Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 66

into the aquatic ecosystem. Second, global warming may accelerate the pho-
toinduced and microbial decomposition of DOM to produce compounds such
as H 2 O 2 , CO 2 , DIC, NO 3

−

, PO 4 3 − , NH 4

+

, LMWDOM and so on (Mostofa and
Sakugawa 2009 ; Johannessen and Miller 2001 ; Ma and Green 2004 ; Xie et al.
2004 ; Johannessen et al. 2007 ; Palenik and Morel 1988 ; Kotsyurbenko et al.
2001 ; Lovley 2006 ). The availability of these compounds can enhance photosyn-
thesis and ultimately increase the primary and secondary production. These pro-
cesses can induce the formation of autochthonous DOM and are usually expected
to deteriorate the quality of natural waters. All these processes are extensively
discussed in chapter “ Impacts of Global Warming on Biogeochemical Cycles in
Natural Waters ” .

7 Possible Mechanisms for Increased and Declined DOM
Contents in Surface Waters

Production of autochthonous DOM is a well-known phenomenon in stagnant
surface waters, particularly in lakes and oceans. The corresponding increase
in autochthonous DOC is, on average, 0-102 % in lakes and 0-194 % in the
oceans' epilimnion compared to the hypolimnion during the summer stratifica-
tion period (Table 2 ). Increased concentrations of DOC in surface waters are a
commonly observed phenomenon in North America and in North and Central
Europe including UK, the Czech Republic, Finland, Norway, Canada, USA and
so on (Hejzlar et al. 2003 ; Worrall et al. 2004a , 2007 ; Evans et al. 2005 , 2006 ;
Monteith et al. 2007 ; Hongve et al. 2004 ; Monteith and Evans 2005 ; Larsen
et al. 2011 ; Clutterbuck and Yallop 2010 ; Yallop et al. 2010 ; Freeman et al. 2001b ;
Bouchard 1997 ; Skjelkvåle et al. 2001 , 2005 ; Driscoll et al. 2003 ; Stoddard
et al. 2003 ; Vuorenmaa et al. 2006 ). The increase in DOC export also enhances
the export of humic DOC from upland peat catchments (Yallop et al. 2010 ). It
is estimated that the increase in mean DOC concentrations between the first and
last 5 years of monitoring in UK's Acid Waters Monitoring Network (AWMN)
streams and lakes are 32-135 % in 11 streams and 31-140 % in 11 lakes (Evans
et al. 2006 ; Monteith and Evans 2005 ). DOC in UK rivers arises from a num-
ber of sources including: decomposition of deep peat if present (McDonald et al.
1991 ), sewage (Eatherall et al. 2000 ), industrial point-source effluents (Tipping
et al. 1997 ) and products of early stages of plant decomposition (Palmer et al.
2001 ). The long-term trends of increasing or decreasing DOC concentration are
not evident in various lakes except at the Experimental Lakes Area, where an
increase in DOC is correlated with a decrease in summer total solar radiation and
an increase in summer precipitation (Zhang et al. 2010 ). The initial DOM contents
are also important to enhance the autochthonous production of DOC in aquatic
ecosystems. Moreover, several mechanisms have been suggested to explain the
enhancement of aquatic DOC including: increased terrestrial vegetation cover
in response to climate change (Larsen et al. 2011 ; Worrall et al. 2003 ; Freeman