
p
CO
2
levels with CDOM and chlorophyll has been
observed in the Southwest Florida Shelf, indicating that CO
2
may be produced
from the photoinduced degradation of CDOM in natural waters (mostly in the
dry season), from microbial respiration and from shifts in the carbonate equilib-
rium (Clark et al.
2004
). The effect is a release of CO
2
into the atmosphere from
DOM that has been formed by primary production (Thomas et al.
2009
). On the
other hand, microbial degradation of DOM and OM in natural waters and sedi-
ment pore waters can release CH
4
to the atmosphere (Mosier et al.
2004
; Cicerone
and Oremiand
1988
; Pepper et al.
1992
; Bastviken et al.
2004
,
2008
; Bergström
et al.
2007
). Anoxia in freshwater sediments contributes to high CH
4
emissions,
and the production of CH
4
in epilimnetic sediments is the main driver of meth-
ane emission from surface waters (Bastviken et al.
2004
,
2008
). Methane produc-
tion can also be enhanced by water temperature and lake level fluctuations. Such
effects can affect carbon balances depending on the predominant plant species
and sediment properties (Bergström et al.
2007
). It is estimated that the contri-
bution of CH
4
to the atmosphere is 100-200 Tg yr
−
1
from wetlands, 5-20 Tg
yr
−
1
from oceans, and 1-25 Tg yr
−
1
from freshwater (Mosier et al.
2004
). N
2
O
can be released from freshwater and oceanic environments (Watson et al.
1992
;
Seitzinger
1990
). Increases in oxygen-deficient regions in the ocean caused by
climate changecould enhance the emissions of nitrous oxide, an important green-
house and ozone-depleting gas (Zepp et al.
2011
).
The upper ocean microbial food web (mostly the autotrophs) is a huge carbon-
processing machine that can remove CO
2
from the atmosphere, but part of the car-
bon fixed by autotrophy is actually respired in situ (Sarmento et al.
2010
). The
heterotrophic bacteria are responsible for the major respiration (>95 %) in the
ocean (del Giorgo and Duarte
2002
), and half of it (approximately 37 Gt of C per
year) takes place in the euphotic layer (del Giorgio and Williams
2005
). Notes that
global ocean respiration is approximately as important as the oceanic primary pro-
duction (del Giorgo and Duarte
2002
; Karl et al.
2003
; Williams PJlB et al.
2004
;
Riser and Johnson
2008
). Increasing temperature will often increase respiration
rates in natural waters (Vázquez-Domínguez et al.
2007
). Increasing aquatic respi-
ration is presumably the result of enhanced photo- and microbial products (H
2
O
2
,
CO
2
, DIC, etc.) derived from the photoinduced and microbial degradation of DOM
and OM in the euphotic zone. The temperature increase accelerates the respira-
tory consumption of organic carbon relative to the autotrophic production, with a
decrease in the biological drawdown of DIC. A decrease of up to 31 % has been
observed in mesocosms warmed by 2, 4 and 6 ºC (Wohlers et al.
2009
). Changes
in the biogenic carbon flow induced by warming have the potential to reduce the
transfer of primary produced OM to higher trophic levels (Vázquez-Domínguez
et al.
2007
; Wohlers et al.
2009
; Laws et al.
2000
). This would weaken the ocean's
biological carbon pump and provide a positive feedback to the rise of atmospheric
CO
2
(Vázquez-Domínguez et al.
2007
; Wohlers et al.
2009
; Laws et al.
2000
).
The photoinduced and microbial activities of DOM and POM in natural surface
waters may act as sources or sinks of N
2
O that is produced via nitrification and deni-
trification (Tranvik et al.
2009
; Mengis et al.
1997
; Huttunen et al.
2003b
,
2004
; Wang
