Environmental Engineering Reference
In-Depth Information
marine fulvic acids (Malcolm
1990
), without a coherent terminology. Recent stud-
ies demonstrate that the two fluorescent components, termed as autochthonous ful-
vic acid (C-like) (Fig.
3
h-j) and as autochthonous fulvic acid (M-like) (Fig.
3
k,
l), are primarily produced under photoinduced or microbial respiration (or assim-
ilation) of algae or phytoplankton biomass (Mostofa et al.
2009b
; Stedmon and
Markager
2005a
; Zhang et al.
2009a
). PARAFAC modeling on EEM spectra of
algal-originated DOM suggests that the fluorescence peaks and the images of
the first fluorescent component are similar to allochthonous fulvic acid (C-like)
(Figs.
2
a,
3
a). Therefore, this component is indicated as autochthonous fulvic acid
(C-like) of algal origin (Fig.
3
h-j). On the other hand, the fluorescence peaks and
the images of the second fluorescent component (Fig.
3
k, l) are similar to alloch-
thonous fulvic acid (M-like) (Fig.
3
c) and to the marine humic-like substances
(Coble
1996
). Therefore, this component is denoted as autochthonous fulvic acid
(M-like) of algal origin. The fluorescence intensities and the excitation-emission
maxima of these two fluorescent components are significantly different depend-
ing on the respective peak positions. Considering the similarities between EEM
images of the algal originated fluorescent component and allochthonous fulvic
acid (C-like), it is suggested to denote the first and the second fluorescent com-
ponents as 'autochthonous fulvic acid (C-like)' and 'autochthonous fulvic acid
(M-like)', respectively (Fig.
3
h-l). Similarly, allochthonous fulvic acids can be
denoted as 'allochthonous fulvic acid (C-like)' and 'allochthonous fulvic acid
(A-like)', respectively (Fig.
3
a, b). The allochthonous fulvic acid (A-like) shows
only one shoulder or strong fluorescence intensity at peak A-region, which may
not be classified as a peak in standard SRFA and SRHA as well as in field obser-
vations (Tables
1
,
2
; Fig.
3
b). Apparently, autochthonous fulvic acids often show
higher fluorescence intensities at peak C-region than at peak A-region (Fig.
3
h-l),
whilst allochthonous fulvic acids (C-like) often show opposite behavior (Fig.
3
a-c).
The differences in fluorescence intensities at peak C- and A-regions could be use-
ful to distinguish between allochthonous and autochthonous fulvic acids using
the fluorescence index (Mostofa et al.
2009b
; Huguet et al.
2009
; Battin
1998
;
Zsolnay et al.
1999
; McKnight et al.
2001
). The fluorescence index (
f
450/500
) is
defined as the ratio of fluorescence intensity at Ex/Em 370/450 nm to that at Ex/
Em
=
370/500 nm, which can provide a basis for estimating the degree of aro-
maticity—and potentially for discriminating the sources—of DOM (Battin
1998
;
McKnight et al.
2001
). However, the index
f
450/500
does not distinguish the
autochthonous fulvic acid (C-like) of algal origin (1.75-2.59) from allochtho-
nous fulvic acid (1.30-3.22) and allochthonous humic acid (1.28-1.51), which
can be identified using PARAFAC modeling of a variety of DOM sources in natu-
ral waters (Fig.
4
; Table
3
) (Mostofa et al.
2005a
,
2007a
,
2005b
; Mostofa KMG
et al., unpublished data; Fu et al.
2010
; Li et al., Characteristics of sediment pore
water dissolved organic matter in four Chinese lakes using EEM spectroscopy
and PARAFAC modeling, unpublished data). Further, this index can also fail
when applied to a variety of natural waters (Huguet et al.
2009
; Jaffé et al.
2004
).
Another fluorescence index (HIX) has been developed to estimate the degree of
maturation of DOM in soil (Zsolnay et al.
1999
). HIX is defined as the ratio H/L
