Geoscience Reference
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of DOM from runoff and precipitation may contribute to the DOM pool, it is clear from
Figure 7.5A-D
that evaporation is concentrating fluorescent DOM. Blue shifting in DOM
fluorescence might occur if the increase in salinity is also causing a metal-quenching effect,
or
if increased salinity is causing conformational changes in DOM, as suggested by a num-
ber of studies (e.g., Lochmuller and Saavedra,
1986
; Reche et al.,
1999
; Boyd and Osburn,
2004
; Batchelli et al.,
2009
; Provenzano et al.,
2010
). However, the opposite effect was
observed. The EEMs show an enhancement in a peak centered at 315 nm excitation and
400-420 nm emission (labeled INT on
Figure 7.5A-D
), intermediate between the region
of the terrestrial humic C peak and the marine humic (or microbial) M peak (Boyd et al.,
2010a
). This peak increased with conductivity (
Figure 7.6A
), but the pH did not change
appreciably in this lake from 2001 to 2005 (data not shown). However, Mg
2+
concentration
in Alkali Lake increased from May to August 2004 (2.46 to 3.41 mmol Mg
2+
, respectively).
It is possible that increasing Mg
2+
concentrations in the lake water displaced quenching
metals, similar to results found by Willey (
1984
) and Cabaniss (1992). Interestingly, the
INT peak coordinates are very similar to ex/em maxima for the 3-hydroxybenzoic acid
and salicylic acid moieties proposed to constitute humic fluorescence (Senesi et al.,
2005
),
as well as amino sugars (Biers et al.,
2007
) and evidence for phytoplankton production
of fluorescent DOM (Romera-Castillo et al.,
2010
). This latter point suggests that micro-
bial processing of DOM, rather than ion displacement, created or transformed fluorescent
DOM in Alkali Lake (Boyd and Osburn,
2004
; Osburn et al.,
2011
).
The results from this saline lake are in contrast to the results from Provenzano et al.
(
2008
) for hydrophilic (HI) and hydrophobic (HO) humic substances isolated from three
increasingly saline soils (
Figure 7.6B
). This study showed a clear decrease in fluorescence
intensity for both DOM fractions, though at different ex/em maxima. Provenzano et al.
(2008) indicated that the suppression of ionization was likely responsible for this effect.
Exchangeable Na
+
potential increased in each soil type with salinity, but information on
Mg
2+
ion was not available in this study. More work is needed on these systems to elucidate
the roles of metal-ligand complexation and suppression of ionization in the DOM chemis-
try of saline lake and salinized soil environments.
EEMs fluorescence changes occurred when DOM was added to solutions of increasing
ionic strength - while maintaining the natural
in situ
milieu - in experiments meant to sim-
ulate coastal mixing and to investigate biological and photochemical degradation. Boyd
and Osburn (
2004
) found that mixing of COM isolated from the Susquehanna River by
ultrafiltration into solutions of increasing salinity quenched the fluorescence intensity of
terrestrial and marine humic peaks (C, A, M) but not protein peaks (T, B). In subsequent
work, in which LMW organic matter fluorescence from several estuaries was studied, Boyd
et al. (
2010a
) observed a relatively linear mixing relationship for freshwater-derived LMW
DOM (<1000 nm) but found mid-estuarine LMW DOM (~16 salinity) became more fluo-
rescent when mixed toward the freshwater end member. HMW DOM (>1000 nm) col-
lected in freshwater and mid-estuarine regions in the same estuaries showed EEM peaks,
peak ratios, and PCA and PARAFAC modeled components which generally varied (had
lower signal) at lower salinities and increased toward the ocean end member. The B peak,
