Environmental Engineering Reference
In-Depth Information
6. Laboratory evidence of CDOM photobleaching
Upon light exposure, CDOM undergoes a complex series of reaction that also
can potentially affect the entire aquatic system (Figure 6)
47
.
SUN
CO
2
, CO, COS
UV-B
M
n+
CDOM
LMW
labile C, N, P
ROS
Living organisms
Figure 6.
Scheme of CDOM photodegradation (Modified from Zepp et al.
47
).
Because CDOM is one of the primary light absorbing species in aquatic systems,
its breakdown affects the optical properties of the whole water body
56
. Its breakdown
further produces potentially important substrates for the plankton growth
57
.
Photochemical degradation of CDOM releases carbon monoxide
58
, carbon dioxide
51,59
and a variety of low molecular weight organic compounds, as well as inorganic
nitrogen
60-61
. Most of these compounds are however released with very low quantum
yields
62
. Biologically available substrates such as pyruvate, other low molecular weight
(LMW) carbonyl compounds are released from the photolytic breakdown of the
biologically refractive CDOM
63-65
. These photochemical products seem to stimulate the
bacterial growth, although to different extents depending upon the DOM source (e.g.,
surface versus deep water DOM)
66-67
. Photochemical and microbial degradation
combined also result in a more efficient consumption of CDOM
40,45,68
.
Overall, absorption of photons by the CDOM can initiate photochemical
reactions in which primary products are produced as well as intermediates.
Photochemical reactions ultimately lead not only to a destruction of the chromophores
and hence to a loss of the CDOM optical properties (called photobleaching)
20,34,49,69-71
but also to transformation of the terrestrial DOM into a new form
42
.
Considerable new research (field and laboratory) has been conducted during the
last few years to further understand the CDOM photobleaching. However, the nature of
the CDOM chromophores and the mechanisms of their photochemical reactions need to
be understood to quantify the importance of photochemistry in natural waters.
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