Environmental Engineering Reference
In-Depth Information
(DOM) in natural waters. Photoinduced or photolytic processes can maintain the
acidity-alkalinity, water transparency, thermal stratification, redox reactions, pro-
duction of bioavailable carbon substrates to enhance biological productivity, nutri-
ent concentrations, production of dissolved inorganic carbon (DIC), autochthonous
production of DOM, photosynthesis, formation of surface chlorophyll
a
maxima
(SCM) and so on (Harvey et al.
1995
; Moran and Zepp
1997
; Laurion et al.
2000
;
Kopacek et al.
2003
; Barbiero and Tuchman
2004
; Huisman et al.
2006
; Mostofa et
al.
2009a
,
b
). Finally, solar radiation is a major source of energy that is of essen-
tial importance in natural water ecosystems. The Photoinduced degradation of
DOM and its consequences on natural waters are significantly dependent on the
spectral range of sunlight under consideration, namely the UV-A (315-400 nm),
UV-B (280-315 nm) or visible light (400-700 nm). Depending on the wave-
length, there are significant variations as far as sunlight penetration in the water
column is concerned (Scully et al.
1996
; Morris and Hargreaves
1997
; Reche
et al.
1999
). DOM is typically able to absorb UV radiation in sea and lake water
(Kirk
1994
; Morris et al.
1995
), thereby controlling the penetration of UV in the
deep water layers. The penetration depths of UV radiation in natural waters are
greatly varied, with typical penetration in clear ocean water of ~20 m for UV-B
and ~50 m for UV-A radiation. In oligotrophic marine waters, penetration of UV-B
radiation is 5-10 m and 0.5-3 m in freshwater (Kirk
1994
; Smith and Baker
1981
;
Waiser and Robarts
2000
). Therefore, any changes in the radiation wavelengths or
an increase in global temperature can greatly impact on the various biogeochemi-
cal processes mentioned earlier. However, researchers do not pay much attention to
photoinduced processes to assess the biogeochemical processes in natural waters.
The microbial process is a well-known observable fact that is typically respon-
sible for the in situ generation of DOM, cycling of nutrients, occurrence of deep
chlorophyll
a
maxima (DCM), photosynthesis, thermal energy and degradation of
organic matter in soil or sediment porewaters, which are vital to water environments
(Mostofa et al.
2009a
,
b
; Conrad
1999
; Guildford and Hecky
2000
; Rochelle-Newall
and Fisher
2002
; Roberts et al.
2004
; Lovley
2006
; Yamashita and Tanoue
2008
).
Microbial process is generally controlled by bacterial cells and microorganisms,
both autotrophs (plants, algae, bacteria) and heterotrophs (animals, fungi, bacteria)
in the aquatic environments. Microbial activity is often taking place at the hypolim-
nion in natural waters as well as in sediment porewaters, and it is most significant in
temperate, Arctic and Antarctic regions when the lake epilimnion is covered by ice.
There is also no photoinduced degradation taking place at the epilimnion during the
ice-covered period. The overall photoinduced and microbial changes in organic mat-
ter and DOM components in waters play a significant role in the global carbon cycle
and in the biogeochemical processes in the aquatic environment.
This review will provide a common overview on biogeochemical functions of
DOM for photoinduced and microbial processes, DOM degradation for a variety of
waters, theoretical model and mechanisms for photoinduced and microbial degradation
of organic matter and DOM components, reaction rate constants by functional group
contribution method, kinetics of photoinduced degradation of DOM, and interactions
between photoinduced and microbial degradation in waters. This study also discusses
