Environmental Engineering Reference
In-Depth Information
of NO 2
, and of other redox reactions in natural waters (Qian et al. 2001 ;
Randall et al. 2005 ). The HO
and NO 3
formation rates from nitrate, as well as DOM plus nitrite
are significantly increased during ozone hole conditions, compared to non-ozone hole
periods (Qian et al. 2001 ). Therefore, a higher production of HO
during ozone hole
events can enhance the photoinduced degradation of DOM. UV transparency of the lake
water column is also greatly enhanced during the summer season due to photoinduced
degradation of DOM in the lake epilimnion (Morris and Hargreaves 1997 ). Diffuse
attenuation coefficients are greatly varied (39-81 %) seasonally at the epilimnion, and
minimum values occur during the summer season (Morris and Hargreaves 1997 ). Thus,
an increase in incident UV radiation (280-400 nm) in response to stratospheric ozone
depletion can increase the transformation of surface DOM and, by increasing the UV
transparency of water, can also induce additional degradation of DOM in the deeper
layers (Qian et al. 2001 ; Randall et al. 2005 ).
3.11 Global Warming
Global warming may expand the summer season (Huisman et al. 2006 ; Sarmiento
et al. 2004 ; Schmittner 2005 ), which might accelerate the photoinduced degrada-
tion of DOM. For example, it might lead high production of HO
because of the
increase in water temperature due to global warming. At the same time, there can
be an increase in UV radiation during ozone hole events (Qian et al. 2001 ; Rex
et al. 1997 ). Global warming may also affect (and possibly enhance) the water col-
umn transparency, which is modified on a variety of time scales, and the depth of the
mixing layer, as well as lead to changes in climatologic factors such as cloud cover,
particulate material and total content of column ozone. These factors may influence
the incident UV radiation (Morris and Hargreaves 1997 ; Morris et al. 1995 ; Scully
and Lean 1994 ). Global warming may affect the seasonal patterns of chlorophyll and
nutrient concentrations in the deep chlorophyll maxima (DCM) in waters (Huisman
et al. 2006 ; Mostofa et al. 2009b ; Letelier et al. 2004 ). The combination of global
warming and photoinduced degradation may significantly impact on primary pro-
duction, species composition, global carbon cycle, biological activities, and finally
the seasonal modifications of the water column in natural waters (Huisman et al.
2006 ; Häder et al. 2011 ). The effects of global warming on photoinduced degrada-
tion of DOM are extensively discussed in the global warming chapter (see chapter
Impacts of Global Warming on Biogeochemical Cycles in Natural Waters ).
3.12 Salinity
DOM photoreactivity is significantly increased with salinity or addition of salts in
natural waters (Osburn et al. 2009 ; Hernes and Benner 2003 ; Osburn and Morris
2003 ; Anastasio and Newberg 2007 ; Grebel et al. 2009 ). Controlled laboratory
studies demonstrate that the presence of seawater concentrations of chloride and
bromide ions can enhance absorbance photobleaching reaction rates by ~40 %,
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