Environmental Engineering Reference
In-Depth Information
et al.
2001b
; Stoddard et al.
2003
; Evans et al.
2005
) and associated increases in
enchytraeid worm activity (Cole et al.
2002
; Carrera et al.
2009
); increasing CO
2
-
mediated stimulation of primary productivity (Freeman et al.
2004
); hydrological
change (Hongve et al.
2004
; Evans et al.
2005
); artificial peat drainage (Worrall et
al.
2003
); the occurrence of severe drought events (Watts et al.
2001
; Worrall and
Burt
2004
); and the removal of decomposition-inhibiting phenolic compounds fol-
lowing prolonged water table drawdown (Freeman et al.
2001a
). However, these
mechanisms are not sufficiently well documented yet to understand the increased
DOC concentrations in natural waters.
One of the possible mechanisms leading to increased DOC concentrations in
surface waters is the enhancement of photosynthesis. Two different processes are
involved, depending on the sources of DOM.
(i) The first issue is that increased soil respiration may increase the decompo-
sition rates of soil OM due to the effect of global warming. Furthermore,
elevated CO
2
enhances DOC supply in peat soils because of elevated net pri-
mary productivity and increased root exudation of DOC in soil environments
(Freeman et al.
2001b
,
2004
; Lavoie et al.
2005
; Fenner et al.
2007a
,
b
; Wolf
et al.
2007
; Kang et al.
2001
; Tranvik and Jasson
2002
; Monteith et al.
2007
;
Evans et al.
2002
; Dorodnikov et al.
2011
). This process ultimately leaches
allochthonous DOM into the aquatic ecosystem. The increased activity of
enchytraeid worms (the dominant invertebrates in upland peats) at higher
temperature increases the microbial activity in peat and enhances nutrient
mineralization (Cole et al.
2002
). The mineralization of C- and N-containing
compounds would increase the losses of nitrate and DOC (Cole et al.
2002
).
(ii) The second issue is that the allochthonous DOM that is increasingly released
into surface waters can undergo photoinduced decomposition to generate
H
2
O
2
, CO
2
and DIC (dissolved CO
2
, H
2
CO
3
, HCO
3
, and CO
3
2-
), or micro-
bial degradation with production of H
2
O
2
, CO
2
, DIC, CH
4
, PO
4
3-
, NH
4
−
+
and so on (Lovley et al.
1996
; Johannessen and Miller
2001
; Ma and Green
2004
; Xie et al.
2004
; Johannessen et al.
2007
; Palenik and Morel
1988
; Clark
et al.
2004
; Kotsyurbenko et al.
2001
). Many of these compounds are able to
enhance photosynthesis (Mostofa et al.
2009a
; Komissarov
1994
,
1995
,
2003
;
Li et al.
2011
; Li
1994
; Zubkov and Tarran
2008
; Beardall et al.
2009a
,
b
;
Wu and Gao
2009
; Liu et al.
2010
). This process can fuel primary and sec-
ondary production, thereby leading to enhanced aquatic OM and DOM. In
fact, algae and phytoplankton can produce autochthonous DOM under both
photoinduced and microbial respiration or assimilation, which contributes
to increasing DOM in natural waters. Under elevated carbon dioxide levels,
the proportion of DOM derived from recently assimilated CO
2
is ten times
higher compared to the control cases (Freeman et al.
2004
). In addition, new
DOC release is far more sensitive to environmental drivers that affect net pri-
mary productivity compared to decomposition alone (Freeman et al.
2004
).
