Geoscience Reference
In-Depth Information
25
γ
Offset = 65nm
20
α
15
10
β
5
0
250
270
290
350
Excitation wavelength (nm)
310
330
370
390
410
Figure 9.5. Synchronous fluorescence spectra with an offset of 65 nm of a freshwater sample (solid
line) and marine water sample (dotted line). Locations of the
α
(humic-like),
β
(marine humic-like),
and
γ
(tyrosine-like) peaks are identified. The marine humic-like peak has been associated with
recently created humic material due to algal activity and is commonly seen with a corresponding
protein peak. The
β
and
α
peaks are used to calculate the freshness index, which is considered an
indicator of the degree of recently produced humic material in coastal systems. (From Parlanti et al.
[
2000
], with minor adaptations.)
et al. (
2009
) investigated filtered estuarine samples, and after collecting EEMs, calculated
the BIX. Low ratios were interpreted in this environment as containing dissolved organic
matter with a low autochthonous component. Increasing ratios were correlated to a high
autochthonous component of greater aquatic microbial origin.
Wilson and Xenopoulos (
2009
) suggested another slight variation on the
β
/
α
ratio.
Riverine dissolved organic matter from 34 watersheds was compared to determine the
impact of agricultural land use on DOM characteristics. Like Huguet et al. (
2009
) the
310 nm excitation wavelength was used. The
β
/
α
index was calculated by dividing the
emission intensity at 380 nm (representing the
β
peak) with the maximum emission inten-
sity observed between 420 and 435 nm (representing the
α
peak). Wilson and Xenopoulos
(
2009
) found an increase in the
β
/
α
ratio with an increase in percent cropland coverage,
corresponding with an increase in total dissolved nitrogen, a nutrient that often increases
biological activity. These results reinforced the association of the freshness index with
recently produced DOM from microbial sources.
9.2.4 Fluorescence Index to identify Precursor Material
in Freshwater DOM (FI)
While studying humics found in lakes and streams, McKnight et al. (
2001
) investigated
the relative importance of plant/soil-derived organic material compared to organic material
derived from microbial biomass as precursor material. They used
13
C-NMR,
1
H-NMR, IR,
