Geoscience Reference
In-Depth Information
9.3.3 Understanding Controls on DOM Source and Quality in Surface Waters
One initial application of the McKnight et al. FI was to understand the controls on DOM
concentration in a system of alpine and subalpine lakes in the Rocky Mountains that are
studied as part of the Niwot Ridge Long Term Ecological Research (LTER) project (Hood
et al.,
2003
). In all the lakes an increase in DOC concentration occurred during snowmelt
and this DOC peak was associated with a high percentage of humics and low FIs, indica-
tive of a plant/soil source of organic material. During the summer algal bloom, the percent
humics decreased and the FI increased. This increase in FI was greater in the alpine lakes
than in the sub-alpine lakes, reflecting the greater initial input of terrestrial DOM from
the subalpine forest compared to the alpine tundra and talus fields. A seasonal change in
FI was not observed in the stream draining an adjacent watershed that does not contain
lakes. The seasonal trend has been observed to occur every spring and summer, and the
greater aromaticity of fulvic acid from the alpine lake in spring snowmelt compared to
the summer was confirmed by chemical characterization of isolated samples by
13
C-NMR
(Hood et al.,
2005
). Further, Miller et al. (
2009
) observed that an unusual 3-day summer
rainstorm caused the FI in the alpine lake to decrease abruptly due to additional runoff, and
then the FI rebounded to higher values when the algal populations increased in abundance
(
Figure 9.13
).
Fluorescence indices have been applied to understand controls on DOM quality asso-
ciated with land cover and land use for 34 temperate watersheds in Canada where the
percentage of riparian cropland varied from 0 to 45% (Wilson and Xenopoulos,
2009
).
These investigators examined filtered water samples collected bimonthly over 2 years and
the spectral data were inner filter corrected before analysis. In the study catchments there
was a correlation between HIX
EM
and moisture conditions, indicating a climate effect on
DOM character. However, the FI and BIX strongly correlated with the amount of riparian
cropland and total dissolved nitrogen respectively, and showed no relationship to mois-
ture conditions, indicating that the chemical signal for source material was independent of
moisture (
Figure 9.14
).
The correlation of the FI with continuous cropland demonstrated, for the first time, a
possible link between land cover, DOM character, and aromaticity. The BIX index, ori-
ginally developed to compare peaks M and C in marine and estuarine environments, was
applied to a terrestrial environment and was shown to correlate with total dissolved nitro-
gen. This is likely to be a co-relationship, with increased microbial productivity leading to
increased fluorescence in the peak M and peak T regions, increasing the value of this index
due to eutrophication.
The McKnight et al. (
2001
) FI has also been used by a number of researchers to inter-
pret spatial and seasonal trends, and often has been combined with a “microbial/autoch-
thonous” versus “terrestrial/allochthonous” interpretation based on the calibration data set
of surface waters (McKnight et al.,
2001
). In a comparison across a diverse set of long-
term research sites primarily in the LTER network, Jaffe et al. (2008) found that the FI
was correlated with the carbon-to-nitrogen ratio in the DOM (
Figure 9.15
), with higher FI
