Environmental Engineering Reference
In-Depth Information
Many studies observe that DOC export is positively correlated with runoff,
and two issues are involved (Sobek et al.
2007
): First, the carbon budget of
the studied landscapes is not in steady state, i.e., increased runoff exports
more DOC than is produced in soil, which implies that observed increases are
temporary. Second, changes in runoff are concomitant to changes in leachable
organic carbon stocks in soil. High runoff indicates a high water table, which
favors DOC leaching and hampers microbial degradation due to anoxia and
humification. In addition, the negative relationship between runoff and lake
DOC concentration indicates that when high runoff prevails over extended
periods of time, the leachable soil organic carbon pool will eventually be
reduced (Sobek et al.
2007
).
(v) Severe drought seasons can either greatly decrease or increase the DOC lev-
els in natural water (Watts et al.
2001
; Meier et al.
2004
; Shaw
1979
; Worrall
et al.
2003
,
2005
,
2006
; Worrall and Burt
2004
; Ward et al.
2007
). DOC con-
centrations in shallow groundwater are very low (1558
μ
M C) under drought
condition compared to spring samples (2583
μ
M C). Correspondingly, the
properties of DOM are largely different (Meier et al.
2004
). Such differences
may be attributed to biogeochemical changes in the DOM pool over the sum-
mer and fall seasons under drought conditions. Studies show that runoff char-
acteristics and flow-paths within peat soils change as a result of severe drought,
which could increase DOC concentrations in runoff water (Evans et al.
1999
;
Holden and Burt
2002
).
In addition, releases of allochthonous DOM are largely dependent on several
catchment properties such as drainage ratio (catchment : lake area), proportion
of wetlands, proportion of upstream lakes, watershed slope, altitude, catchment
area, % peat cover, water area, wetland cover, and soil C:N ratio in the catchment
(Sobek et al.
2007
; Eckhardt and Moore
1990
; Xenopoulos et al.
2003
; Rasmussen
et al.
1989
; Kortelainen
1993
; Hope et al.
1997
; Aitkenhead and McDowell
2000
).
The export of DOC from catchments is often related to the organic carbon stocks
in the catchment soils (Hope et al.
1994
; Aitkenhead et al.
1999
). Studies show
that DOC fluxes are small: 0.8
±
0.2 % relative to gross primary productivity,
1.0
±
0.3 % relative to ecosystem respiration, and (2.4
±
0.4 %) relative to soil
respiration, when the DOC fluxes are considered relative to the gross ecosystem
carbon fluxes in a specific catchment (Gielen et al.
2011
).
6.3 Effect of Temperature
Temperature can affect DOM in two ways: First, water temperature (WT), linked
with solar radiation, is one of the most important variables in the production of
autochthonous DOM in natural waters because it affects the physical, pho-
toinduced, microbial and ecological processes (Mostofa et al.
2009a
; Sobek et
al.
2007
; Gielen et al.
2011
; Gudasz et al.
2010
). The mineralization of organic
