Environmental Engineering Reference
In-Depth Information
et al.
2000
; Winter et al.
2007
; Mostofa KMG et al., unpublished data; Brown
1977
).
After 13 days incubation, CDOM absorption has been found to increase over the
entire spectrum in upstream waters. Absorption increase was 20-81 % at 340-350
nm and 4-38 % at 600-700 nm, but it was highest at 312-410 nm (77-88 %) for
Nishi-Mataya upstream and at 440-570 nm (39-49 %) for Kago upstream (Fig.
1
;
labile
1
). In downstream waters, CDOM absorption was decreased (6-7 %) at 340-
350 nm but increased (8-49 %) at 600-700 nm. The maximum decrease occurred at
390-415 nm (9-11 %) (Fig.
1
; Table
1
). CDOM absorption was also decreased in
pond (4 % at 340 nm), lakes (1-32 % at 340 nm), estuaries (2-4 % at 350 nm and
4-11 % at 250-500 nm). In other cases, very small increases have been observed in
pond (3 % at 340 nm), marsh (2-3 % at 340 nm), lakes (2-3 % at 340 nm) and estu-
aries (1 %) (Table
1
) (Moran et al.
2000
; Winter et al.
2007
). These results show a
significant microbial effect on CDOM in natural waters (Table
1
; Fig.
1
).
Such an effect shows several characteristic phenomena. First, an increase
in CDOM absorption over the entire spectrum in upstream waters might be due
to a microbial alteration of the composition of fulvic acids. In fact, it has been
known that upstream CDOM is mainly composed of fulvic acids (Mostofa et al.
2007
; Mostofa et al.
2005
). Second, an increase in absorption at longer wavelength
and a decrease at shorter wavelength in the waters of downstream river might be
due to the presence of various CDOM sources. Note that upstream water is one
of the sources of the downstream one, and downstream fulvic acids (FA) might
derive from upstream FA upon transformation induced by photochemistry and/
or by microorganisms. The latter process is rather slow but could account for
the increase of downstream CDOM absorption in the longer wavelength region,
because a similar phenomenon is also observed in upstream waters. Conversely,
the autochthonous and agricultural CDOM in downstream waters are likely to
undergo rapid microbial degradation that, given the different nature of this kind
of CDOM, might result in a decrease of CDOM absorption in the shorter wave-
length region. Downstream DOM is in fact derived from several sources includ-
ing autochthonous (protein-like or tryptophan-like), allochthonous (mostly fulvic
acids of upstream origin) and agricultural DOM that is released from nearby agri-
cultural fields (Mostofa et al.
2007
; Mostofa et al.
2005
).
CDOM absorption is also found either to decrease or to increase in ponds,
lakes, marshes and estuaries that are relatively similar to downstream river envi-
ronments. The CDOM in these natural waters generally consists of both alloch-
thonous (mostly fulvic acids) and autochthonous material. Autochthonous organic
substances are microbially labile and their absorption is decreased by microbial
degradation, differently from allochthonous fulvic substances. This can explain the
rather complex effects of microbial processing on CDOM absorption in the differ-
ent wavelength ranges.
Experimental studies also show that a large amount of high molecular weight
CDOM is produced during phytoplankton lysis.
S
300-500
is decreased in the first
9 days when CDOM composition is changed due to increasing microbial activity,
which is expected to decrease the molecular weight of organic substances (Fig.
4
b)
(Zhang et al.
2009
; Mostofa KMG et al., unpublished data).
